Nitrous Oxide Emissions and the Use of Wetlands for Water Quality Amelioration CHRIS FREEMAN,* ,²,‡ MAURICE A. LOCK, ² STEVE HUGHES, ‡ AND BRIAN REYNOLDS ‡ School of Biological Sciences and Institute of Terrestrial Ecology, University of Wales, Bangor, LL57 2UW, U.K. JIM A. HUDSON Institute of Hydrology, Staylittle, Powys, SY19 7DB, U.K. Wetlands ameliorate nitrate pollution but have also been recognized as a source of the greenhouse gas nitrous oxide. Nitrate and N 2 O fluxes were studied in an experimental wetland in mid-Wales. Diversion of water inflows caused a 200% increase in nitrate release and a >95% decline in nitrous oxide emission over a 20-week period. The responses were attributed to the onset of drier (more aerobic) conditions causing (i) aerobic mineralization and nitrification of N-containing compounds that had previously been immobilized within the wetlands (releasing nitrate) and (ii) the absence of anaerobic denitrification (a potent mech- anism for nitrate elimination and source of N 2 O), allowing NO 3 -N to leave the wetland. The responses were instantaneously reversible upon re-initiation of the nitrate inflow, indicating a close hydrological coupling between nitrate removal and nitrous oxide emission processes. Introduction Global riverine exports of nitrate from terrestrial systems [20- 60 Tg of N yr -1 (1)] are causing concern due to the increased eutrophication of our coastal waters (2, 3). These exports are rising, largely due to changes in land-use practices and the intensification of agriculture, compounded by aerial deposi- tion of nitrogenous pollutants derived from other anthro- pogenic activities (4). When wetlands lie between terrestrial and aquatic ecosystems, they can act as “filters” and may eliminate pollutants before their entry into the recipient ecosystems (5, 6). Wetlands have the potential to control nitrate pollution with efficiencies of up to 90% (7) and are increasingly being restored or artificially created as a means to improve water quality (e.g., refs 8-10). Sweden has announced plans to reduce their nitrogen exports by 50% and has recognized that wetlands could play a pivotal role in achieving that goal (11). Various mechanisms contribute to the wetland nitrate removal mechanism including sediment deposition, deni- trification, ammonium adsorption, and plant uptake (11). In natural wetlands receiving low nitrate inputs, such as rain- fed bogs, plant uptake appears to be the major nitrate sink (12). However, denitrification has been found to dominate the N-elimination mechanism in wetlands under evaluation for water quality amelioration (11), and denitrification is a major source of nitrous oxide production. Terrestrial N 2O emissions have attracted considerable interest following recognition of their contribution to the greenhouse effect and possible role in stratospheric ozone depletion (13). Molecule for molecule, N2O has an atmospheric warming potential ca. 200 times greater than that of CO 2 (14), and atmospheric concentrations are rising by 0.2-0.3% per annum (15). The following study considers the potential mechanisms by which constructed wetlands could influence those emis- sions (16, 17) in an investigation of the impact of manipulating water flow through an experimental wetland. Methods The experimental site at Cerrig-yr-Wyn, Plynlimon, mid-Wales (U.K. Natural Grid Reference SN 820 866) consists of a discontinuous series of peat-accumulating freshwater wet- lands (ca. 30 m × 5 m), which are dominated by Sphagnum and Juncus species. The organic-rich flushed-peat soil (pH 4.2-5.1) lies along the base of a small gully and is underlain with a further layer of mineral deposits at the base. The natural discontinuities facilitate isolation of various subsec- tions of the wetland, such that the site can be used as a natural laboratory where different manipulations can be applied to the different wetland subunits. Our manipulation involved reducing water inflow to a subsection of the wetland by diversion through a system of pipes for 20 weeks between May and October, followed by re-introduction of water flow for the remainder of the year (Figure 1). Nitrous oxide emissions were estimated on a weekly basis while the bypass was in operation and then monthly during the remainder of the year, using a closed chamber technique (18). The chambers were constructed from wide-necked medium-weight polyethylene bottles (Fisher Scientific) of 4.5 L capacity inserted into the peat to a depth of 2.5 cm. Collections were made by first purging the systems with external air and then sealing each chamber. The increase in N 2O concentration over a 2-h period was related to back- ground concentrations to give an estimate of flux from the two experimental wetlands. Gas samples were collected in gas-tight syringes (SGE) and analyzed using an Ai Cambridge Model 92 gas chromatograph with an electron capture detector (16). Samples of the water draining from the two wetlands were collected using EPIC Products autosamplers that collected 5-mL samples every 30 min. The samples were bulked into a 2.3‚L container under the control of a data logger (Figure 1). Nitrate concentrations were analyzed by ion chroma- tography using a Dionex 2000i IC system using an AS4A anion column with 1.7 mM NaHCO 3/1.8 mM Na2CO3 eluent at 2 mL min -1 flow rate and 25 mN H2SO4 regenerant (19). Results and Discussion Manipulation of water flow through the experimental wetland impacted on both the fluxes of nitrate and nitrous oxide from the site. The mechanisms controlling N 2O emissions are known to be complex, for the gas is produced as both an intermediate in denitrification and as a byproduct of nitri- fication. Nitrous oxide production is known to be particularly favored by (i) medium-high soil water content, (ii) high organic carbon availability, and (iii) high mineral N availability (20, 21). The first two of these are present in both the control and the manipulated wetlands. However, the third, the availability of inorganic N, differs markedly between the sites due to the diversion of the inflowing N supply. When nitrate inflows were diverted around the outside of the site, a dramatic and immediate decline in N 2O emissions occurred. Over the period of diversion, N 2O emission fell by >95% when compared to the control wetland. Confirmation that the * Corresponding author fax: +44 1248 370731; e-mail: [email protected]. ² School of Biological Sciences. ‡ Institute of Terrestrial Ecology. Environ. Sci. Technol. 1997, 31, 2438-2440 2438 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 8, 1997 S0013-936X(96)00467-1 CCC: $14.00 1997 American Chemical Society
Transcript
Nitrous Oxide Emissions and theUse of Wetlands for Water QualityAmeliorationC H R I S F R E E M A N , * , † , ‡ M A U R I C E A . L O C K , †
S T E V E H U G H E S , ‡ A N D B R I A N R E Y N O L D S ‡
School of Biological Sciences and Institute of TerrestrialEcology, University of Wales, Bangor, LL57 2UW, U.K.
J I M A . H U D S O N
Institute of Hydrology, Staylittle, Powys, SY19 7DB, U.K.
Wetlands ameliorate nitrate pollution but have also beenrecognized as a source of the greenhouse gas nitrousoxide. Nitrate and N2O fluxes were studied in an experimentalwetland in mid-Wales. Diversion of water inflows causeda 200% increase in nitrate release and a >95% decline innitrous oxide emission over a 20-week period. Theresponses were attributed to the onset of drier (more aerobic)conditions causing (i) aerobic mineralization and nitrificationof N-containing compounds that had previously beenimmobilized within the wetlands (releasing nitrate) and (ii)the absence of anaerobic denitrification (a potent mech-anism for nitrate elimination and source of N2O), allowingNO3-N to leave the wetland. The responses wereinstantaneously reversible upon re-initiation of the nitrateinflow, indicating a close hydrological coupling betweennitrate removal and nitrous oxide emission processes.
IntroductionGlobal riverine exports of nitrate from terrestrial systems [20-60 Tg of N yr-1 (1)] are causing concern due to the increasedeutrophication of our coastal waters (2, 3). These exports arerising, largely due to changes in land-use practices and theintensification of agriculture, compounded by aerial deposi-tion of nitrogenous pollutants derived from other anthro-pogenic activities (4). When wetlands lie between terrestrialand aquatic ecosystems, they can act as “filters” and mayeliminate pollutants before their entry into the recipientecosystems (5, 6). Wetlands have the potential to controlnitrate pollution with efficiencies of up to 90% (7) and areincreasingly being restored or artificially created as a meansto improve water quality (e.g., refs 8-10). Sweden hasannounced plans to reduce their nitrogen exports by 50%and has recognized that wetlands could play a pivotal rolein achieving that goal (11).
Various mechanisms contribute to the wetland nitrateremoval mechanism including sediment deposition, deni-trification, ammonium adsorption, and plant uptake (11). Innatural wetlands receiving low nitrate inputs, such as rain-fed bogs, plant uptake appears to be the major nitrate sink(12). However, denitrification has been found to dominatethe N-elimination mechanism in wetlands under evaluationfor water quality amelioration (11), and denitrification is amajor source of nitrous oxide production. Terrestrial N2O
emissions have attracted considerable interest followingrecognition of their contribution to the greenhouse effectand possible role in stratospheric ozone depletion (13).Molecule for molecule, N2O has an atmospheric warmingpotential ca. 200 times greater than that of CO2 (14), andatmospheric concentrations are rising by 0.2-0.3% per annum(15). The following study considers the potential mechanismsby which constructed wetlands could influence those emis-sions (16, 17) in an investigation of the impact of manipulatingwater flow through an experimental wetland.
MethodsThe experimental site at Cerrig-yr-Wyn, Plynlimon, mid-Wales(U.K. Natural Grid Reference SN 820 866) consists of adiscontinuous series of peat-accumulating freshwater wet-lands (ca. 30 m × 5 m), which are dominated by Sphagnumand Juncus species. The organic-rich flushed-peat soil (pH4.2-5.1) lies along the base of a small gully and is underlainwith a further layer of mineral deposits at the base. Thenatural discontinuities facilitate isolation of various subsec-tions of the wetland, such that the site can be used as a naturallaboratory where different manipulations can be applied tothe different wetland subunits. Our manipulation involvedreducing water inflow to a subsection of the wetland bydiversion through a system of pipes for 20 weeks betweenMay and October, followed by re-introduction of water flowfor the remainder of the year (Figure 1).
Nitrous oxide emissions were estimated on a weekly basiswhile the bypass was in operation and then monthly duringthe remainder of the year, using a closed chamber technique(18). The chambers were constructed from wide-neckedmedium-weight polyethylene bottles (Fisher Scientific) of 4.5L capacity inserted into the peat to a depth of 2.5 cm.Collections were made by first purging the systems withexternal air and then sealing each chamber. The increase inN2O concentration over a 2-h period was related to back-ground concentrations to give an estimate of flux from thetwo experimental wetlands. Gas samples were collected ingas-tight syringes (SGE) and analyzed using an Ai CambridgeModel 92 gas chromatograph with an electron capturedetector (16).
Samples of the water draining from the two wetlands werecollected using EPIC Products autosamplers that collected5-mL samples every 30 min. The samples were bulked intoa 2.3‚L container under the control of a data logger (Figure1). Nitrate concentrations were analyzed by ion chroma-tography using a Dionex 2000i IC system using an AS4A anioncolumn with 1.7 mM NaHCO3/1.8 mM Na2CO3 eluent at 2mL min-1 flow rate and 25 mN H2SO4 regenerant (19).
Results and DiscussionManipulation of water flow through the experimental wetlandimpacted on both the fluxes of nitrate and nitrous oxide fromthe site. The mechanisms controlling N2O emissions areknown to be complex, for the gas is produced as both anintermediate in denitrification and as a byproduct of nitri-fication. Nitrous oxide production is known to be particularlyfavored by (i) medium-high soil water content, (ii) highorganic carbon availability, and (iii) high mineral N availability(20, 21). The first two of these are present in both the controland the manipulated wetlands. However, the third, theavailability of inorganic N, differs markedly between the sitesdue to the diversion of the inflowing N supply. When nitrateinflows were diverted around the outside of the site, a dramaticand immediate decline in N2O emissions occurred. Over theperiod of diversion, N2O emission fell by >95% whencompared to the control wetland. Confirmation that the
† School of Biological Sciences.‡ Institute of Terrestrial Ecology.
Environ. Sci. Technol. 1997, 31, 2438-2440
2438 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 8, 1997 S0013-936X(96)00467-1 CCC: $14.00 1997 American Chemical Society
response was due to the experimental manipulation (andnot simply a difference between sites) was accomplished byre-establishing the inflow of nitrate; N2O emissions wereinstantly restored to the levels found in the control site (Figure2). The impacts on N cycling were demonstrated to be readilyreversible, and the immediate restoration of high N2O fluxes(or more specifically the absence of a lag period) upon re-introduction of the in-flowing waters suggests that the originalpopulation of anaerobic denitrifiers were able to survive the
drier conditions over what was effectively a 20-week periodof drought.
It is interesting to note, however, that between weeks 12and 20 a small flux of N2O was emitted from the manipulatedwetland. Drier conditions have been found to favor min-eralization and would also allow the establishment of apopulation of aerobic nitrifying bacteria (22). As such, thenitrous oxide emitted may have been a byproduct ofnitrification of ammonium released during mineralization.It could also be proposed that denitrification of nitrateproducts (following their diffusion into the lower anaerobiczones of the soil) could have contributed to the N2O flux (16,19, 23). However, the treatment also caused a dramatic 200%increase in the nitrate content of the waters draining fromthe site (Figure 3a). When considered alongside the excep-tionally low N2O flux, this suggests that the nitrate eliminationmechanism (denitrification) had been severely impaired, thusallowing nitrates produced within the wetland and anyexogenous nitrates entering from the sides of the valley topass unhindered through the wetland and to escape out intothe drainage stream.
Re-establishing the water flow caused a rapid decline inthe outflow nitrate concentration to levels below those of theupstream control (Figure 3b). The fall in nitrate releasecoincided exactly with the return of high nitrous oxide fluxesand attests to the significant hydrological coupling betweenthe nitrate amelioration and N2O production. The observa-tion also lends support to the hypothesized importance ofdenitrification in the wetland nitrate elimination mechanism(11).
In the present study, 0.53% of the NO3-N that was removedby the wetland was released as N2O-N. This relatively lowvalue may be due to the abundance of organic carbon inthese wetlands, for while DOC promotes denitrification, italso decreases the ratio of N2O to N2 that is evolved (24). Theacid pH of the soil (pH < 5) would also have limited the N2Orelease, for while denitrification occurs over a wide range of
FIGURE 1. Experimental field site (schematic, not to scale).
FIGURE 2. Cumulative N2O release from control (9) and experimental(b) wetlands.
pH values, its rate is particularly slow below pH 6 (25).However, it should be noted that far higher N2O release wouldbe anticipated in constructed wetlands containing mineralsoils of circum-neutral pH.
The oxidized N in deposition at the site is mainly derivedfrom vehicle emissions and fossil fuel combustion (i.e., NOx
emissions). Nitrate-N in wet deposition at the site has a meanof 0.17 mg of N L-1, although annual means (rainfall weighted)varied between 0.13 and 0.21 mg of N L-1 between 1980 and1989. Nitrate-N makes up 43% of the inorganic N loading inthat deposition (26). In lowland agricultural areas, nitrateinputs are likely to be far higher as a consequence of fertiliserusage, and thus the emissions of nitrous oxide would becorrespondingly higher. Our observation of nitrate removal
from waters passing through the experimental wetlandconfirms that wetlands could have value for reducing thecurrent 20-60 Tg of N yr-1 (1) global riverine export of nitrate.However, the findings also indicate that nitrate removal islikely to be accompanied by an increased release of N2O tothe atmosphere, suggesting a longer term potential risk thata water quality problem is simply being converted into anatmospheric pollution problem.
AcknowledgmentsThis project was supported by the Natural EnvironmentResearch Council, the Royal Society, and the Welsh Office,U.K. We are grateful to P. Hill, A. Hughes, H. Guasch, andC. Swanson for field/lab assistance. C.F. is currently a RoyalSociety University Research Fellow.
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Received for review May 30, 1996. Revised manuscript re-ceived April 9, 1997. Accepted April 11, 1997.X
ES9604673
X Abstract published in Advance ACS Abstracts, June 15, 1997.
FIGURE 3. Nitrate release from upstream control (9) and downstreamexperimental (b) wetlands, with inflowing waters diverted aroundthe wetland (a) and with inflow re-initiated (b).
