The inﬂuence of Mediterranean riparian forests on stream ...

Limnetica 29 (2) x-xx (2011)Limnetica 36 (2) 507-523 (2017) DOI 1023818limn3618ccopy Asociacioacuten Ibeacuterica de Limnologiacutea Madrid Spain ISSN 0213-8409

The influence of Mediterranean riparian forests on stream nitrogendynamics a review from a catchment perspective

Anna Lupon12lowast Francesc Sabater13 and Susana Bernal14

1 Departament de Biologia Evolutiva Ecologia i Ciegravencies Ambientals Universitat de Barcelona Av Diagonal643 08028 Barcelona Spain2 Department of Forest Ecology and Management Swedish University of Agricultural Sciences (SLU) Skogs-marksgraumlnd 901 83 Umearing Sweden3 Centre for Ecological Research and Forestry Applications (CREAF) Campus de Bellaterra Edifici C 08193Cerdanyola del Vallegraves Spain4 Integrative Freshwater Ecology Group Center for Advanced Studies of Blanes (CEAB-CSIC) Acceacutes a la CalaSant Francesc 14 17300 Blanes Spain

lowast Corresponding author annaluponsluse2

Received 221116 Accepted 310117

ABSTRACT

The influence of Mediterranean riparian forests on stream nitrogen dynamics a review from a catchment perspective

Riparian zones are considered natural filters of nitrogen (N) within catchments because they can substantially diminish theexports of N from terrestrial to aquatic ecosystems However understanding the influence of riparian zones on regulatingN exports at the catchment scale still remains a big challenge in ecology mainly because upscaling plot scale results isdifficult as it is disentangling the effects of riparian upland and in-stream processes on stream water chemistry In this reviewwe summarize previous studies examining key hydrological and biogeochemical processes by which Mediterranean riparianzones regulate catchment water and N exports We focus on Mediterranean regions because they experience a marked climaticseasonality that facilitates disentangling the close link between climate riparian hydrology and stream N exports We showthat Mediterranean riparian soils can be hot spots of N mineralization and nitrification within catchments given their relativelymoist conditions and large stocks of N-rich leaf litter Extremely large nitrification rates can occur during short-time periods(ie hot moments) and lead to increases in stream N loads suggesting that riparian soils can be a potential source of N toadjacent aquatic systems Moreover riparian trees can contribute to decrease riparian groundwater level during the vegetativeperiod and promote reverse fluxes from the stream to the riparian zone During periods of high hydrological retention streamwater exports to downstream ecosystem decrease while stream water chemistry is mostly determined by in-stream processesRiparian tree phenology can also affect catchment N exports by shaping the temporal pattern of both light and litter inputsinto the stream In spring light enhances in-stream photoautotrophic N uptake before riparian leaf-out while riparian leaflitter inputs promote in-stream N mineralization in summer and fall Finally we illustrate that the impact of Mediterraneanriparian zones on stream hydrology and biogeochemistry increases along the stream continuum and can ultimately influencecatchment N exports to downstream ecosystems Overall findings gathered in this review question the well-established ideathat riparian zones are efficient N buffers at least for Mediterranean regions and stress that an integrated view of uplandriparian and stream ecosystems is essential for advancing our understanding of catchment hydrology and biogeochemistry

Key words Soil nitrogen cycle evapotranspiration riparian canopy catchment hydrology in-stream biogeochemical pro-cesses

RESUMEN

La influencia de los bosques de ribera mediterraacuteneos en la dinaacutemica del nitroacutegeno en los riacuteos una revisioacuten desde unaperspectiva de cuenca

Las zonas de ribera son filtros naturales de nitroacutegeno (N) y disminuyen substancialmente los aportes terrestres de N que llegana los riacuteos Sin embargo entender coacutemo las riberas regulan el exporte de N a escala de cuenca es auacuten un reto porque extrapolarlos estudios de parcela a escala de cuenca es difiacutecil asiacute como tambieacuten lo es discernir la contribucioacuten de los procesos que

17140_Limnetica 36(2) pagravegina 507 28092017

508 Lupon Sabater and Bernal

ocurren en las cabeceras riberas y en los propios riacuteos en la quiacutemica del agua En esta revisioacuten se resumen distintos estudiosque examinan los procesos hidroloacutegicos y biogeoquiacutemicos mediante los cuales los bosques de ribera mediterraacuteneos regulanel exporte de agua y N aguas abajo La revisioacuten se centra en las zonas mediterraacuteneas ya que su marcada estacionalidadclimaacutetica permite discernir la estrecha relacioacuten entre el clima la hidrologiacutea de la ribera y las exportaciones de N Losestudios analizados muestran que los suelos riberentildeos pueden ser puntos calientes de mineralizacioacuten y nitrificacioacuten dentrode las cuencas mediterraacuteneas gracias a las condicionas relativamente huacutemedas del suelo y a la hojarasca enriquecida en NTasas de nitrificacioacuten extremadamente altas suceden puntualmente (ie momentos calientes) e incrementan las exportacionesde N sugiriendo que los suelos riberentildeos son fuentes de N para los riacuteos Ademaacutes los aacuterboles riberentildeos contribuyen a ladisminucioacuten del nivel freaacutetico durante el periodo vegetativo y promueven el movimiento de agua del riacuteo hacia la riberaDurante eacutepocas de alta retencioacuten hidroloacutegica el exporte de agua disminuye y su sentildeal quiacutemica depende principalmente de losprocesos biogeoquiacutemicos fluviales El dosel riberentildeo tambieacuten puede afectar al exporte de N aguas abajo ya que controla lasentradas de luz y hojarasca En primavera la asimilacioacuten fotoautotroacutefica de N aumenta justo antes de que las hojas brotenmientras que los aportes de hojarasca pueden incentivar los procesos de mineralizacioacuten en verano y otontildeo Finalmente lainfluencia de las riberas mediterraacuteneas sobre la hidrologiacutea y biogeoquiacutemica del riacuteo incrementa a lo largo del riacuteo y modula elexporte de N a escala de cuenca En conjunto los resultados de esta revisioacuten cuestionan la idea de que los ecosistemas deribera mediterraacuteneos son filtros eficientes de N y ponen de manifiesto la importancia de integrar el funcionamiento de losbosques de cabecera las zonas de ribera y los riacuteos para avanzar en el conocimiento sobre la hidrologiacutea y biogeoquiacutemica aescala de cuenca

Palabras clave Ciclo del nitroacutegeno suelos forestales evapotranspiracioacuten dosel riberentildeo hidrologiacutea de cuenca procesosbiogeoquiacutemicos fluviales

INTRODUCTION

During the last decade anthropogenic activitieshave doubled the amount of available nitrogen(N) in freshwater ecosystems leading to severalenvironmental problems such as eutrophicationacidity toxicity or biodiversity declines (Vi-tousek et al 1997 Schlesinger 2009) The en-vironmental issues derived from N excesses maybe intensified in the future because increasedwarming and dryness would probably reduce wa-ter availability as well as the dilution capacity offluvial ecosystems (Cooper et al 2013) Withincatchments riparian buffer strips have beenconsidered as an economical environmentallyefficient tool for protecting freshwaters from dif-fuse N pollution because they can contribute todecrease N fluxes from terrestrial to aquatic en-vironments (McClain et al 2003 Vidon et al2010)The high capacity of riparian zones to reduce

terrestrial dissolved inorganic N (DIN) inputs de-rives from the topographic hydrologic and bio-geochemical conditions at their unique interfacelocation between upland and streams Flat to-pographies permanent upland-riparian hydrolo-gic connectivity shallow riparian groundwater

tables and carbon (C) enriched soils usually fa-vor ammonium (NH+4 ) and nitrate (NO

minus3 ) removal

via denitrification and biological uptake (Pinayet al 2000 Dosskey et al 2010) Converselyriparian zones that are seasonally disconnectedfrom uplands have large annual water table draw-downs that dry out riparian soils (Ocampo et al2006 Vidon amp Hill 2004) which enhance mi-crobial N mineralization and nitrification andincrease soil NOminus3 availability (Harms amp Grimm2010 Duncan et al 2015) In some cases thehydrologic disconnection between uplands andriparian systems promote the loss of water fromthe stream to the riparian zone (ie stream hydro-logical retention) (Rassam et al 2006) whichcan favor the assimilation of stream DIN by biotaat the stream-riparian edge and consequentlydecrease DIN concentrations (Schade et al2002 Dent et al 2007 Bernal amp Sabater 2012)Therefore the variability in space and timeof soil moisture conditions groundwater tableelevation and water flow paths can substan-tial impact the fate and transport of N throughriparian zonesMediterranean systems (Med systems from

here on) are a unique natural laboratory tounderstand the close link between catchment hy-

Limnetica 36 (2) 507-523 (2017)


Riparian influences on stream N dynamics 509

drology and riparian N buffer capacity becausethey are characterized by a marked seasonal pat-tern in both temperature and precipitation Medregions are subjected to a seasonal alternationof wet (spring) dry (summer) and rewetting(early-fall) periods which can affect N removal

in riparian zones by altering the riparian ground-water table elevation as well as the hydrologicalconnectivity between uplands riparian andstream systems (Fig 1) During rewetting eventsshallow groundwater levels and moist soils canpromote pulses of N mineralization assimila-

Figure 1 Main fluxes of (a) water and (b) nitrogen in Mediterranean riparian forests During rewetting events (solid arrows)water inputs from rainfall and uplands increase groundwater levels and moist the soil Increased water availability enhances soil Nprocessing (mineralization nitrification biological assimilation and denitrification) as well as the transport of inorganic nitrogenfrom riparian soils to streams During dry periods (dashed arrows) riparian evapotranspiration promotes the movement of waterfrom the stream to the riparian zone (ie stream hydrological retention) which can favor biological assimilation and denitrificationat the stream-riparian interface Moreover the riparian canopy is an important source of organic nitrogen to both riparian soils andstreams in summer and fall (gray arrows) Principales flujos de (a) agua y (b) nitroacutegeno en los bosques de ribera mediterraacuteneosTras los eventos de lluvia (liacuteneas solidas) las entradas de agua de lluvia y de las zonas de cabecera incrementan el nivel freaacuteticoy humedecen el suelo El incremento en la disponibilidad de agua favorece el ciclado del nitroacutegeno en el suelo (mineralizacioacutennitrificacioacuten asimilacioacuten bioloacutegica y desnitrificacioacuten) asiacute como el transporte de nitroacutegeno inorgaacutenico desde el suelo riberentildeo hastael riacuteo Durante los periacuteodos secos (liacuteneas discontinuas) la evapotranspiracioacuten de los aacuterboles riberentildeos promueve la entrada de aguadel riacuteo hacia la ribera (ie retencioacuten hidroloacutegica) la cual favorece los procesos de asimilacioacuten de nitroacutegeno y desnitrificacioacuten en lainterface riacuteo-ribera Ademaacutes la hojarasca de los aacuterboles puede ser una fuente importante de nitroacutegeno orgaacutenico para la ribera y elriacuteo en verano y otontildeo (liacuteneas grises)

Limnetica 36 (2) 507-523 (2017)



tion and leaching which can have a direct effecton N pools not only in riparian soils but also inriparian groundwater and stream compartments(Butturini et al 2003 Lupon et al 2016a) Atthe other end low moisture conditions can limitsoil biological activity during summer reduc-ing DIN removal and favoring the storage of Nin riparian soils (Butturini et al 2003) More-over stream hydrological retention induced byriparian evapotranspiration (ET) can decreasestream water export and enhance DIN retentionat the stream-riparian edge (Bernal et al 2013Vazquez et al 2013 Lupon et al 2016b) Thisdual behavior of Med riparian zones between thewet and dry seasons pinpoints that their buffercapacity is extremely vulnerable to changes inclimate and therefore future alterations in pre-cipitation and temperature regimes may severelyaffect the magnitude and temporal pattern of Nexports from Med continental systemsMuch of our current understanding of riparian

hydrology and biogeochemistry in Med-regionshas been obtained from studies carried out at plotor reach scale However the capability of ripar-ian zones to modify catchment N exports is stillpoorly understood limiting our ability for an in-tegrated conservation and management of theseecosystems within the landscape (Vidon et al2010 Pinay et al 2015) Quantifying the roleof riparian zones at catchment scale is complexand not always easy to achieve because streamwater chemistry integrates biogeochemical pro-cesses co-occurring within upland riparian andfluvial ecosystems Furthermore processes oc-curring in different landscape units mutuallyinfluence each other and thus both uplands andstream can affect the buffer capacity of riparianzones by regulating the amount of water carbonand DIN entering to riparian zones (Pinay et al2000 Dent et al 2007) For instance previousstudies have shown that the capability of ripar-ian zones to regulate catchment N export maychange from headwaters to the valley bottom asa result of changes in upland-riparian hydrologicconnectivity (Ocampo et al 2006 Jencso et al2009) and stream hydrological retention (Covinoet al 2010 Montreuil et al 2010) Thereforean integrated view of hydrological and biogeo-

chemical processes occurring across landscapeunits within the catchment is needed to assessthe potential influence of riparian processes onstream N dynamics at catchment scale (Bormannamp Likens 1967)The present review aims to explore the influ-

ence of Med riparian zones on regulating bothstream hydrology and N dynamics at catchmentscale To this end we gleaned different empiricaland modelling studies from the literature in or-der to (i) summarize the current knowledge of Nbiogeochemistry in Med riparian zones and (ii)discuss their potential implications at catchmentscale and within the context of climate changeSpecifically this review focuses on three ma-jor processes by which Med riparian zones canshape catchment N exports (i) the role of soilN transformations on stream N exports duringrewetting events (ii) the influence of ripariantree ET on stream water and N retention duringthe vegetative period and (iii) the capability ofriparian canopy dynamics to shape stream N pro-cesses The studies summarized here comprisedifferent monitoring strategies and include dif-ferent catchment pools which ultimately shedlight on the relevance of the riparian systemwithin the upland-riparian-stream context

RIPARIAN SOILS AS POTENTIALSOURCES OF N TO STREAMS

Soil microbial activity is essential to understandsoil N availability in catchments especially inthose regions experiencing low atmospheric Ninputs (Kendall et al 2007) The biogeochemi-cal processes involved in the soil N cycle dependprimarily on N and water availability Soil or-ganic matter can be quickly mineralized to NH+4under either relatively oxic or anoxic condi-tions nitrification can only occur in aerated soils(water filled pore space (WFPS) lt 80) andboth denitrification and dissimilatory nitrate re-duction require saturated soils (WFPS gt 60)(Linn amp Doran 1984) Furthermore soils thatare N-enriched due to natural or artificial Nfertilization (ie N2 fixation atmospheric depo-sition agriculture) can hold higher rates of soil

Limnetica 36 (2) 507-523 (2017)



N processes than N-poor soils (Vitousek et al1997 LeBauer amp Treseder 2008) Ultimatelythe combination of all these processes mediatesthe amount of inorganic N that is available to beleach out and consequently the temporal andspatial patterns of catchment N exports (Goodaleet al 2009 Ross et al 2012)Within Med catchments riparian zones ex-

hibit larger net N mineralization (07-23 mg Nkgminus1 dminus1) and net nitrification rates (06-15 mg Nkgminus1 dminus1) compared to surrounding upland soils(lt 05 mg N kgminus1 dminus1) (Davis et al 2011 Smithet al 2012 Lupon et al 2016a) Increased mi-crobial N production in Med riparian soils hasbeen attributed to the surplus of organic N fromthe leaf litter of N2-fixing species (CN ratio lt20) and to their relatively wet conditions (WFPS= 40-80) compared to upland soils (Medici etal 2010 Lupon et al 2015) However deni-trification rates are usually low in Med ripariansoils (0-024 mg N kgminus1 dminus1) as a result of waterunsaturated soils (Bernal et al 2007 Davis etal 2011 Hinshaw amp Dahlgren 2016) In factthe contribution of denitrification to N depletion

from riparian groundwater is thought to be neg-ligible at annual and seasonal scales (Sabater ampBernal 2011)Ultimately high nitrification and low denitri-

fication rates lead to high soil NOminus3 availability(5-20 mg N kgminus1) in Med riparian zones (Bernalet al 2007 Smith et al 2012 Lupon et al2016a) and thus Med riparian zones can beconsidered hot spots of soil microbial N sup-ply within catchments Noteworthy the role ofriparian zones on whole catchment N budgetscan vary widely among biomes In arid ripar-ian zones water scarcity (WFPS lt 30) usuallylimits soil microbial activity and therefore thecontribution of riparian zones to catchment Nproduction and losses is thought to be small(Harms amp Grimm 2010 Dijkstra et al 2012)On the other extreme temperate and tropicalsystems usually show waterlogged riparian soils(WFPS gt 70) where denitrification rates areenhanced (02-08 mg N kgminus1 dminus1) Hence hu-mid riparian zones usually become hot spots ofN removal at catchment scale (McClain et al2003 Vidon et al 2010)

Figure 2 Relationship between soil net nitrification rates and stream nitrate export (expressed by catchment area) for (a) oak (b)beech and (c) riparian forests coexisting in a Med catchment of NE Spain Net nitrification rates and stream nitrate exports weremeasured simultaneously every 2-4 weeks from March 2010 to February 2011 Black circles represent pulses of net nitrification (iedisproportionally high rates compared to the median of the distribution) and solid lines indicate the best fitting model The influenceof nitrification rates on stream nitrate loads differed among forest types most of the nitrate produced in upland soils was retainedwithin the catchment while riparian soils were potential nitrogen sources to streams Adapted from Lupon et al (2016a) Relacioacutenentre las tasas netas de nitrificacioacuten en el suelo y las cargas (expresadas por aacuterea especiacutefica) de nitrato del riacuteo para (a) un encinar(b) un hayedo y (c) un bosque de ribera en una cuenca mediterraacutenea del NE de Espantildea Las tasas netas de nitrificacioacuten y lascargas de nitrato del riacuteo se midieron simultaacuteneamente cada 2-4 semanas desde Marzo 2010 hasta Febrero 2011 Los ciacuterculos negrosrepresentan pulsos de nitrificacioacuten (ie tasas desproporcionadamente maacutes altas que la media de la distribucioacuten) y las liacuteneas soacutelidasindican el modelo de mejor ajuste La figura muestra que la influencia de las tasas de nitrificacioacuten sobre las cargas de nitrato del riacuteodifiere entre los bosques la mayor parte del nitrato producido en los suelos forestales es retenida dentro de la cuenca mientras quelos suelos riberentildeos pueden ser fuentes potenciales de nitroacutegeno para los riacuteos Figura adaptada de Lupon et al (2016a)

Limnetica 36 (2) 507-523 (2017)



Soil microbial activity in Med-regions notonly vary across forest types but also overtime Several studies have reported large rates ofsoil microbial processes immediately after rain

events which can be gt 10 fold higher than thoseobserved in the antecedent days (Serrasolses1999 Rey et al 2002) Despite these pulses ofmicrobial N supply may occur during short time

(a)

(b)

(c)

Figure 3 Temporal pattern of (a) monthly riparian evapotranspiration based on sap-flow measurements (b) daily variations instream discharge and (c) mean daily net riparian groundwater inputs for two contiguous reaches during the period 2010-2012 Theheadwater reach had a poor developed riparian zone (5-10 m wide) while it was well-developed in the valley reach (30 m wide)In panel (c) mean daily groundwater inputs gt 0 and lt 0 indicate when the stream reach was net gaining and net losing waterrespectively Riparian ET promoted diel discharge variations and stream hydrological retention (ie the displacement of water fromthe stream to the riparian zone) especially in the valley reach where the riparian zone had higher water requirements V vegetativeperiod D dormant period Adapted from Lupon et al (2016b) Variacioacuten temporal de (a) la tasa mensual de evapotranspiracioacuten enla ribera medida a partir del flujo de salvia (b) los ciclos diarios del caudal y (c) las entradas netas de agua freaacutetica al riacuteo para dostramos de riacuteo contiguos durante el periodo 2010-2012 El tramo de cabecera teniacutea una escasa zona de ribera (5-10 m de anchura)mientras que eacutesta estaba bien desarrollada en el fondo del valle (30 m de anchura) En el panel (c) los valores gt 0 correspondea periodos durante los cuales el tramo fluvial recibe agua de forma neta mientras que los valores lt 0 indican lo contrario Laevapotranspiracioacuten de los aacuterboles riberentildeos causoacute ciclos diarios en el caudal y promovioacute la retencioacuten hidraacuteulica (ie la entrada deagua del riacuteo hacia la ribera) siendo especialmente notoria en el tramo del fondo de valle donde la ribera teniacutea maacutes requerimientosde agua V periodo vegetativo D periodo durmiente Figura adaptada de Lupon et al (2016b)

Limnetica 36 (2) 507-523 (2017)



periods novel studies suggest that they can con-tribute up to 25-40 of annual mineralizationand nitrification (Lupon et al 2016a) Moreoversuch biogeochemical pulses (or hot moments)can increase catchment N exports because rain-fall events are usually associated with a largemobilization of water and N from the soil pool tothe stream (Bernal et al 2013) However rewet-ting events can also induce pulses of microbialimmobilization (Harms amp Grimm 2010 Dijk-stra et al 2012) denitrification (Butturini et al2003 Tiemann amp Billings 2012) and biologi-cal uptake (Harms amp Grimm 2010 Jongen etal 2013) if there is enough time for biota andsolute media to interact Thus the effect of mi-crobial pulses on stream N export will dependon how quickly water is transferred from terres-trial systems to streams (Meixner amp Fenn 2004Lohse et al 2013) This idea is well illustratedby Lupon et al (2016a) who simultaneous quan-tified stream N loads and net nitrification ratesin three forest types (oak beech and riparian)of a Med catchment under a wide range of an-tecedent moisture conditions The study showeda contrasting influence of net nitrification rateson stream NOminus3 loads between Med uplands andriparian soils As expected for N-limited ecosys-tems there was a weak relation between uplandnitrification and stream N loads suggesting thatmost of the NOminus3 produced in upland soils tendedto be retained within the catchment (Fig 2a and2b) The capacity of terrestrial ecosystems toretain N was especially noticeable during hotmoments when stream N loads did not increasedespite the extremely high nitrification rates ob-served in the upland soils Conversely streamNOminus3 loads were strongly related to nitrificationin riparian soils (Fig 2c) highlighting their po-tential to enhance catchment N losses due totheir proximity and strong hydrological connec-tion with the adjacent aquatic ecosystems Thesefindings are in agreement with recent modellingapproaches which suggest that riparian soilscan be critical for understanding the temporalpattern of N budgets and exports in Med catch-ments (Medici et al 2010 Lupon et al 2015)In particular model simulations suggest that theinfluence of riparian soils on N exports may be

maxima in summer and early-fall when warmand well oxygenated soils can enhance nitrifica-tion rates and NOminus3 leaching All together thesestudies highlight that despite upland systemshave a strong influence on catchment N exportsthe role of riparian hydrology and biogeochem-istry on modulating stream N exports can beequally important

RIPARIAN EVAPOTRANSPIRATION AS ADRIVER OF STREAM N EXPORTS

In most Med systems upland water requirementsare high and thus riparian ET (450-600 mmyrminus1) contributes minimally (lt 5) to the to-tal annual catchment water depletion (Sabater ampBernal 2011 Lupon et al 2016b) Nonethelessriparian ET can strongly influence the temporalpattern of stream hydrology as well as the hy-drological connectivity between riparian zonesand streams (Fig 3) On a sub-daily basis ripar-ian vegetation can induce a variation in streamdischarge up to 20 by taking up water fromthe riparian aquifer or directly from the stream(Lundquist amp Cayan 2002 Lupon et al 2016b)(Fig 3b) Moreover during the vegetative pe-riod riparian ET can contribute to decreasethe riparian groundwater elevation and increasestream hydrological retention (ie the displace-ment of water from the stream to the riparianzone) (Rassam et al 2006 Lupon et al 2016b)(Fig 3c) The seasonal influence of riparian ETon catchment hydrology becomes more accen-tuated in drier climates where drawbacks in thegroundwater table can induce premature abscis-sion of riparian tree leafs (Sabater amp Bernal2011) and the complete desiccation of the streamchannel (Butturini et al 2003 Medici et al2008)From a catchment perspective several studies

have shown that stream hydrological retentionincrease from headwaters to the valley bottom(Covino et al 2010 Montreuil et al 2010Bernal amp Sabater 2012) Yet there are fewempirical evidences linking the longitudinalvariation of stream hydrology with riparian wa-ter requirements A recent study conducted in

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the NE of the Iberian Peninsula showed thatduring the vegetative period stream hydrologi-cal retention occurred often at the valley bottomof a headwater catchment (60 of time) wherea well-developed riparian forest ensured high ETrates compared to headwaters (Fig 3) (Luponet al 2016c) In this line of thought pioneermodelling approaches indicate that the ripariancompartment is crucial for successfully simu-late the non-linear behavior of stream hydrologyat the valley bottom of Med catchments (Medici

et al 2008 Lupon 2015) These results con-trast with those found in temperate streamswhere water exports mostly depend on seasonalchanges in precipitation and upland ET (Futteret al 2014 Kim et al 2014) and suggest thatriparian ET could be critical to predict water andnutrients exports in regions experiencing somewater limitationRiparian ET can not only influence stream

discharge but also stream N concentrations be-cause the water moving towards the riparian zone

(a)

(b)

Figure 4 Temporal pattern of the relative difference in monthly volume-weighted stream nitrate concentration (ΔNOndash3) betweenthe headwaters and the valley bottom of (a) a semi-arid Med catchment (Fuirosos) and (b) a sub-humid Med catchment (Font delRegagraves) The two catchments located lt 50 km apart were mostly forested and received similar amounts of atmospheric N deposition(15 kg N yearndash1) However catchments had different annual precipitation (613 mm in Fuirosos vs 980 mm in Font del Regagraves) andhydrologic regime (temporal in Fuirosos vs permanent in Font del Regaacutes) For each month ΔNOndash3 = (Cvalley minus Cheadwater)Cheadwaterand was expressed by km of reach length Values of ΔNOndash3 gt 0 indicate when stream nitrate concentration increased along the reachwhile ΔNOndash3 lt 0 indicates the opposite The figure shows the contrasting behavior of both reaches during the vegetative period Fontdel Regagraves release nitrate while Fuirosos uptake nitrate Adapted from Bernal and Sabater (2012) and Lupon et al (2016b) Variacioacutentemporal de la diferencia relativa en las concentraciones de nitrato (ΔNOndash

3 ponderadas por el volumen) entre la cabecera y el fondode valle de un tramo de riacuteo en (a) una cuenca semi-aacuterida (Fuirosos) y (b) una cuenca sub-huacutemeda (Font del Regagraves) Ambas cuencassituadas a lt 50 km de distancia eran mayormente forestadas y recibiacutean una cantidad similar de deposicioacuten atmosfeacuterica (15 kg Nantildeondash1) Las cuencas diferiacutean en la precipitacioacuten anual (613 mm en Fuirosos vs 980 mm en Font del Regagraves) y en el reacutegimen hidroloacutegico(temporal en Fuirosos vs permanente en Font del Regagraves) Para cada mes ΔNOndash

3 = (CvalleyminusCheadwater)Cheadwater y es expresada en kmde longitud de tramo fluvial Valores de ΔNOndash

3 gt 0 indican periodos durante los cuales las concentraciones de nitrato incrementana lo largo del tramo mientras que ΔNOndash

3 lt 0 indica lo contrario La figura muestra el comportamiento contrastado de ambos riacuteosdurante el periacuteodo vegetativo Font del Regagraves libera nitrato mientras que Fuirosos retiene nitrato Figura adaptada de Bernal andSabater 2012 y Lupon et al (2016b)

Limnetica 36 (2) 507-523 (2017)



can enhance the biological N assimilation nearthe stream-riparian edge and reduce up to 60the stream DIN concentration (Schade et al2002 2005) Moreover riparian ET can favorthe interaction between the water column andthe hyporheic zone because during periods ofhydrological retention the dominance of sub-surface flow increase respect to the surface flow(Dahm et al 1998) Presumably hyporheiczones in Med streams are regions of high hetero-trophic activity and oxygen poor environmentsand therefore a sharp decrease of stream NOminus3concentration is expect to occur due to high ratesof N uptake and denitrification (Kemp amp Dodds2002 Brooks amp Lemon 2007 Bernal et al2013) However it has been shown that somearid streams can hold well-oxygenated hyporheiczones even during low discharges periods whichcan favor in-stream N mineralization and nitri-fication and consequently increase rather thandecrease stream NOminus3 concentrations (Holmes etal 1994 Jones et al 1995)Ultimately the combination of biogeochemi-

cal processes occurring within streams riparianand hyporheic zones would determine down-stream N fluxes In some Med catchment a largedrop in stream NOminus3 concentrations and fluxeshas been observed from headwaters to the val-ley bottom likely as a result of the biologicalN assimilation in the stream-riparian edge thestream water column andor the hyporheic zone(Meixner amp Fenn 2004 von Schiller et al 2008Bernal amp Sabater 2012) (Fig 4a) Converselyother studies have reported longitudinal in-creases in stream NOminus3 concentrations when NO

minus3

release processes (ie nitrification) overwhelmbiological N assimilation (Fig 4b) (Dent et al2007 Bernal et al 2015 Lupon et al 2016c)The reasons for differences in the N process-ing along the river continuum remain unclearthough these different patterns likely respondto differences in riparian vegetation streambedsubstrate organic matter availability redox con-ditions and water residence time (Brooks ampLemon 2007 Abbott et al 2016) Moreoverthe degree of hydrological interactions amongthe riparian hyporheic and stream water com-partments may be fundamental to understand the

efficiency of riparian biota to mediate stream Nfluxes (Abbott et al 2016) This idea was wellillustrated by Dent et al (2007) who showedthat the capacity of riparian zones to remove Nfrom an arid stream in Arizona varied strongly(from 7-67) depending on whether stream wa-ter entered uniformly or only at specific locationsto the riparian zone Overall the abovemen-tioned studies pinpoint that the timing and extentof hydrological connectivity between catchmentunits is key for improving our ability to predict Nretention and exports from catchments (Dent etal 2007 Pinay et al 2015 Abbott et al 2016)

RIPARIAN CANOPY AS A REGULATOROF IN-STREAM N CYCLING

Riparian canopy can play a fundamental role incontrolling seasonal changes in stream metabo-lism in Med regions because it regulates bothlight and organic matter inputs to the streamchannel (Guasch amp Sabater 1995 von Schilleret al 2007) However our understanding of howriparian canopy influences stream N cycling bydriving stream metabolism is limited In a pio-neer study Sabater et al (2000) found that bothalgae biomass and NH+4 uptake rates were higherin an open-canopy than in a riparian shadedstream reach suggesting that riparian canopymay limit the in-stream capacity to take up Nfrom the water column More recently Luponet al (2016c) showed that gross primary pro-duction and associated diel variations in streamNOminus3 concentrations decreased as the ripariancrown closed and limited light inputs into thestream (Fig 5) Interestingly the study showedno diel NOminus3 variations in riparian groundwaterevidencing that in-stream photoautotrophic ac-tivity alone was responsible for the diel cyclesin NOminus3 concentration (Fig 5c) This result isimportant because allowed separating the influ-ence of riparian vs in-stream processes on streamN dynamics and further highlights that streamprimary production is directly linked to the phe-nology of the riparian trees Moreover Luponet al (2016c) nicely illustrated the importanceof stream metabolism in regulating catchment

Limnetica 36 (2) 507-523 (2017)



N exports by showing that in-stream photoau-totrophic activity reduced by 10 the catchmentNOminus3 export in spring These results are com-

parable to those reported for high productivityrivers (Grimm 1987 Heffernan amp Cohen 2010)suggesting that photoautotrophs can substan-

Figure 5 Temporal pattern of (a) environmental conditions (b) stream metabolism and (c) diel nitrate variations (expressed asthe relative difference between midnight and noon concentrations) during spring at the valley bottom of a Med catchment Panel(a) shows daily photosynthetically active radiation (ΣPAR) (grey shadow) and mean daily stream water temperature (black line)Panel (b) shows data for gross primary productivity (GPP black line) and ecosystem respiration (ER gray line) Panel (c) showsdata for stream water (black circles) and riparian groundwater (white circles) Solid lines represent the predicted day-night variationsin stream nitrate concentration calculated from GPP rates (black line) and from hydrological mixing with groundwater (grey line)Light inputs to the stream favored in-stream GPP and photoautotrophic N uptake before the riparian canopy closure Adapted fromLupon et al (2016c) Variacioacuten temporal de (a) los factores ambientales (b) las tasas metaboacutelicas y (c) la variacioacuten diaria de nitrato(expresada como la diferencia relativa entre las concentraciones medidas en la medianoche y el mediodiacutea) durante la primavera enel fondo del valle de una cuenca mediterraacutenea El panel (a) muestra los valores diarios de radiacioacuten solar (ΣPAR) (sombra gris) ytemperatura media (liacutenea negra) El panel (b) muestra las tasas de produccioacuten primaria bruta (GPP liacutenea negra) y respiracioacuten delecosistema (ER liacutenea gris) El panel (c) muestra los valores de ΔNOndash

3 en el riacuteo (ciacuterculos negros) y en el freaacutetico de ribera (ciacuterculosblancos) Las liacuteneas soacutelidas representan la variacioacuten diacutea-noche calculada a partir de las tasas de GPP (liacutenea negra) y a partir de lamezcla hidroloacutegica con el agua del freaacutetico (liacutenea gris) Las entradas de luz favorecen la produccioacuten primaria bruta y la asimilacioacutenfotoautotroacutefica de nitroacutegeno justo antes de cerrarse el dosel riberentildeo Figura adaptada de Lupon et al (2016c)

Limnetica 36 (2) 507-523 (2017)



tially contribute to transitorily reduce catchmentN losses even in highly heterotrophic forestedstreamsRiparian leaf litter abscission during late-

summer and early-fall has also a strong influenceon stream hydrology and nutrient biogeochem-istry Large stocks of organic matter increasewater transient storage zones and promote the in-teraction between stream biota and fresh organicmatter which can favor the development of mi-crobial communities and lead to high values ofecosystem respiration in-stream N mineraliza-tion and stream NH+4 concentrations (Acuntildea etal 2004 Argerich et al 2008 Bernal et al2012) In some cases the rapid mineralizationof leachates may also increase the N demandof stream biota and favor in-stream NH+4 uptake(Argerich et al 2008 Bernal et al 2012) andnitrification (Acuntildea et al 2005 Bernal et al2015 Lupon et al 2016b) Altogether the pre-vious studies suggest that Med riparian zonescan be important sources of organic N via lit-terfall which can be mineralized and nitrifiedwithin the stream under favorable conditionsMoreover the presence of N2-fixing speciessuch as Alnus glutinosa or the invasive Robineapseudoacacia can enhance stream N cycling byproviding N-rich leaf litter (Starry et al 2005Mineau et al 2011) and thus natural or humaninduced changes in riparian species compositioncould have a strong impact on stream nutrientdynamics

CONCLUSIONS AND FINAL REMARKS

Riparian zones can play a key role in regulat-ing the N cycle in Med continental systems yetunderstanding their influence on catchment Nexports is still limited In this review we haveshown that fundamental differences exist in thebiogeochemistry of Med riparian zones com-pared to more humid ones that precludes thedirect application of existing knowledge fromtemperate regions For instance we showedthat riparian soils can be hot spots of N supplywithin Med catchments because they are N-richwell oxygenated and relatively wet Moreover

Med riparian soils can be potential sources ofDIN to the streams due to their proximity andstrong hydrological connection with adjacentaquatic ecosystems which contrast with the Nsink behavior typically reported in more humidriparian zones (McClain et al 2003) Interest-ingly the contribution of Med riparian soils tocatchment N export is expected to increase inthe future because they are highly responsiveto warming (Duncan et al 2015 Lupon et al2015) In particular simulations from a mecha-nistic model suggest that N mineralization andnitrification rates in Med riparian soils could in-crease by 6-11 over the next century whichwould increase the amount of NOminus3 that can beleach out to fluvial ecosystems (Lupon et al2015) Moreover future change in climate mayalter the composition and structure of riparianforests thus affecting the soil N pool and exports(Medina-Villar et al 2015 Bruno et al 2016)Taken together the previous studies suggest thatriparian soils may be essential to understandpresent and future temporal patterns of N exportsin Med catchments and stress the importanceto consider this catchment pool as a potentialsource of other essential nutrientsThe results presented here illustrate that ri-

parian ET can influence catchment N export bymediating both stream discharge and N con-centrations From a hydrological perspectiveriparian ET can have a disproportionately largeimpact on water resources by dropping downriparian groundwater levels promoting streamhydrological retention and decreasing streamdischarge Previous studies have shown that rel-atively small decreases in annual precipitationcan markedly increase the relative contribu-tion of riparian ET to catchment water budgetssuggesting that future climate alterations couldexacerbate the impact of Med riparian zones oncatchment water resources (Lupon et al 2016b)Therefore we propose that this catchment poolshould be considered to a further extent whenmodeling stream hydrology as well as for asound and integrated management of catchmentwater resourcesFrom a biogeochemical point of view the ex-

change of water between streams and riparian

Limnetica 36 (2) 507-523 (2017)



zones induced by riparian ET can promote theN filter capacity of riparian ecosystems by en-hancing biotic N uptake at the stream-riparianedge However in-stream processes (either Nuptake andor release) can screen to some extendthe influence of riparian processes on stream Ndynamics Indeed a whole-reach mass balanceapproach based on monthly samplings revealedthat in-stream N processing was at least as im-portant as net riparian groundwater inputs forunderstanding the longitudinal pattern of streamDIN concentrations (Bernal et al 2015) Over-all the studies condensed in this review suggestthat Med riparian zones may have a limited ca-pacity to decrease catchment N export and ques-tion the well-established idea that riparian zonesare efficient N buffers at least for catchmentsexperiencing some degree of water limitationRiparian phenology can also control in-stream

N cycling and even catchment N export duringsome periods We have shown that canopy leaf-out controls the magnitude of in-stream photo-autotrophic activity and the associated N uptakewhile leaf litterfall burst ecosystem respirationin summer and fall Climate change is alreadyinfluencing riparian tree phenology by promot-ing an earlier emergence of riparian tree leavesand longer vegetative periods (Perry et al 2012)These alterations may have implications for Ncycling in Med streams because photoautotro-phic N uptake can only occur when both lightand temperature conditions are favorable (Luponet al 2016c) Moreover future warming anddrying conditions may influence the annual dis-tribution of riparian leaves abscission as theamount of leafs falling in summer is inverselyrelated to precipitation and stream flow in Medstreams (Sanpera-Calbet et al 2016) The pre-mature leaf abscission of riparian trees togetherwith warmer temperatures could increase N re-lease processes and dissolved N concentrationsin Med streams in late-summer leading to en-vironmental issues such as downstream eutro-phicationFinally findings gathered in this review sug-

gest changes in the structural or functional traitsof riparian zones can influence in-stream N cy-cling along the river continuum thus affecting

catchment N exports from headwaters to the val-ley bottom We have shown that the influence ofriparian ET on stream hydrology as well as thecapacity of riparian trees to regulate stream lightand leaf litter inputs increase from headwaters tothe valley bottom and have a substantial effecton in-stream N cycling Overall we have learnedabout the importance of studying a particularbiogeochemical process or ecosystem within abroader context in order to get a more completepicture of their ecological role at relevant spatialand temporal scales However and paradoxi-cally most catchment studies focus mostly onupland ecosystems (Goodale et al 2009 Rosset al 2012) while studies assessing in-streamnutrient cycling do not consider the interactionbetween the stream and riparian groundwater(Heffernan amp Cohen 2010 Bernal et al 2012)The simplification of empirical approximationsis unavoidable and it has been shown to be help-ful for understanding some patterns and driversof complex systems such as forest riparian andstreams Yet the implications of the obtained re-sults are constrained to the scale of observationand difficult to link with processes occurring atlarger scales more relevant from an ecosystemperspective Hence we suggest that future catch-ment research should take into account as muchas possible the links between upland riparianand in-stream biogeochemical cycles to be ableto quantify their potential role as regulators ofwater nutrients sediments and pollutants withinlandscapes This is a true challenge for both for-est and stream ecologists and at the same timean essential exercise in order to advance catch-ment biogeochemistry and develop integratedmanagement strategies that successfully mitigatefuture increments in anthropogenic N inputs

ACKNOWLEDGMENTS

We are thankful to Siacutelvia Poblador Ada PastorLiacutedia Cantildeas Dani Nadal and Miquel Ribot fortheir invaluable field and lab assistance and toEugegravenia Martiacute and Stefan Gerber for their inspi-rational contributions at various studies summa-rized here We also express our deep gratitude to

Limnetica 36 (2) 507-523 (2017)



the Iberian association of Limnology (AIL) forthe award to the PhD thesis and the support re-ceived to disseminate the results therein Specialthanks are extended to two anonymous reviewersfor helpful comments on an earlier version of themanuscript Financial supported was providedby the Spanish Government through the projectsMONTES-Consolider (CSD2008-00040-MON-TES) MEDFORESTREAM (CGL2011-30590)and MEDSOUL (CGL2014-59977-C3-2) ALwas supported by a FPU PhD fellowship fromthe Spanish Ministry of Education and Sci-ence (AP-2009-3711) and a Kempe stipendSB work was funded by the Spanish ResearchCouncil (JAE-DOC027) the Spanish CICT(Juan de la Cierva contract JCI-2008-177) Eu-ropean Social Funds (FSE) and the NICUS(CGL-2014-55234-JIN) project

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ABBOTT BW V BARANOV C MENDOZA-LE-RA M NIKOLAKOPOULOU A HARJUNGT KOLBE MN BALASUBRAMANIAN TNVAESSEN F CIOCCA A CAMPEAU MBWALLIN P ROMEIJN M ANTONELLI JGONSALVES T DATRY AM LAVERMANJR DE DREUZY DM HANNAH S KRAUSEC OLDHAM amp G PINAY 2016 Using multi-tracer inference to move beyond single-catchmentecohydrology Earth-Science Reviews 160 19ndash42 DOI101016jearscirev201606014

ACUNtildeA V A GIORGI I MUNtildeOZ U UEHLIN-GER amp S SABATER 2004 Flow extremes andbenthic organic matter shape the metabolism of aheadwater Mediterranean stream Freshwater Bi-ology 49 960ndash971 DOI101111j1365-2427200401239x

ACUNtildeA V I MUNtildeOZ A GIORGI M OMELLAF SABATER amp S SABATER 2005 Droughtand postdrought recovery cycles in an intermittentMediterranean stream structural and functionalaspects Journal of the North American Bentholog-ical Society 24 919ndash933 DOI10189904-0781

ARGERICH A E MARTIacute F SABATER M RI-BOT D VON SCHILLER amp JL RIERA 2008Combined effects of leaf litter inputs and a floodon nutrient retention in a Mediterranean mountain

stream during fall Limnology and Oceanography53 631ndash641 DOI104319lo20085320631

BERNAL S F SABATER A BUTTURINI ENIN amp S SABATER 2007 Factors limiting deni-trification in a Mediterranean riparian forest SoilBiology and Biochemistry 39 2685ndash2688 DOI101016jsoilbio200704027

BERNAL S amp F SABATER 2012 Changes in dis-charge and solute dynamics between hillslope andvalley-bottom intermittent streamsHydrology andEarth System Sciences 16 1595ndash1605 DOI105194hess-16-1595-2012

BERNAL S D VON SCHILLER E MARTIacute ampF SABATER 2012 In-stream net uptake regu-lates inorganic nitrogen export from catchmentsunder base flow conditions Journal of Geophysi-cal Research Biogeosciences 117 1ndash10 DOI1010292012JG001985

BERNAL S D VON SCHILLER F SABATER ampE MARTIacute 2013 Hydrological extremes modu-late nutrient dynamics in mediterranean climatestreams across different spatial scalesHydrobiolo-gia 719 31ndash42 DOI101007s10750-012-1246-2

BERNAL S A LUPON M RIBOT F SABATERamp E MARTIacute 2015 Riparian and in-stream con-trols on nutrient concentrations and fluxes in a head-water forested stream Biogeosciences 12 1941ndash1954 DOI105194bg-12-1941-2015

BORMANN F amp G LIKENS 1967 Nutrient cy-cling Science 155 424ndash429

BROOKS PD amp MM LEMON 2007 Spatial va-riability in dissolved organic matter and inorganicnitrogen concentrations in a semiarid stream SanPedro River Arizona Journal of Geophysical Re-search Biogeosciences 112 1ndash11 DOI1010292006JG000262

BRUNOD C GUTIEacuteRREZ-CAacuteNOVAS D SAacuteN-CHEZ-FERNAacuteNDEZ J VELASCO amp C NILS-SON 2016 Impacts of environmental filters onfunctional redundancy in riparian vegetation Jour-nal of Applied Ecology 53 846ndash855 DOI1011111365-266412619

BUTTURINI A S BERNAL E NIN C HELLINL RIVERO S SABATER amp F SABATER 2003Influences of the stream groundwater hydrology onnitrate concentration in unsaturated riparian areabounded by an intermittent Mediterranean streamWater Resources Research 39 1ndash13 DOI1010292001

COOPER SD PS LAKE S SABATER JM ME-LACKamp JL SABO 2013 The effects of land use

Limnetica 36 (2) 507-523 (2017)



changes on streams and rivers in mediterraneanclimates Hydrobiologia 719 383ndash425 DOI101007s10750-012-1333-4

COVINO TP B MCGLYNN amp MA BAKER2010 Separating physical and biological nutri-ent retention and quantifying uptake kinetics fromambient to saturation in successive mountainstream reaches Journal of Geophysical Research115 G04010 DOI1010292009JG001263

DAHM CN NB GRIMM P MARMONIERHM VALETT amp P VERVIER 1998 Nutrientdynamics at the interface between surface watersand groundwaters Freshwater Biology 40 427ndash451 DOI101046j1365-2427199800367x

DAVIS JH SM GRIFFITH amp PJ WIGINGTONJR 2011 Surface water and groundwater nitrogendynamics in a well drained riparian forest withina poorly drained agricultural landscape Journal ofEnvironmental Quality 40 505ndash516

DENT CL NB GRIMM E MARTIacute JW ED-MONDS JC HENRY amp JR WELTER 2007Variability in surface-subsurface hydrologic inter-actions and implications for nutrient retention inan arid-land stream Journal of Geophysical Re-search 112 G04004 DOI1010292007JG000467

DIJKSTRA FA DJ AUGUSTINE P BREWER ampJC VON FISCHER 2012 Nitrogen cycling andwater pulses in semiarid grasslands are microbialand plant processes temporally asynchronousOe-cologia 170 799ndash808 DOI101007s00442-012-2336-6

DOSSKEYMG P VIDON NP GURWICKCJ ALLAN TP DUVAL amp R LOWRANCE2010 The role of riparian vegetation in protect-ing and improving chemical water quality instreams Journal of the American Water ResourcesAssociation 46 261ndash277 DOI101111j1752-1688201000419x

DUNCAN JM LE BAND PM GROFFMAN ampES BERNHARDT 2015 Mechanisms drivingthe seasonality of catchment scale nitrate exportevidence for riparian ecohydrologic controls Wa-ter Resources Research 51 3982ndash3997 DOI1010022015WR016937

FUTTER MN MA ERLANDSSON D BUT-TERFIELD PG WHITEHEAD SK ONI amp AJWADE 2014 PERSiST A flexible rainfall-runoffmodelling toolkit for use with the INCA familyof models Hydrology and Earth System Sciences18 855ndash873 DOI105194hess-18-855-2014

GOODALE CL SA THOMAS G FREDRIK-SEN EM ELLIOTT KM FLINN TJ BUT-LER amp MT WALTER 2009 Unusual seasonalpatterns and inferred processes of nitrogen reten-tion in forested headwaters of the Upper Susque-hanna River Biogeochemistry 93 197ndash218 DOI101007s10533-009-9298-8

GRIMM NB 1987 Nitrogen dynamics during suc-cession in a desert stream Ecology 68 1157ndash1170 DOI1023071939200

GUASCH H amp S SABATER 1995 Seasonal vari-ations on photosynthesis-irradiance responses bybiofilms in Mediterranean streams Journal of Phy-cology 31 727ndash735 DOI101111j0022-3646199500727x

HARMS TK amp NB GRIMM 2010 Influence ofthe hydrologic regime on resource availability in asemi-arid stream-riparian corridor Ecohydrology3 349ndash359 DOI101002eco119

HEFFERNAN JB amp MJ COHEN 2010 Directand indirect coupling of primary production anddiel nitrate dynamics in a subtropical spring-fedriver Limnology and Oceanography 55 677ndash688DOI104319lo20105520677

HINSHAW SE amp RA DAHLGREN 2016 Ni-trous oxide fluxes and dissolved N gases (N2 andN2O) within riparian zones along the agricultur-ally impacted San Joaquin River Nutrient Cyclingin Agroecosystems 105 85ndash102 DOI101007s10705-016-9777-y

HOLMES RM SG FISHER amp NB GRIMM1994 Parafluvial nitrogen dynamics in a desertstream ecosystem Journal of North AmericanBenthological Society 13 468ndash478 DOI1023071467844

JENCSO KG BL MCGLYNN MN GOOSEFFSM WONDZELL KE BENCALA amp LAMARSHALL 2009 Hydrologic connectivity be-tween landscapes and streams Transferring reach-and plot-scale understanding to the catchment sca-le Water Resources Research 45 W04428 DOI1010292008WR007225

JONES JB SG FISHER amp NB GRIMM 1995Nitrification in the hyporheic zone of a desertstream ecosystem Journal of the North AmericanBenthological Society 14 249ndash258 DOI1023071467777

JONGENM X LECOMTE S UNGER D FAN-GUEIRO amp JS PEREIRA 2013 Precipitationvariability does not affect soil respiration and ni-trogen dynamics in the understorey of a Medi-

Limnetica 36 (2) 507-523 (2017)



terranean oak woodland Plant and Soil 372 235ndash251 DOI101007s11104-013-1728-7

KEMP MJ amp WK DODDS 2002 The influence ofammonium nitrate and dissolved oxygen concen-trations on uptake nitrification and denitrificationrates associated with prairie stream substrata Lim-nology and Oceanography 47 1380ndash1393 DOI104319lo20024751380

KENDALL C EM ELLIOTT amp SD WANKEL2007 Tracing anthropogenic inputs of nitrogento ecosystems In Stable isotopes in ecology andenvironmental science R Michener amp K Lajtha(eds) 375ndash449 Wiley-Blackwell Oxford UK

KIM Y LE BAND amp C SONG 2014 The Influ-ence of Forest Regrowth on the Stream Dischargein the North Carolina Piedmont Watersheds Jour-nal of the American Water Resources Association50 57ndash73 DOI101111jawr12115

LEBAUER DS amp KK TRESEDER 2008 Nitro-gen limitation of net primary productivity Ecol-ogy 89 371ndash379 DOI10189006-20571

LINN DM amp JW DORAN 1984 Effect of water-filled pore space on carbon dioxide and nitrousoxide production in tilled and nontilled soils SoilScienceSociety ofAmerica Journal 48 1267ndash1272

LOHSE KA J SANDERMAN amp R AMUND-SON 2013 Identifying sources and processesinfluencing nitrogen export to a small stream usingdual isotopes of nitrate Water Resources Re-search 49 5715ndash5731 DOI101002wrcr20439

LUNDQUIST JD amp DR CAYAN 2002 Seasonaland spatial patterns in diurnal cycles in streamflowin the western United States Journal of Hydrome-teorology 3 591ndash603 DOI1011751525-754

LUPON A 2015 The influence of Mediterranean ri-parian zones on stream nitrogen dynamics A catch-ment approach PhD Thesis Universitat de Bar-celona Spain

LUPON A S GERBER F SABATER amp S BER-NAL 2015 Climate response of the soil nitrogencycle in three forest types of a headwater Mediter-ranean catchment Journal of Geophysical Re-search Biogeosciences 120 859ndash875 DOI1010022014JG002791

LUPON A F SABATER A MINtildeARROamp S BER-NAL 2016a Contribution of pulses of soil nitro-gen mineralization and nitrification to soil nitrogenavailability in three Mediterranean forests Euro-pean Journal of Soil Science 67 303ndash313 DOI101111ejss12344

LUPON A S BERNAL S POBLADOR E MAR-TIacute amp F SABATER 2016b The influence of ripar-ian evapotranspiration on stream hydrology andnitrogen retention in a subhumid Mediterraneancatchment Hydrology and Earth System Sciences20 3831ndash3842 DOI105194hess-20-3831-2016

LUPON A E MARTIacute F SABATERampS BERNAL2016c Green light Gross primary production in-fluences seasonal stream N export by controllingfine-scaleN dynamics Ecology 97 133ndash144 DOI10189014-22961

McCLAINME EW BOYER CL DENT SEGERGEL NB GRIMM PM GROFFMANSC HART JW HARVEY CA JOHNSTONE MAYORGA WH McDOWELL amp G PINAY2003 Biogeochemical hot spots and Hot momentsat the interface of terrestrial and aquatic ecosys-tems Ecosystems 6 301ndash312 DOI101007s10021-003-0161-9

MEDICI C A BUTTURINI S BERNAL F SA-BATER amp F FRANC 2008 Modelling the non-linear hydrological behaviour of a small Mediter-ranean forested catchment Hydrological Pro-cesses 3828 3814ndash3828 DOI101002hyp6991

MEDICI C S BERNAL A BUTTURINI F SA-BATER M MARTIN AJ WADE amp F FRAN-CES 2010 Modelling the inorganic nitrogenbehaviour in a small Mediterranean forested catch-ment Fuirosos (Catalonia) Hydrology and EarthSystem Sciences 14 223ndash237 DOI105194hess-14-223-2010

MEDINA-VILLAR S P CASTRO-DIacuteEZAALON-SO I CABRA-RIVAS IM PARKER amp E PEacute-REZ-CORONA 2015 Do the invasive trees Ailan-thus altissima and Robinia pseudoacacia alterlitterfall dynamics and soil properties of riparianecosystems in Central Spain Plant and Soil 396311ndash324 DOI101007s11104-015-2592-4

MEIXNER T amp M FENN 2004 Biogeochemicalbudgets in a Mediterranean catchment with highrates of atmospheric N deposition-Importance ofscale and temporal asynchrony Biogeochemistry70 331ndash356 DOI101007s10533-003-4106-3

MINEAUMM CV BAXTER amp AM MAR-CARELLI 2011 A non-native riparian tree (Elae-agnus angustifolia) changes nutrient dynamics instreams Ecosystems 14 353ndash365 DOI101007s10021-011-9415-0

MONTREUIL O P MEROT amp P MARMONIER2010 Estimation of nitrate removal by riparianwetlands and streams in agricultural catchments

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effect of discharge and stream order Freshwa-ter Biology 55 2305ndash2318 DOI101111j1365-2427201002439x

OCAMPO CJ M SIVAPALAN amp C OLDHAM2006 Hydrological connectivity of upland-ripa-rian zones in agricultural catchments Implicationsfor runoff generation and nitrate transport Jour-nal of Hydrology 331 643ndash658 DOI101016jjhydrol200606010

PERRY LG DC ANDERSEN LV REYNOLDSSM NELSON amp PB SHAFROTH 2012 Vul-nerability of riparian ecosystems to elevated CO2and climate change in arid and semiarid west-ern North America Global Change Biology 18821ndash842 DOI101111j1365-2486201102588x

PINAY G VJ BLACK A-M PLANTY-TABAC-CHI B GUMIERO amp H DEacuteCAMPS 2000 Ge-omorphic control of denitrification in large riverfloodplain soils Biogeochemistry 50 163ndash182DOI101023A1006317004639

PINAY G S PEIFFER JR DE DREUZY SKRAUSE DM HANNAH JH FLECKEN-STEIN M SEBILO K BISHOP amp L HUBERT-MOY 2015 Upscaling nitrogen removal ca-pacity from local hotspots to low stream ordersrsquodrainage basins Ecosystems 18 1101ndash1120 DOI101007s10021-015-9878-5

RASSAMDW CS FELLOWS R DE HAYR HHUNTER amp P BLOESCH 2006 The hydrologyof riparian buffer zones two case studies in anephemeral and a perennial stream Journal of Hy-drology 325 308ndash324 DOI101016jjhydrol200510023

REY A E PEGORARO V TEDESCHI I DE PA-RRI P JARVIS amp R VALENTINI 2002 Annualvariation in soil respiration and its components in acoppice oak forest in Central Italy Global ChangeBiology 8 851ndash866 DOI101046j1365-2486200200521x

ROSS DS JB SHANLEY JL CAMPBELL GBLAWRENCE SW BAILEY GE LIKENS BCWEMPLE G FREDRIKSEN amp AE JAMISON2012 Spatial patterns of soil nitrification and ni-trate export from forested headwaters in the north-eastern United States Journal of Geophysical Re-search 117 1ndash14 DOI1010292011JG001740

SABATER F amp S BERNAL 2011 Keeping healthyriparian and aquatic ecosystems in the Med- iter-ranean challenges and solutions through riparianforest management In Water for forests and peo-ple in the Mediterranean region A challenging

balance Y Boirot C Gracia amp M Palahiacute (eds)151ndash155 European Forest Institute Joensuu Fin-land

SABATER F A BUTTURINI E MARTIacute amp I MU-NtildeOZ 2000 Effects of riparian vegetation removalon nutrient retention in a Mediterranean streamJournal of the North American Benthological So-ciety 19 609ndash620 DOI1010292011JG001740DOI1023071468120

SANPERA-CALBET I V ACUNtildeA A BUTTU-RINI R MARCEacute amp I MUNtildeOZ 2016 El Nintildeosouthern oscillation and seasonal drought drive ri-parian input dynamics in a Mediterranean streamLimnology and Oceanography 61 214ndash226 DOI101002lno10211

SCHADE JD E MARTIacute JR WELTER SG FI-SHER amp NB GRIMM 2002 Sources of nitrogento the riparian zone of a desert stream Implica-tions for riparian vegetation and nitrogen retentionEcosystems 5 68ndash79 DOI101007s10021-001-0056-6

SCHADE JD JR WELTER E MARTIacute amp NBGRIMM 2005 Hydrologic exchange and N up-take by riparian vegetation in an arid-land streamJournal of the North American Benthological So-ciety 24 19ndash28

SCHLESINGERWH 2009 On the fate of anthro-pogenic nitrogen Proceedings of the NationalAcademy of Sciences of the United States of Amer-ica 106 203ndash208 DOI101073pnas0810193105

SERRASOLSES I 1999 Soil nitrogen dynamicsIn Ecological Studies Ecology of Mediterraneanevergreen oak forests F Rodagrave J Retana CAGracia amp J Bellot (eds) 223ndash235 SpringerBerlin Heidelberg Berlin Germany

SMITHM P CONTE AE BERNS JR THOM-SONampTR CAVAGNARO 2012 Spatial patternsof and environmental controls on soil propertiesat a riparian-paddock interface Soil Biology andBiochemistry 49 38ndash45 DOI101016jsoilbio201202007

STARRY OS HM VALETTampME SCHREIBER2005 Nitrification rates in a headwater stream in-fluences of seasonal variation in C and N supplyJournal of the North American Benthological So-ciety 24 753ndash768 DOI10189905-0151

TIEMANN LK amp SA BILLINGS 2012 TrackingC and N flows through microbial biomass with in-creased soil moisture variability Soil Biology andBiochemistry 49 11ndash22 DOI101016jsoilbio201201030

Limnetica 36 (2) 507-523 (2017)



VAZQUEZ E V ACUNtildeA J ARTIGAS S BER-NAL E EJARQUE A GAUDES I YLLAE MARTIacute E MAS-MARTIacute A GUARCH IMUNtildeOZ A ROMANI S SABATER F SABA-TER D VON SCHILLER amp A BUTTURINI2013 Fourteen years of hydro-biogeochemicalmonitoring in a Mediterranean catchment Die Bo-denkultur 64 13ndash20

VIDON PGF amp AR HILL 2004 Landscape con-trols on the hydrology of stream riparian zonesJournal of Hydrology 292 210ndash228 DOI101016jjhydrol200401005

VIDON P CJ ALLAN D BURNS TP DUVALN GURWICK S INAMDAR R LOWRANCEJ OKAY D SCOTT amp S SEBESTYEN 2010Hot spots and hot moments in riparian zones Po-tential for improved water quality managementJournal of the American Water Resources Associ-ation 46 278ndash298 DOI101111j1752-1688201000420x

VITOUSEK PM JD ABER RW HOWARTHGE LIKENS PA MATSON DW SCHIND-LER WH SCHLESINGER amp DG TILMAN1997 Human alteration of the global nitrogen cy-cle sources and consequences EcologicalApplica-tions 7 737ndash750 DOI1018901051-0761(1997)007[0737HAOTGN]20CO2

VON SCHILLER D E MARTIacute JL RIERA amp FSABATER 2007 Effects of nutrients and light onperiphyton biomass and nitrogen uptake inMedite-rranean streams with contrasting land uses Fresh-water Biology 52 891ndash906 DOI101111j1365-2427200701742x

VON SCHILLER D E MARTIacute JL RIERA MRIBOT A ARGERICH P FONOLLAgrave amp F SA-BATER 2008 Inter-annual annual and seasonalvariation of P and N retention in a perennial andan intermittent stream Ecosystems 11 670ndash687DOI101007s10021-008-9150-3

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ocurren en las cabeceras riberas y en los propios riacuteos en la quiacutemica del agua En esta revisioacuten se resumen distintos estudiosque examinan los procesos hidroloacutegicos y biogeoquiacutemicos mediante los cuales los bosques de ribera mediterraacuteneos regulanel exporte de agua y N aguas abajo La revisioacuten se centra en las zonas mediterraacuteneas ya que su marcada estacionalidadclimaacutetica permite discernir la estrecha relacioacuten entre el clima la hidrologiacutea de la ribera y las exportaciones de N Losestudios analizados muestran que los suelos riberentildeos pueden ser puntos calientes de mineralizacioacuten y nitrificacioacuten dentrode las cuencas mediterraacuteneas gracias a las condicionas relativamente huacutemedas del suelo y a la hojarasca enriquecida en NTasas de nitrificacioacuten extremadamente altas suceden puntualmente (ie momentos calientes) e incrementan las exportacionesde N sugiriendo que los suelos riberentildeos son fuentes de N para los riacuteos Ademaacutes los aacuterboles riberentildeos contribuyen a ladisminucioacuten del nivel freaacutetico durante el periodo vegetativo y promueven el movimiento de agua del riacuteo hacia la riberaDurante eacutepocas de alta retencioacuten hidroloacutegica el exporte de agua disminuye y su sentildeal quiacutemica depende principalmente de losprocesos biogeoquiacutemicos fluviales El dosel riberentildeo tambieacuten puede afectar al exporte de N aguas abajo ya que controla lasentradas de luz y hojarasca En primavera la asimilacioacuten fotoautotroacutefica de N aumenta justo antes de que las hojas brotenmientras que los aportes de hojarasca pueden incentivar los procesos de mineralizacioacuten en verano y otontildeo Finalmente lainfluencia de las riberas mediterraacuteneas sobre la hidrologiacutea y biogeoquiacutemica del riacuteo incrementa a lo largo del riacuteo y modula elexporte de N a escala de cuenca En conjunto los resultados de esta revisioacuten cuestionan la idea de que los ecosistemas deribera mediterraacuteneos son filtros eficientes de N y ponen de manifiesto la importancia de integrar el funcionamiento de losbosques de cabecera las zonas de ribera y los riacuteos para avanzar en el conocimiento sobre la hidrologiacutea y biogeoquiacutemica aescala de cuenca

Palabras clave Ciclo del nitroacutegeno suelos forestales evapotranspiracioacuten dosel riberentildeo hidrologiacutea de cuenca procesosbiogeoquiacutemicos fluviales

INTRODUCTION

During the last decade anthropogenic activitieshave doubled the amount of available nitrogen(N) in freshwater ecosystems leading to severalenvironmental problems such as eutrophicationacidity toxicity or biodiversity declines (Vi-tousek et al 1997 Schlesinger 2009) The en-vironmental issues derived from N excesses maybe intensified in the future because increasedwarming and dryness would probably reduce wa-ter availability as well as the dilution capacity offluvial ecosystems (Cooper et al 2013) Withincatchments riparian buffer strips have beenconsidered as an economical environmentallyefficient tool for protecting freshwaters from dif-fuse N pollution because they can contribute todecrease N fluxes from terrestrial to aquatic en-vironments (McClain et al 2003 Vidon et al2010)The high capacity of riparian zones to reduce

terrestrial dissolved inorganic N (DIN) inputs de-rives from the topographic hydrologic and bio-geochemical conditions at their unique interfacelocation between upland and streams Flat to-pographies permanent upland-riparian hydrolo-gic connectivity shallow riparian groundwater

tables and carbon (C) enriched soils usually fa-vor ammonium (NH+4 ) and nitrate (NO

minus3 ) removal

via denitrification and biological uptake (Pinayet al 2000 Dosskey et al 2010) Converselyriparian zones that are seasonally disconnectedfrom uplands have large annual water table draw-downs that dry out riparian soils (Ocampo et al2006 Vidon amp Hill 2004) which enhance mi-crobial N mineralization and nitrification andincrease soil NOminus3 availability (Harms amp Grimm2010 Duncan et al 2015) In some cases thehydrologic disconnection between uplands andriparian systems promote the loss of water fromthe stream to the riparian zone (ie stream hydro-logical retention) (Rassam et al 2006) whichcan favor the assimilation of stream DIN by biotaat the stream-riparian edge and consequentlydecrease DIN concentrations (Schade et al2002 Dent et al 2007 Bernal amp Sabater 2012)Therefore the variability in space and timeof soil moisture conditions groundwater tableelevation and water flow paths can substan-tial impact the fate and transport of N throughriparian zonesMediterranean systems (Med systems from

here on) are a unique natural laboratory tounderstand the close link between catchment hy-

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