Phosphorus fractions and retention in drainage ditch sediments receiving surface runoff and...

$Page 1: Phosphorus fractions and retention in drainage ditch sediments receiving surface runoff and subsurface drainage from agricultural catchments in the North Island, New Zealand$
Agriculture, Ecosystems and Environment 92 (2002) 49–69

Phosphorus fractions and retention in drainage ditch sedimentsreceiving surface runoff and subsurface drainage from agricultural

catchments in the North Island, New Zealand

Long Nguyen∗, James SukiasNational Institute of Water& Atmospheric Research Ltd., P.O. Box 11-115, Hamilton, New Zealand

Received 12 July 2000; received in revised form 4 July 2001; accepted 16 July 2001

Abstract

Drainage ditches used to remove surface runoff or connected to subsurface tile and mole drains may act as a major conduitof phosphorus (P) from agricultural lands to receiving waters. The extent of P transport via drainage ditches is potentiallygoverned by the P status and retention characteristics of drainage ditch sediments. Twenty-six surface (0–5 cm) and subsurface(5–15 cm) sediment cores (10 cm diameter) from 26 drainage ditches in four major New Zealand pastoral catchments weretherefore characterised for P fractions and P retention capacities. Both surface and subsurface sediments were found to contain asignificant amount of P and possess a range of P retention capacities. Phosphorus retention capacities in surface and subsurfacesediments ranged from 2467 to 4197 and 2225 to 3891 mg P kg−1 sediment, respectively. They were significantly correlated(r = 0.638–0.918;P ≤ 0.001) with sediment chemical characteristics (pH, organic matter, and oxalate-extractable Al and Fe).Approximately 42–57% (±S.E.M. of 1.1–2.8) of P in drainage sediments was present as loosely bound fractions (non-occludedAl/Fe-P and carbonate-bound P), suggesting that drainage sediments may temporarily store P originating from agriculturalcatchments, and that P held in this storage pool may be readily released into the overlying drainage water. Calcium-bound P andoccluded Al/Fe-P represented minor fractions in the drainage sediments, accounting for less than 10% of total P in the sediments(2–7 and 5–9% of total P, respectively). Drainage sediments also acted as a long-term P sink, since residual P represented asubstantial fraction (6–39% of total P) in some drainage sediments. Drainage water contained not only soluble P but also partic-ulate and dissolved organic P fractions, indicating that drainage management for P pollution control needs to consider all thesethree P fractions. Soluble P concentrations (0.006–0.019 mg P l−1) in drainage waters were not significantly correlated withloosely bound sediment P, suggesting a non-equilibrium status between sediment and overlying drainage water P. Although thesurveyed sediments had high P retention capacities (44–84% of added 5000 mg P kg−1 sediment), up to 64–68% of P sorptionsites on sediments were saturated with P, attributed to a long history of P loadings from agricultural runoff and subsurface flows.The results suggest that water-extractable or Olsen-extractable sediment P should be used in conjunction with the sediment Psaturation index (degrees of P saturation) to identify drainage ditches that act as a potential source of P to receiving waters. In-formation obtained could then be used in targeting appropriate management practices to minimise P release from these ditches.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:Phosphorus; Fractionation; Retention; Phosphorus forms; Drainage; Sediments; Drainage ditches; New Zealand drainage

∗ Corresponding author. Tel.:+64-7-856-7026;fax: +64-7-856-0151.E-mail address:[email protected] (L. Nguyen).

1. Introduction

Agricultural activities play a major role as pointand non-point (diffuse) sources of pollutants such as

0167-8809/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0167-8809(01)00284-5

50 L. Nguyen, J. Sukias / Agriculture, Ecosystems and Environment 92 (2002) 49–69

phosphorus (P) to receiving waters in New Zealand(Smith et al., 1993; Wilcock et al., 1999) and manyother countries around the world (Grant et al., 1996;Sims et al., 1998; Haygarth and Jarvis, 1999). Withincreased worldwide legislation controlling pointsource inputs to receiving waters, P derived from dif-fuse sources such as drainage networks has become amajor environmental issue and possibly a significantP contributor to receiving waters (Grant et al., 1996;The Ministry for the Environment, 1997; Sims et al.,1998; Addiscott et al., 2000).

Drainage networks of surface (open drains) andsubsurface drains (tile and mole drains and corru-gated, flexible high density polyethylene pipes) areintegral components for sustaining economic growthin many pastoral lowland areas of New Zealand.These networks help to remove surface water pond-ing and lower water tables below the major rootingzone (50–100 cm depth) for optimum plant produc-tion. However, they may potentially act as a majorconduit of agricultural pollutants (sediments, faecalmaterial and nutrients) to receiving waters and po-tentially become a major source of dissolved andparticulate P to streams (Grant et al., 1996; Brookeset al., 1997). This has implications for water resourceand environmental managers, since a soluble P levelas low as 0.010–0.015 mg P l−1 has been reported toenhance nuisance growth of algae and aquatic weedsand hence potentially accelerate freshwater eutroph-ication, which restricts water use for fisheries andrecreational activities (Smith et al., 1993; Foy andWithers, 1998). The extent of P transport in drainagewaters, and hence potential eutrophication in streams,is partly governed by sediment P status and P re-tention and release characteristics within drainagesediments (Sims et al., 1998; Reddy et al., 1999).

Research has shown that sediment reduction caneither increase or decrease P sorption capacity. Un-der anaerobic (reducing) conditions, sediments mayrelease P into solutions of low P concentrations, butsorb P from solutions of high P concentrations (Patrickand Khalid, 1974; Reddy et al., 1999). Greater P sorp-tion capacity is attributed to the reduction–dissolutionof crystalline ferric oxyhydroxide to amorphous fer-rous forms with a much greater surface area for Psorption than ferric forms (Patrick and Khalid, 1974).In the Delaware Inland Bays’ agricultural watershedin the USA, Sallade and Sims (1997a,b) reported a

significant proportion of sediment P in drains exists inloosely bound fractions (non-occluded aluminium-Pand iron-P) that are readily released into drainage wa-ters under reducing conditions. This suggests that theuse of drainage ditches as wetlands to process drainagewater P (Bowmer et al., 1994) needs careful consid-eration. Ditches with high P status and non-occludedP fractions may require regular water monitoring andappropriate management practices to minimise sig-nificant P releases into receiving waters (Sallade andSims, 1997b).

Phosphorus fractions in drainage sediments arelikely to vary from one drainage catchment to an-other, depending on inter-related factors such as soilphysical and chemical properties, sediment pH anddissolved P concentrations in drainage waters (Salladeand Sims, 1997a,b). It is therefore important to quan-tify P status, characterise P fractions and investigatethe P sorption–release characteristics of New Zealanddrainage sediments. This is particularly relevant formany lowland areas of New Zealand where there isa significant peat content in the soils. In these areas,water control structures are installed in drains to im-pound drainage water during late spring and summermonths to minimise peat oxidation and shrinkage.This practice is likely to enhance the reducing condi-tions within drainage ditches and thus modify the Pretention–release characteristics of sediments (Salladeand Sims, 1997b).

To predict the potential P retention in soils andhence the potential risk of P losses from sedimentsto drainage waters, several workers (e.g. Sallade andSims, 1997a) have suggested the use of P sorptionindices and the degree of P saturation. Phosphorussorption index (PSI) is the measure of P sorption bysediments, while the degree of P saturation (DPS)represents the extent of P accumulation in sedi-ments relative to their maximum P sorption capacity.A sediment containing a high level of adsorbed P(non-occluded Al and Fe-P fractions) and hence highPSI and DPS may potentially become a source ofsoluble P in drainage waters. A critical DPS value of40% has been used to establish a water quality indexfor drainage sediments in the Delaware Inland Baysof the USA (Sallade and Sims, 1997b). Sedimentswith DPS values greater than 40% were suggestedto have a higher susceptibility to P losses than thosewith lower DPS values. Thus, an estimate of the

L. Nguyen, J. Sukias / Agriculture, Ecosystems and Environment 92 (2002) 49–69 51

proportion of sediment P sorption sites saturated bynon-occluded Al and Fe-P fractions, coupled with aninventory of drainage vegetation and hydrology, mayprovide significant scope for identifying drains thathave a potential to release P into overlying waters.

The objectives of this study were: (i) to identify Pforms and characterise P retention capacity of drainsediments obtained from New Zealand agriculturalcatchments with different sediment physical andchemical properties; (ii) to examine the use of PSI andDPS as indicators of sediment P retention–release po-tential; and (iii) to determine sediment chemical andphysical properties that reflect sediment P sorptioncapacity.

Drain clearance is likely to modify environmen-tal conditions (e.g. macrophyte habitat and sedimentredox status) which influence plant P uptake and Psorption–desorption processes (Bowmer et al., 1994).Thus, sediments from drains that had been recentlydredged (<3–6 months) to remove vegetation and sed-iment deposits were also collected and characterisedfor P forms and P retention capacity.

2. Materials and methods

2.1. Experimental sites

Twenty-six surface (0–5 cm) and subsurface(5–15 cm) sediments were collected in September1998 from drainage ditches of 12 pastoral farms.Only the top 15 cm of sediment was collected, since Pstored below this depth is unlikely to play a major rolein P exchange with the overlying drainage water. The0–5 cm layer was usually dark black, while the lowerlayer was a greyish black, suggesting that differentamounts of organic matter accumulation and oxygendiffusion existed in these two sediment layers. Sinceboth organic matter and sediment oxidation–reductionstatus are known to influence sediment Fe chemistryand hence P retention (e.g. Patrick and Khalid, 1974;Axt and Walbridge, 1999), P status in the 0–5 cmsediment layer was expected to differ markedly fromthose in the 5–15 cm layer.

Farms were selected as representative of four ma-jor drainage catchments (Toenepi, Pokeno, Bay ofPlenty and Northland; Fig. 1) in the North Island ofNew Zealand with networks of surface (open drains)

Fig. 1. Map of the North Island of New Zealand showing catchmentsites where drainage sediments were collected.

and subsurface (tile/mole/corrugated high densitypolyethylene pipe) drains that empty surface runoffand subsurface drainage water into drainage ditcheswhich then carry water to receiving waters. The catch-ments chosen represent intensive dairy or beef farmingwith year-round grazing. Pasture herbage consistedmainly of ryegrass (Lolium perenne) and white clover(Trifolium repens) and received annual applicationsof single superphosphate (SSP with 9.2% total P) ata rate of 100–200 kg SSP ha−1 per year. The four se-lected areas cover a range of soil types with a rangeof parent material, texture and structure (Table 1).

At the time of sediment collection, the depth ofdrainage ditches and vegetation type were recorded.Using surface ground level as a reference point,

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shallow and deep drainage ditches were defined asthose between 50 and 75 cm, and 1 and 2 m deep,respectively. In the Toenepi catchment, sediments(0–5 and 5–15 cm layers) were also collected fromthree drainage ditches approximately 3–6 months af-ter they had been dredged to remove vegetation andapproximately the top 50–75 cm layer of sediment.

2.2. Sample collection and sample processing

In each farm, two to three representative ditcheswere selected and nine soil cores (10 cm diameter)were then taken from each ditch at approximately 5 mintervals along the ditch to a 15 cm depth using a1 m long piston corer (10 cm diameter). Samples weresectioned into 0–5 and 5–15 cm depths. They werethen bulked on a depth basis, sieved through a 2 mmsieve to remove organic debris and plant material,sealed and stored at 4◦C until determination (within3 days after sampling) for pH, Olsen-extractable P,water-extractable P, and P fractions. Another threesubsamples (10 g wet sediment) were oven-dried at105◦C for approximately 48–72 h for water contentdetermination. Subsamples of the oven-dried materi-als were ground to<150�m and analysed for totalcarbon (TC).

Prior to sediment collection, drainage water (50 ml)was collected from each sediment sampling point atthe sediment-water interface. However, in ditches witha water depth of >50 cm, drainage water was collectedat a depth of 50 cm. Water samples collected fromeach ditch were then bulked. Both water and sedimentsamples were stored and transported on ice to the lab-oratory.

Overall, the number of drainage ditches from eachcatchment where sediments were collected was 8, 7,7 and 4 for Toenepi, Pokeno, Northland and Bay ofPlenty, respectively.

2.3. Sediment and drainage water characterisation

Within 1 day after the collection, water sampleswere filtered through a 0.45�m membrane and thefiltrates were determined for pH, dissolved reactiveP (soluble P), ammonium, nitrate and total Kjeldahlnitrogen (TKN). Drainage water pH and sediment pH(equilibrated under anaerobic conditions at a sedi-ment:water ratio of 1:2.5 for 30 min) were measured

using a Radiometer 26 m and combination glass elec-trode. Total P (TP) in drainage waters was measuredafter persulphate digestion of unfiltered samples in anautoclave (Koroleff, 1983), followed by colorimetricdetermination of dissolved reactive P (DRP) using theMurphy and Riley procedure (1962). The differencebetween TP and DRP was defined as the sum of partic-ulate P (PP) and dissolved organic P (DOP) fractions.

Suspended solids (SS), ammonium (NH4-N),nitrate (NO3-N) and TKN were determined us-ing the following APHA (1995) methods: SS bygravimetric determination after filtration througha 0.45�m filter and drying at 104◦C; NH4-N byindophenol blue colorimetry; NO3-N (plus anynitrite-nitrogen; NO2-N) by the automated cadmiumcolumn reduction method after complexation with1-napthyl-ethylenediamine dihydrochloride, followedby analysis on an auto-analyser; and TKN by sulphuricacid and potassium sulphate digestion, followed byindophenol blue colorimetry. Total N (TN) was calcu-lated as the sum of TKN and NO3-N, while the sumof particulate N (PN) and organic N fractions was cal-culated as the difference between TKN and NH4-N.

Wet sediments (except where specified) were usedfor P characterisation, since drying may affect P trans-formations and composition in the sediments. Wetsediments (on a 5 g oven-dried basis) were extractedfor Olsen-extractable P by equilibrating with 25 ml of0.5 M sodium bicarbonate (NaHCO3; pH = 8.5) on anend-over-end shaker at 25◦C for 30 min (Olsen et al.,1954). A similar extraction procedure was used forwater-extractable P by equilibrating 1 g (oven-driedbasis) of wet sediment with 20 ml of water for 30 min(Olsen and Dean, 1965). After the equilibration pe-riod, sediment-reagent suspensions were centrifuged(1360×g for 10 min), filtered (0.45�m) and analysedfor DRP in the filtered extracts using the colorimetricmethod of Murphy and Riley (1962) at 880 nm with aShimadzu UV-160A spectrophotometer.

Sediment P retention capacity was determined byshaking duplicate 5 g (oven-dried basis) of field moistsediment for 16 h with 25 ml of 0.2 M sodium acetatesolution (CH3COONa; pH= 4.65), one with, and theother without added P as KH2PO4 (1000 mg P l−1 or5000 mg P kg−1 soil; Saunders, 1965). Toluene (twodrops) was added to soil suspensions to inhibit micro-bial activity. The amounts of added P remaining in thesolution were determined by the method of Murphy


and Riley (1962). Soil P retention capacity was thencalculated as the difference between the amount ofP added and that remaining in soil solution and ex-pressed as a percentage of the P originally added aftertaking into account the amount of native soil P ex-tracted by the CH3COONa reagent.

The sediment P sorption index (PSI; l kg−1) wasdetermined asx log C−1, wherex is the amount of Psorbed by the sediment (mg P kg−1 sediment) basedon mg P l−1 added, and C is the resulting equilib-rium P concentration (mg P l−1) in solution after 16 hof equilibration (Sallade and Sims, 1997a; Axt andWalbridge, 1999).

The degree of P saturation (DPS) of sedimentswas determined as the percentage of P retention assediment extractable P (the sum of non-occludedAl/Fe-P and carbonate-bound P with and withoutthe inclusion occluded Al/Fe-P). Phosphorus reten-tion capacity was calculated from the equilibrationstudy of sediments with CH3COONa-KH2PO4 solu-tion (Saunders, 1965). The approach of determiningDPS in which only the sum of non-occluded Al/Fe-Pand carbonate-bound P was considered, was similarto the method used by Sallade and Sims (1997a) inwhich DPS was determined as the percentage of max-imum phosphate retention as Mehlich 1-extractable P(0.05 M HCl+ 0.025 M H2SO4) or 0.1 M sodium hy-droxide (NaOH)-extractable P. Sediment P extractedby Mehlich or NaOH reagents is likely to originatemainly from the sediment non-occluded and occludedAl/Fe-P fractions (Sallade and Sims, 1997a,b).

Sediments were passed through a nest of sieves,which categorised sediment particle sizes into2–0.2 mm (coarse sand), 0.2–0.02 mm (fine sand),0.02–0.002 mm (silt) and<0.002 mm (clay).

Total C (TC) in oven-dried sediment samples(<150�m) was determined by the combustion methodusing a Perkin-Elmer 2400 CHN Elemental Analyser.

Amorphous (non-crystalline) and poorly crystallinealuminium (Al) and iron (Fe) contents in sedimentswere determined by shaking fresh sediments (2 g on anoven-dried basis) with 50 ml of 0.175 M ammoniumoxalate (soil:solution ratio of 1:25; adjusted to pH 3)for 4 h (Shukla et al., 1971). After 4 h of equilibra-tion, the suspension was centrifuged, filtered througha 0.45�m filter, and determined for Al and Fe by in-ductively coupled plasma emission spectroscopy-massspectrometry.

2.4. Phosphorus fractionation

Freshly-collected and sieved sediments (equivalentto 5 g air-dried sediment) were sequentially extractedfor P fractions using the Olsen and Sommers (1982)procedure. The extraction procedure involved sequen-tial extraction as follows:

1. sodium hydroxide (0.1 M NaOH) to removenon-occluded Al and Fe-bound P (labile P);

2. sodium chloride (1 M NaCl) and citrate-bicarbonate(CB) to remove P sorbed by calcium carbonatesduring the preceding NaOH extraction;

3. citrate-dithionite-bicarbonate (CDB) to remove Poccluded within Al and Fe oxides and hydrous ox-ide;

4. 1 M HCl to remove calcium (Ca)-bound P;5. digestion of soil residue with nitric-perchloric acid

mixture to remove residual (recalcitrant) P.

Total sediment-extractable P was estimated as thesum of all the above forms. This approach was likelyto underestimate the total sediment P, since organic Pcomponents in all extractants (as shown in Nguyen,2000) were not determined.

All sediments were analysed in triplicate for P frac-tions, oxalate-extractable Al and Fe, and P retentioncapacity.

2.5. Expression of results and statistical analyses

Results obtained from sediment analyses were cor-rected for sediment water content. Data on sediment Pfractions and drainage water P and N concentrationswere presented as box plots. For each box, the dia-mond represents the mean values. The bottom, midlineand top of the boxes represent the 25th, 50th (medianvalues), and 75th percentiles. The whiskers encompassthe main body of the data (highest and lowest valueswithin 1.5× interquartile range) and extreme values(outliers) are denoted with circles.

Data points from four catchments were combinedinto a single dataset and the following relationshipswere investigated:

1. Sediment P retention capacity and sediment chem-ical or physical properties.

2. Sediment P extracted by Olsen or distilledwater and the sum of the non-occluded and


carbonate-bound P fractions with and without theinclusion of the occluded Al/Fe-P.

3. The sum of the above (2) sediment P frac-tions and oxalate-extractable sediment Al oroxalate-extractable sediment Fe content.

3. Results and discussion

3.1. General sediment and drainage watercharacteristics

Drainage waters contained both soluble (DRP)and particulate plus dissolved organic P (PP+ DOP)(Fig. 2). Concentrations of soluble P in most of thesurveyed drainage waters (21 out of 26) were belowthe level (0.015–0.030 mg P l−1) associated with sur-face water eutrophication (Smith et al., 1993; Foy andWithers, 1998). Thus, most of the drainage ditchesstudied would not pose a significant contribution tothe eutrophication of receiving waters at the time ofthe survey. This is assuming that the final P concen-

Fig. 2. Summary statistics for drainage water phosphorus chemistry in four catchments. The bottom, midline and top of each box representthe 25th, 50th, and 75th percentiles. The diamonds represents the mean values. The whiskers encompass the main body of the data (+1.5×interquartile range) and extreme values are denoted with circles.

tration at drain outlets has not been greatly modifiedby processes that affect sediment P retention–releasecharacteristics along the drainage ditch network.These processes (e.g. microbial immobilisation andP diffusion within drainage sediments), as discussedlater in this section, need to be taken into account,in conjunction with water seepage along drainageditches, ditch vegetation and the length of drainageditches.

Soluble P concentrations in drainage water ac-counted for 6.3–49.6% of drainage water TP (Table 2),suggesting that PP and DOP were major P con-stituents in some drainage waters. Similar results werefound for N fractions in drainage waters. Not onlynitrate but also ammonium, PN and possibly somedissolved organic N (DON) were present in drainagewaters (Fig. 3). Particulate N was the predominantN fraction in drainage waters where P was presentpredominantly as PP (Table 2). Nitrate (NO3-N) con-centrations in drainage waters (Fig. 3) were below thecritical guidelines for water consumption by humans


Table 2Concentrations of suspended solids (SS) and pH in drainage waters and proportions of drainage water total phosphorus (TP) and nitrogen(TN) in soluble and particulate fractions for samples taken from four catchments at the time of sediment collection

Catchment pH SS(mg l−1)

Proportions (%) of drainagewater TP as

Proportions (%) of drainagewater TN as

Soluble P(dissolvedreactive P)

Particulateand dissolvedorganic P

Ammonium Nitrate Particulate N

Toenepi Mean 5.56 (8)a 8.54 (8) 49.6 (8) 50.4 (8) 4.3 (8) 72.9 (8) 22.9 (8)Median 5.57 2.20 47.6 52.4 0.4 79.4 20.1S.E.M.b 0.04 4.64 7.8 7.8 2.4 5.9 3.6

Pokeno Mean 5.19 (7) 7.47 (7) 10.4 (7) 89.6 (7) 23.3 (7) 4.1 (7) 72.6 (7)Median 5.15 4.10 11.0 89.0 27.1 1.9 71.0S.E.M. 0.08 3.80 2.1 2.1 5.5 2.1 7.0

Northland Mean 5.15 (7) 3.70 (7) 20.1 (7) 79.9 (7) 10.2 (7) 48.7 (7) 41.1 (7)Median 4.95 2.50 20.0 80.0 2.1 54.3 43.9S.E.M. 0.28 0.81 4.7 4.7 6.7 16.6 13.2

Bay of plenty Mean 6.38 (4) 5.13 (4) 6.3 (4) 93.8 (4) 13.3 (4) 16.9 (4) 69.8 (4)Median 6.37 4.25 3.5 96.5 11.9 6.6 72.7S.E.M. 0.23 1.74 3.3 3.3 6.3 12.3 11.3

a Values in parentheses indicate number of data points.b Standard error of the mean.

Fig. 3. Summary statistics for drainage water nitrogen chemistry in four catchments. The bottom, midline and top of each box representthe 25th, 50th, and 75th percentiles. The diamonds represents the mean values. The whiskers encompass the main body of the data (+1.5×interquartile range) and extreme values are denoted with circles.


(11 mg N l−1) and stock (30 mg N l−1) (Wilcock et al.,1999). Drainage water NO3-N concentrations wereextremely low (0.012–0.16 mg N l−1; Fig. 3) in thePokeno and Bay of Plenty catchments, where peatwas the predominant parent material and impound-ment of drainage water was a common practice toavoid peat shrinkage during summer months. Thispractice probably enhanced denitrifying activity inboth upland catchment soils and drainage sediments,particularly in the peat sediments, which had highTC contents (Pokeno and Bay of Plenty catchments;Table 3).

Ammonium (NH4-N) concentrations found indrainage waters (Fig. 3) were unlikely to generate am-monia (NH3) above the toxic level (0.035 mg NH3 l−1)for salmonid fish (USEPA, 1985) for two reasons:(i) drainage water pH was acidic (pH= 5.15–6.38;Table 2) and (ii) drainage water temperature at thesampling time was 18–24◦C. Under these conditions,<0.13% of total ammoniacal nitrogen (NH3-N) wasestimated to exist as dissolved ammonia gas (NH3)and the remaining as ammonium (NH4-N). Theseestimates were based on the ionisation relationshipbetween NH3-N, NH3 gas and NH4-N over a rangeof drainage water pH and temperatures as describedby Erickson (1985). With concentrations of NH4-Nin drainage waters at most sites of<0.46 mg N l−1

(Fig. 3), concentrations of toxic NH3 gas wereunlikely to exceed 0.0006 mg NH3 l−1.

The presence of both particulate and dissolvednutrient fractions in drainage ditches probably re-flects nutrient inputs from productive farming sys-tems via surface runoff and subsurface drainage(House et al., 1998; Haygarth and Jarvis, 1999;Nash and Halliwell, 1999). These inputs may bemodified by processes within the drainage ditchessuch as the erosion of the drainage bank, the re-turn of in situ plant litter within drainage ditches,the resuspension-sedimentation of particulate mate-rials, P diffusion within drainage sediments, micro-bial immobilisation, and nutrient retention–releasecharacteristics of drainage sediments. Studies con-ducted in wetland ecosystems have demonstrated theimportance of these processes on sediment and nu-trient retention in wetland sediments (e.g. Johnston,1991; Reddy et al., 1999). Since drainage ditchesare likely to behave in a similar fashion to wetlands(Bowmer et al., 1994), these processes are expected

to affect the distribution and forms of P in drainagesediments.

Drainage water SS concentrations were mostly<10 mg l−1 (Table 2), probably reflecting the lowdrainage flow at the time of water sampling (drainagedischarges from tiles, moles and corrugated polyethy-lene pipes into ditches were normally<2 l s−1). Inaddition, submerged vegetation in drains may alsotrap suspended materials. The main vegetation inshallow (50–75 cm deep) drainage ditches in the fourstudied catchments consisted of soft brome (Bro-mus hordeaceous), duckweed (Lemna minorL.), andfloating reed sweet grasses (Glyceria maxima) withbuttercup (Ranunculus flammulaL. and Ranuncu-lus trichophyllus) around the drain margins. In thedeep (100–200 cm deep) drains, the main vegetationconsists of stonewort (Nitella hookeri), pondweed(Potamogeton ochreatusand Potamogeton crispusL.), reed sweet grass (G. maxima) and swamp willowweed (Polygonumsp.). The annual die-off of theseplants is expected to contribute both organic matterand nutrients (e.g. N and P) to drainage sediments.

Sediment pH was acidic (pH of 5.0–5.64), partic-ularly in the 5–15 cm sediment layer (Table 3). Un-der acid conditions, Fe-P and Al-P would be expectedto be the dominant P fractions (Reddy et al., 1996;Phillips, 1998).

Sediment TC content, particularly in the top 5 cmlayer was high, ranging from 3.8 to 14.1% (Table 3).This high TC content in accumulated drainage sedi-ments is attributed to organic detritus resulting fromthe death and decay of drainage wetland plants. Otherpossible sources include runoff of agricultural topsoiland faecal materials. In ditches that had been recentlydredged, sediment TC was low (2.5–2.9%; data notshown in Table 3), particularly in the lower sedimentlayer (1.8–2.1%), even 3–6 months after dredging, at-tributed to the removal of organic detritus, plant litterand the top sediment layer.

Sediment Al/Fe oxide content and fine sedimentparticle sizes (silt and clay fractions) were greaterin the 0–5 cm than the 5–15 cm layer (Table 4). Thehigher Al and Fe content in the 0–5 cm layer probablyreflected the sorption of Al and Fe by fine sedimentand organic particles, both of which were higher inthis layer than the 5–15 cm layer. The significant rela-tionships between sediment oxalate-extractable Fe/Aland sediment TC (Table 5) suggest that organic C

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Table 5Correlation coefficients (r) between sediment total carbon (TC) content or sediment phosphorus (P) retention capacity (% of added P) andsediment chemical and physical properties at different depths

Sediment properties Sediment TC Sediment P retention capacity

0–5 cm depth 5–15 cm depth 0–15 cm depth 0–5 cm depth 5–15 cm depth 0–15 cm depth

Clay 0.066 (26)a NSb 0.045 (26) NS 0.077 (52) NS 0.175 (26) NS 0.072 (26) NS 0.144 (52) NSSilt 0.337 (26)∗ 0.499 (26)∗∗∗ 0.413 (52)∗∗∗ 0.344 (26)∗ 0.457 (26)∗ 0.393 (52)∗∗∗Oxalate-extractable Al 0.395 (26)∗ 0.665 (26)∗∗∗ 0.524 (52)∗∗∗ 0.487 (26)∗ 0.778 (26)∗∗∗ 0.602 (52)∗∗∗pH 0.809 (26)∗∗∗ 0.662 (26)∗∗∗ 0.772 (52)∗∗∗ 0.800 (26)∗∗∗ 0.655 (26)∗∗∗ 0.718 (52)∗∗∗Total C 1 1 1 0.886 (26)∗∗∗ 0.892 (26)∗∗∗ 0.888 (52)∗∗∗Oxalate-extractable Fe 0.812 (26)∗∗∗ 0.863 (26)∗∗∗ 0.836 (52)∗∗∗ 0.901 (26)∗∗∗ 0.937 (26)∗∗∗ 0.914 (52)∗∗∗

a Values in parentheses indicate number of data points used in the correlation analyses.b Not significant.∗ P < 0.05.∗∗∗ P < 0.001.

could be associated with Fe and Al, possibly forminghumic-Al/Fe complexes (Axt and Walbridge, 1999).These complexes are thought to play an importantrole in controlling P sorption potentials in sediments(Paludan and Jensen, 1995; Axt and Walbridge, 1999).Humus coating of Al/Fe complexes may inhibit thecrystallisation of Al and Fe minerals, thus maintain-ing Al/Fe compounds in a non-crystalline state for Psorption (e.g. Axt and Walbridge, 1999).

Approximately 63–67% of the drainage sedimentswere in fine particle size fractions (0.02–0.002 and<0.002 mm; Table 4). This probably reflected theselective deposition of silt and clay particles and fineparticulate matter from the runoff of soil organic mat-ter and animal excreta from upland catchments. Or-ganic detritus from the death and decay of vegetationand algal biomass within the ditches also probablycontributed fine particulate matter to drain sediments.Phosphorus retention is often directly proportional tothe surface area of the particle sizes (Cooke, 1988;Sharpley, 1980). Phosphorus retention is thereforeexpected to be significant in drainage sediments witha high proportion of fine particles. However, fineparticles are also likely to be suspended and prefer-entially transported along a water flow path (Houseet al., 1998). Thus, any drainage management andenvironmental factors (e.g. dredging and straight-ening drainage channels) that affect water flow andthe transport or sedimentation rate of fine particles,are likely to influence P retention characteristics ofdrainage systems.

3.2. Sediment phosphorus status and phosphorusretention characteristics

Substantial amounts of P were present in drainagesediments (Fig. 4), indicating that drainage sedimentsact as storage of P from agricultural runoff and sub-surface drains. Management of drainage ditches istherefore an increasingly important issue in minimis-ing P inputs to receiving waters both in New Zealand(Nguyen et al., 1998) and overseas (Sallade and Sims,1997a; Sims et al., 1998). Studies with drainage andwetland sediments (e.g. Olila et al., 1997; Sallade andSims, 1997b; Young and Ross, 2001) have found thatsediment wetting-drying caused a large P flux to theoverlying water column, as a result of changes in or-ganic matter decomposition and sediment redox status.Thus, drainage management may need to take into ac-count seasonal water table fluctuations to avoid draindrying and intensive aerobic decomposition. In NewZealand, some drainage ditches are dry during the latespring–summer period, while others remain floodedthroughout the year. Future research is therefore re-quired to investigate P release-retention characteristicsof drainage sediments under prolonged flooding andalternating waterlogged-drying conditions.

Drain clearance is another management consid-eration. The results showed that approximately 3–6months after drain clearance, sediments from thesedrains had a lower P content and P retention thanthose in which drains had not been cleared for 5years (Table 6). This was attributed to: (i) the removal


Fig. 4. Phosphorus concentrations (mg P kg−1) in non-occluded (A), carbonate-bound (B), occluded (C), calcium-bound (D) and residual(E) fractions of drain sediments collected from four catchments at 0–5 and 5–15 cm depths.


Table 6Chemical characteristics of drainage sediments and soluble phosphorus (P) concentration in drainage waters collected from Toenepicatchment drainage ditches that have or have not been recently dredged

Chemical characteristics Drainage ditches with and without recent dredging

3–6 months after dredging No dredging for the last 5 years

0–5 cm depth 5–15 cm depths 0–5 cm depth 5–15 cm depth

Sedimenta

Non-occluded P 33.0± 11.95 (9) 30.4± 9.69 (9) 272± 168.6 (15) 158± 82.6 (15)Carbonate P 4.6± 2.45 (9) 5.3± 3.39 (9) 35.7± 22.4 (15) 19.3± 10.3 (15)Occluded P 6.9± 3.98 (9) 5.8± 3.23 (9) 51.9± 20.8 (15) 32.3± 13.2 (15)Ca-P 5.2± 2.05 (9) 5.8± 3.03 (9) 24.4± 14.9 (15) 13.3± 3.81 (15)Residual P 44.9± 5.84 (9) 40.3± 3.46 (9) 145± 39.3 (15) 107± 35.8 (15)Total sediment extractable P 94.6± 24.9 (9) 87.7± 20.96 (9) 528± 251.2 (15) 330± 120.9 (15)P retention (%) 37.0± 4.17 (9) 36.6± 3.99 (9) 56.8± 10.79 (15) 49.2± 5.36 (15)Degree (%) of P saturation (DPS) basedon loosely bound and carbonate P

2.0 ± 0.67 (9) 1.9± 0.69 (9) 10.1± 5.24 (15) 7.1± 3.26 (15)

DPS based on loosely bound andoccluded P (%)

2.4 ± 0.84 (9) 2.3± 0.81 (9) 11.9± 5.66 (15) 8.4± 3.70 (15)

Oxalate-extractable Al 0.96± 0.117 (9) 0.83± 0.106 (9) 2.4± 0.45 (15) 1.2± 0.22 (15)Oxalate-extractable Fe 2.8± 0.68 (9) 2.6± 0.59 (9) 6.7± 1.05 (15) 4.3± 1.14 (15)

Water qualityb

Soluble P (dissolved reactive P) 0.0047± 0.00185 (3) 0.027± 0.0209 (5)Particulate P (PP) and dissolved organicP (DOP)

0.011± 0.0142 (3) 0.036± 0.0383 (5)

Total P 0.016± 0.0146 (3) 0.063± 0.0578 (5)Suspended solids 1.73± 0.321 (3) 12.6± 15.69 (5)

a Unit: mg P kg−1 sediment, except where indicated. Values in parentheses indicate number of data points.b Unit: mg P l−1. Values in parentheses indicate number of data points.

of P accumulated in sediments; and (ii) the lossof P sorption sites resulting from the removal ofP-retaining reactants such as organic C (Section 3.1)and oxalate-extractable Al and Fe (Table 6) fromsediments by drain clearance. All sediment P frac-tions, particularly non-occluded P, carbonate P andoccluded P, were lower in drains that had been re-cently dredged, compared to those that had not beencleaned out for 5 years (Table 6), suggesting thatdrain clearance removed not only loosely bound butalso strongly sorbed P fractions from sediments. Suchremoval may explain the observed differences in solu-ble P concentration in drainage waters and the degreeof P saturation (DPS) in sediments collected fromditches with and without recent drain clearance in theToenepi catchment (Table 6). The higher level of SS indrainage water collected from ditches with no dredg-ing for the past 5 years (Table 6) probably reflected

the accumulation of fine particles in sediments origi-nating from agricultural runoff, slumping of drainagebanks and organic detritus within the ditches.

In all sediment samples, the non-occluded Fe- andAl-bound P fractions were predominant (42–56% ofsediment P; Table 7). Much of the remaining sedi-ment P was associated with either carbonate (6–30%)or residual (6–39%) fractions, with less than 10% ofsediment P as either occluded Al/Fe-P or Ca-boundP (Table 7). The non-occluded Fe- and Al-bound Pfractions (0.1 M NaOH-extractable P) are consideredto represent bioavailable P (BAP; Sharpley, 1993),which includes not only sediment soluble P but alsoa portion of sediment PP that is biologically avail-able to algae and other aquatic biota (Sharpley andRekolainen, 1997). The results show that BAP rangedfrom 20 to 1625 mg P kg−1 and 18 to 875 mg P kg−1

in the 0–5 and 5–15 cm sediment layers, respectively


Table 7Proportions of non-occluded phosphorus (P), carbonate-bound P, occluded P, calcium-bound P and residual P fractions in sedimentscollected from four catchments and at two sampling depths

Catchment Sedimentdepth (cm)

Proportion (%) of sediment P as

Non-occluded P Carbonate-P Occluded P Calcium-bound P Residual P

Toenepi 0–5 Mean 41.9 (24)a 5.6 (24) 9.1 (24) 5.0 (24) 38.5 (24)Median 38.7 5.8 8.6 4.8 37.0S.E.M. 2.3 0.4 0.7 0.3 2.8

5–15 Mean 41.1 (24) 5.5 (24) 8.6 (24) 5.1 (24) 39.6 (24)Median 38.2 5.3 8.4 4.7 40.7S.E.M. 1.9 0.4 0.7 0.4 2.4

Pokeno 0–5 Mean 46.7 (21) 15.5 (21) 8.1 (21) 7.4 (21) 22.4 (21)Median 46.2 14.5 7.7 4.5 18.3S.E.M. 1.6 1.4 0.9 1.3 2.2

5–15 Mean 44.2 (21) 13.2 (21) 7.8 (21) 5.9 (21) 28.9 (21)Median 46.9 12.9 7.0 4.3 29.6S.E.M. 2.3 1.4 0.9 0.7 3.3

Northland 0–5 Mean 56.7 (21) 17.9 (21) 5.3 (21) 3.1 (21) 17.0 (21)Median 59.2 13.7 4.6 3.7 16.6S.E.M. 2.8 2.8 0.7 0.3 1.2

5–15 Mean 58.4 (21) 15.9 (21) 5.9 (21) 2.9 (21) 16.9 (21)Median 57.3 13.9 4.6 3.0 16.1S.E.M. 2.5 1.8 0.8 0.2 1.2

Bay of Plenty 0–5 Mean 56.4 (12) 29.6 (12) 5.2 (12) 2.3 (12) 6.4 (12)Median 53.9 31.1 6.1 2.3 6.3S.E.M. 1.7 0.9 0.5 0.2 0.2

5–15 Mean 53.7 (12) 23.7 (12) 5.5 (12) 4.7 (12) 12.5 (12)Median 53.0 22.1 4.8 4.8 13.3S.E.M. 1.1 1.9 0.9 0.2 0.8

a Values in parentheses indicate number of data points.

(Fig. 4). These levels were comparable to those re-ported by Sallade and Sims (1997b) for drainage sed-iments in the Delaware Inland Bays and were higherthan those commonly found in water runoff fromagricultural soils (Sharpley, 1993). The top sedimentlayer had higher BAP than the 5–15 cm layer, withmean BAP values in Toenepi, Pokeno, Northland andthe Bay of Plenty catchments of 182–1533 mg P kg−1

in the 0–5 cm depth and 110–634 mg P kg−1 in the5–15 cm depth (Fig. 4). Since the sediment-water in-terface is likely to play a major role in P diffusion andP sorption–desorption processes (House and Deni-son, 2000; Young and Ross, 2001), the top sedimentswith their high BAP values may have a higher poten-tial impact on the biological productivity of surfacewaters than the 5–15 cm sediment layer.

The significant (P ≤ 0.001) correlation betweenoxalate-extractable Fe/Al and the sum of non-occluded

Al/Fe-P and carbonate-bound P sediment fractions(r values of 0.593 and 0.502 for oxalate-extractableFe and oxalate-extractable Al, respectively; data notshown) indicates that the high proportion of looselybound P in drainage sediments (Table 7) was associ-ated with non-crystalline Fe compounds and slightlyless with Al as P reactants. Organic matter is alsolikely to be preferentially involved with Fe as P re-actants, particularly in the top 5 cm sediment layer,since sediment TC had a much higher correlation withoxalate-extractable Fe than with oxalate-extractableAl (Table 5). This suggestion is supported by thehighly significant correlations between sediment Pretention capacity and: (a) sediment TC and (b)oxalate-extractable sediment Fe (Table 5).

Sediment Ca-P (Fig. 4) probably originated fromeroded pastoral topsoil and P fertilisers that mightbe lost as suspended solids in runoff or via soil


macropores to tile and mole drains (McDowell andCondron, 2000; Addiscott et al., 2000). Phosphorusfertilisers annually applied to New Zealand pastoralecosystems as superphosphate (monocalcium phos-phate) or reactive phosphate rock have been shownto enhance P accumulation in pastoral soils and asignificant amount of accumulated soil P was presentas a Ca-P fraction (McDowell and Condron, 2000).Calcium in animal faeces and liming practices every5–6 years in New Zealand farming to maintain opti-mum pH (5.7–5.8) for a vigorous grass-clover swardmay also explain the presence of Ca-P in drainagesediments.

The precipitation of P as Ca-P is unlikely to bethe major process in the studied drainage sediments,since pH levels in drainage water and sediments (Ta-bles 2 and 3) did not exceed a pH of 8, which is nec-essary for significant precipitation (Diaz et al., 1994).Diurnal variations in drainage water pH, as measuredwith DataSonde loggers (Hydrolab Corp., Austin, TX,USA) over a 3-day period in four drainage ditches pereach catchment also confirmed the spot sampling data(Table 2) in showing that the pH range (5.25–6.26 atmidnight to the highest peak of 5.75–6.55 at mid-day;data not shown) was too low to cause Ca-P precip-itation. Since diurnal changes in drainage water pHwere monitored during the late spring–summer pe-riod, when plant growth and photosynthetic activity indrainage ditches were probably at their most active,pH levels at other times of the year are also unlikelyto reach the level (pH> 8) that causes P precipitation(Diaz et al., 1994). The prevailing acidic nature of thestudied sediments (Table 3) suggests that solubiliza-tion of Ca-P might occur (Diaz et al., 1994; Salladeand Sims, 1997b) although the extent of this solubi-lization is unknown.

The low proportion of occluded Al/Fe-P (Table 7)is not surprising in an anaerobic environment ofdrainage sediments, since this form is often associ-ated with ferric ion, which is not predominant underanaerobic conditions (Paludan and Jensen, 1995).Residual P represented a significant proportion of P insome sediment (Table 7). Most of this is likely to beorganic, since residual P was significantly correlatedwith sediment TC (r = 0.59; P ≤ 0.001).

The amounts of organic P bound to Al/Fe, carbonateand Ca are unknown, since organic P in NaOH, NaCl,CDB and HCl extracts was not determined. However,

they are likely to be substantial, based on the recentresults reported for wetlands and peat soils with a sig-nificant organic C enrichment (Qualls and Richardson,1995; Nguyen, 2000).

All sediments, particularly the top 5 cm layer, werefound to possess a significant P retention capacityand PSI (Table 3). A significant correlation (r =0.602–0.914;P ≤ 0.001; Table 5) was found betweenthe maximum P retention capacity (as measured bya single point P sorption capacity after equilibratingwith 5000 mg P kg−1 soil according to the methodof Saunders, 1965) and a range of sediment param-eters (oxalate-extractable Al, oxalate-extractable Fe,sediment pH and sediment TC content).

Sediment P retention capacity was also highly cor-related with the silt fraction (r = 0.392; P ≤ 0.001;Table 5), while insignificant correlation was found forthe clay fraction. The lack of a significant correlationbetween P sorption capacity and sediment clay contentin the study is rather surprising, since clays are knownto contain broken edges that act as exchange sites be-tween P and clay-associated hydroxyl (–OH) groups(Axt and Walbridge, 1999). This lack of significancewas attributed to the predominant effect of low sedi-ment pH on the ligand exchange between P and Al/Feoxides-associated hydroxyl (–OH) groups. At low pH,protons (H+) from sediment porewater can be ad-sorbed by –OH groups to produce positively-chargedaquo (–OH2+) groups. This results in a net positivecharge on the surface of Al and Fe oxides and hydrox-ides for sorption of negatively-charged ions (anions)such as P (Parfitt, 1978).

Stepwise multiple regression showed that sedi-ment TC and oxalate-extractable Fe accounted forapproximately 88.6% of the variability in sediment Psorption capacity (Table 8). The addition of sedimentpH, oxalate-extractable Al, silt and clay contentsonly accounted for an additional variability of<1%in sediment P sorption capacity (Table 8). These re-sults suggest that sediment organic matter (TC) andoxalate-extractable Fe could be used as an index ofsediment P retention capacity for the studied drainagesediments.

The study demonstrated that drainage sedi-ments retained approximately 44–84% of added P(5000 mg PO4-P added kg−1 soil) over a range ofequilibrium P concentrations of 161–507 mg P l−1

(Table 3). The amounts of P potentially sorbed (2467–


Table 8Parameters of the linear regressions obtained from single variable and multivariate regression analyses showing the relationships betweensediment phosphorus (P) retention (% of added P) and sediment chemical and physical characteristicsa

Sediment characteristics Sediment P retention (% of added P) capacity

Intercept Slope Partialr2 Model r2

Oxalate-extractable Fe 11.06 9.04 0.836 (52)∗∗∗

Oxalate-extractable Fe 18.74 5.64+ Total C 1.46 0.886 (52)∗∗∗

Oxalate-extractable Fe 56.74 5.68+ Total C 1.18+ pH −6.69 0.890 (52)∗∗∗

Oxalate-extractable Fe 50.93 6.21+ Total C 1.18+ pH −5.39+ Oxalate-extractable Al −2.50 0.892 (52)∗∗∗

Oxalate-extractable Fe 45.86 6.25+ Total C 1.18+ pH −6.16+ Oxalate-extractable Al −1.88+ Silt 0.23 0.893 (52)∗∗∗

Oxalate-extractable Fe 47.76 6.26 0.836 0.836∗∗∗+ Total C 1.17 0.050 0.886∗∗∗+ pH −6.33 0.040 0.890∗∗∗+ Oxalate-extractable Al −1.86 0.002 0.892∗∗∗+ Silt 0.21 0.001 0.893∗∗∗+ Clay −0.02 0.001 0.894 (52)∗∗∗

a Values in parentheses indicate number of data points used in regression analyses.∗∗∗ P < 0.001.

4197 mg P kg−1 soil or PSI of 931–2328 l kg−1 soil)by sediments were substantially higher than thesum of non-occluded and occluded Al/Fe-P (241–2492 mg P kg−1 soil; Fig. 4) in each of the drain sedi-ments. Thus, P sorption sites on sediments may still beavailable for further P removal from drainage water.

Both sediment PSI (mean= 816–2328 l kg−1 sedi-ment) and DPS (7–68%) values, which are indicatorsof sediment P retention capacity (Axt and Walbridge,1999) were found to be comparable to those reportedby Sallade and Sims (1997a). Thus, all the studiedsediments were assumed to have similar P retentioncapacities and hence presumably similar EPC0 valuesto those reported by Sallade and Sims (1997a). TheEPC0 is defined as the sediment P concentration atwhich P sorption on the sediment surface is in equi-librium with P release from sediments (i.e. at net zerosorption–desorption) to the overlying waters (Salladeand Sims, 1997a; House et al., 1998). Sediments are

assumed to release P into drainage waters if the EPC0is greater than the P concentration in drainage waterand vice versa (Sallade and Sims, 1997a,b).

Unpublished data of P sorption isotherms over arange of added P concentrations (0, 5, 10, 20, 40,60, 80 and 100 mg P l−1) for 16 out of the 26 col-lected surface (0–5 cm) sediments showed that theEPC0 values for these 16 randomly selected sedi-ments were 0.03–0.55 mg P l−1. These values werecomparable to those (0.02–0.50 mg P l−1) reported bySallade and Sims (1997a,b) and were higher than sol-uble P concentrations (0.006–0.019 mg P l−1; Fig. 2)in most of the drainage water samples collected (24out of 26). This suggests that most of the surveyedsediments (assuming that the obtained EPC0 values of0.03–0.55 mg P l−1 were applicable to the remainingsediments) would release P until soluble P concen-trations in drainage waters reached the EPC0 values(Sallade and Sims, 1997a; House et al., 1998).


Similar to findings reported by various workers (e.g.Axt and Walbridge, 1999; Sallade and Sims, 1997a),PSI was significantly related (r = 0.284–0.830;P ≤ 0.001; data not shown) to sediment constituents(oxalate-extractable Fe/Al, TC and pH), which greatlyinfluenced sediment P sorption (Table 5), suggest-ing that these sediment constituents can be used toestimate sediment PSI.

The study did not investigate the extent of P satura-tion (i.e. critical DPS value) above which drainageditch sediments have a potential impact on the eutro-phication of the overlying waters. To the knowledge ofthe authors, the only information on the critical DPSvalues for New Zealand soils was that reported byMcDowell et al. (2001). In their study, the poten-tial losses of P to drainage waters from some NewZealand, UK and USA topsoils (varying from 0–5 to0–23 cm depth) sharply increased when DPS valueswere above 29–30%. These values are similar to thatreported by Sallade and Sims (1997b) for drainageditch sediments in the Delaware Inland Bays. Theyfound that drainage sediments with critical DPS valueof over 40% had the potential to release P to the overly-ing waters in excess of the levels (0.01–0.03 mg P l−1)associated with surface water eutrophication in theDelaware Inland Bays.

With limited DPS information on New Zealandsoils/sediments as outlined above, it is assumed thata DPS value of 30–40% as reported by Sallade andSims (1997b) and McDowell et al. (2001) serves as anindex of the sediments’ susceptibility for P losses inour studied drainage sediments. This assumption wasbased on the following considerations: (i) the PSI andDPS values in the studied sediments were comparableto those reported by Sallade and Sims (1997b); (ii)the soluble P limit for surface water eutrophication inNew Zealand (0.015–0.030 mg P l−1) was similar tothat (0.01–0.03 mg P l−1) used for setting the criticalDPS value in Sallade and Sims’ study Sallade andSims (1997a,b); and (iii) the critical DPS values forsome New Zealand soils (≥29–30%; McDowell et al.,2001) were comparable to those (≥40%) reported bySallade and Sims (1997b).

Using the DPS value of≥30–40% as a crit-ical limit for the surface water quality standard(0.015–0.030 mg P l−1), most of the sediments col-lected (22 out of 26), except for the top 5 cm layerof drainage sediments from the Bay of Plenty catch-

ment, had DPS values of<30% and hence they wereconsidered to have a low potential to release P todrainage waters.

Although drainage sediments with DPS values of64–68% (Table 3) were expected to be associated withhigh P concentration in drainage waters (assumingequilibrium exists between sediment and drainage wa-ter P), the results demonstrated that drainage waterP concentrations in these sediments were still belowthe level associated with surface water eutrophication(Fig. 2). This suggests that a critical DPS value of≥64–68%, instead of≥30–40% may be more appro-priate for the studied sediments. Thus, further studyis required to define these critical values for NewZealand sediments.

Sediment P extracted by either water or Olsenreagent (Table 4) was significantly correlated withsediment P which constituted either non-occludedand carbonate-bound P fractions (r = 0.801–0.805;P ≤ 0.001) or non-occluded, carbonate-bound andoccluded P fractions (r = 0.795–0.804;P ≤ 0.001).Both water-extractable P and Olsen-extractableP were also significantly correlated with PSI(r = 0.467–0.578;P ≤ 0.001) and DPS (r =0.663–0.752;P ≤ 0.001). Thus, water-extractable orOlsen-extractable sediment P could be used to iden-tify drainage ditches where sediments are saturatedwith P and potentially act as a non-point source of P.Information obtained could then be used to target ap-propriate management practices to areas where thesedrainage ditches exist. In these areas, farm practicessuch as P fertiliser application, grazing intensity andland application of dairy wastewaters should be care-fully managed to minimise P losses in runoff andsubsurface flow to these ditches (e.g. Sims et al.,1998; Gburek et al., 2000).

Although routine soil P tests (water or Olsen-extractable P) potentially accounted for the saturationof P sorption sites in the studied sediments, these soiltests alone do not differentiate the extent of sedimentP saturation as non-occluded (loosely-bound andreadily desorbed) and occluded P (slowly desorbed)fractions. Although the P saturation approach (DPS)has the potential to rank sediments in terms of Psaturation of sediment P sorption sites, the study hasnot established the relationship between sediment Psaturation and the extent of P release from a range ofNew Zealand sediments. Future research is therefore


required to calibrate sediment P saturation and the ex-tent of P release from sediments with a range of DPS,PSI, EPC0 and added P values, particularly for sedi-ments with high DPS values under aerobic-anaerobicconditions. In the calibration study, the DPS valueswill be based on a range of soil test methods thatreflect either the pools of P retained in sediment Psorption sites or loosely-bound extractable P. Table 3showed that the DPS values of the studied sedimentsincreased by only<15% with the inclusion of oc-cluded P (strongly-sorbed P). This indicates that most(85–95%) of the DPS in the studied sediments wasattributed to the saturation of P sorption sites with theloosely bound P form, which is known to be readilyreleased from sediments than the occluded P fraction(Parfitt, 1978).

It is important to note that routine soil P testsand laboratory techniques used to obtain PSI, DPSand EPC0 values were performed under laboratoryconditions where overlying water was in maximumcontact with drainage sediments via continuous shak-ing. Thus, the values obtained may not adequatelypredict sediment P status and sediment potential forP release under field conditions, where equilibriummay not establish between sediment P and soluble Pconcentration in drainage water. This non-equilibriumcould explain the observed finding in the presentstudy in which soluble P concentration in drainagewater was not significantly correlated to the looselybound sediment P fraction (data not presented). Suchnon-equilibrium might be caused by variable environ-mental factors (e.g. water flow, biological activity, Ploading and turbulence) which govern P diffusion andredox status at the sediment–water interface, which inturn influence sediment P release-retention character-istics (Reddy et al., 1999; House and Denison, 2000;Young and Ross, 2001).

4. Conclusions

Agricultural drainage ditches were found to con-tain a substantial amount of P (mean= 240–2775 mgP kg−1 soil), probably originating from surface runoffand subsurface drainage of upland agricultural catch-ments. Phosphorus retained in drainage networks is apotential source of P to drainage waters, since a sub-stantial proportion (mean= 45–89%) of sediment P

was loosely bound and hence could potentially be re-leased to the overlying waters. Water-extractable orOlsen-extractable sediment P can be used to identifydrainage ditches with a high proportion of looselybound and strongly sorbed P status. These soil P meth-ods may need to be used in conjunction with a sed-iment P retention test to identify drainage sedimentswith a high P saturation capacity (DPS) and hence ahigh susceptibility to P losses to the overlying waters.In this study, the critical DPS value for identifyingdrain sediments with the potential for P loss has notbeen established for sediments with a range of P re-tention capacity and phosphate saturation. Future re-search is required to establish this critical DPS valueand calibrate it across a range of sediment P tests andsediment equilibrium P concentrations at net zero Psorption–desorption.

Soluble P concentrations in drainage waters weregenerally below the level associated with surface wa-ter eutrophication of 0.015–0.030 mg P l−1, probablyreflecting the high potential for P sorption in the sur-veyed sediments (P sorption capacities of 44–84%of added 5000 mg P kg−1 sediment). However, the Psorption sites on sediments may eventually be satu-rated with loosely bound and strongly sorbed P as aresult of a continuing loading of P pollutants from sur-face runoff and subsurface flows.

It is important to determine not only the extentof DPS in drainage sediments but also the propor-tion of DPS as loosely bound and strongly sorbed Pforms. Sediments that are saturated with loosely boundP may act as temporary P storage, which then re-lease P into the overlying waters, depending on vari-able environmental factors that affect the equilibriumstatus between sediment P and drainage water P. Incontrast, sediments that are saturated with occluded(strongly sorbed) P may not readily release P into, orremove P, from the overlying waters. Most (85–95%)of the DPS in the studied sediments was attributed tothe saturation of P sorption sites with loosely boundP form.

Organic matter accumulation in drainage sedi-ments, contributed from catchment runoff or fromwithin the drainage network, may provide addi-tional P sorption sites by complexation as humic-Al/Fe compounds. Management of drainage networksas vegetative wetlands to optimise P sorption–comple-xation–sedimentation processes is an important issue


in drainage pollution control. The relative importanceof these processes may need to be tested under fieldconditions for a range of sediments with different Pstatus.

The non-equilibrium between sediment anddrainage water P, as shown by the insignificant cor-relation between soluble P concentration in drainagewaters and the loosely bound fraction in drainagesediments, highlights the importance of measuringnot only sediment P solubility (soil P tests) and sedi-ment P sortion–desorption potential (DPS, EPC0 andPSI) but also accounting for environmental factorswhich potentially affect P retention and release withindrainage ditches.

Future research is required to investigate the in-teraction of P in drainage sediment and overlyingdrainage water under a range of environmental condi-tions (e.g. flooding, drying and wetting and P loadingsin drainage water) in order to improve our under-standing of drainage ditches as a sink-source of P inaquatic environments.

Acknowledgements

Funding for this study was provided by the NewZealand Foundation for Research, Science and Tech-nology. The authors are grateful to Drs David Loweand Peter Singleton for their assistance with soil clas-sification, Kerry Costley for technical assistance, PaulChampion for plant identification, Dr. Chris Tanner forassistance with sediment samplings, Sharon Nguyenfor editing, and the farmers on whose properties thestudy was conducted.

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