Spatial and temporal variations of nutrien

Spatial and temporal variations of nutrientconcentration in the groundwater of a floodplain:eect of hydrology, vegetation and substrate

Naima Takatert,1 Jose Miguel Sanchez-Perez2* and Miche`le Tremolie`res11Centre dEtudes et de Recherches Eco-Geographiques (CEREG ULP-CNRS), Laboratoire de Botanique et dEcologie Vegetale, Institut

Franco-Allemand de Recherche sur lenvironement (IFARE), Institut de botanique,28, rue Goethe F-67083 Strasbourg, Cedex, France

2CEREG ULP-CNRS, IFARE, Universite Louis Pasteur, 3, rue de lArgonne, F-67083 Strasbourg, Cedex, France

Abstract:Spatio-temporal variations in nitrogen and phosphorus concentrations in groundwater were analysed andrelated to the variations in hydrological conditions, vegetation type and substrate in an alluvial ecosystem. Thisstudy was conducted in the Illwald forest in the Rhine Plain (eastern France) to assess the removal of nutrients

from groundwater in a regularly flooded area. We compared both forest and meadow ecosystems on clayey-siltysoils with an anoxic horizon (pseudogley) at 1.52 m depth (eutric gley soil) and a forest ecosystem on a clayey-silty fluviosoil rich in organic matter with a gley at 0.5 m depth (calcaric gley soil). Piezometers were used to

measure the nutrient concentrations in the groundwater at 2 m depth in the root layer and at 4.5 m depth, belowthe root layer. Lower concentrations of nitrate and phosphate in groundwater were observed under forest thanunder meadow, which could be explained by more ecient plant uptake by woody species than herbaceous

plants. Thus NO3-N inputs by river floods were reduced by 73% in the shallow groundwater of the forestedecosystem, and only by 37% in the meadow. Compared with the superficial groundwater layer, the lowest levelof nitrate nitrogen (NO3-N) and the highest level of ammonium nitrogen (NH4-N) were measured in the deeplayer (under the gley horizon at 2.5 m depth), which suggests that the reducing potential of the anoxic horizon

in the gley soils contributes to the reduction of nitrate. Nitrate concentrations were higher in the groundwater ofthe parcel rich in organic matter than in the one poorer in organic matter. Phosphate (PO4-P) concentrations inboth shallow and deep groundwater are less than 62 to 76% of those found in surface water which can be related

to the retention capacity of the clay colloids of these soils. Moreover, the temporal variations in nutrientconcentrations in groundwater are directly related to variations in groundwater level during an annualhydrological cycle. Our results suggest that variations in groundwater level regulate spatio-temporal variations

in nutrient concentrations in groundwater as a result of the oxidationreduction status of soil, which createsfavourable or unfavourable conditions for nutrient bioavailability. The hydrological variations are much moreimportant than those concerning substrate and type of vegetation. Copyright# 1999 John Wiley & Sons, Ltd.

KEY WORDS floodplain; groundwater; hydrology; nitrogen; phosphorus; riparian zone

INTRODUCTION

In recent years, increasing attention has been focused on terrestrialaquatic ecotones owing to theirimportant role in regulation of the fluxes of energy and material in hydrosystems (Holland et al., 1990;Naiman and Decamps, 1990). Floodplains are known to control large exchanges of nutrients and organic

CCC 08856087/99/10151116$1750 Received 24 July 1998Copyright # 1999 John Wiley & Sons, Ltd. Revised 20 December 1998

Accepted 10 December 1998

HYDROLOGICAL PROCESSESHydrol. Process. 13, 15111526 (1999)

*Correspondence to: Dr J. M. Sanchez-Perez, CEREG ULP-CNRS, IFARE, Universite Louis Pasteur, 3 rue de lAgonne, F-67083Strasbourg, Cedex, France. E-mail: [email protected]

matter between aquatic and terrestrial ecosystems (Peterson and Rolfe, 1982; Swanson et al., 1982; Brinsonet al., 1983, 1984). Thus the riparian forest limits the transfer of nutrients from groundwater to river water(Peterjohn and Correll, 1984; Pinay and Decamps, 1988). Cristofor et al. (1993) showed the importance offlood zones in reducing eutrophication of aquatic ecosystems (lakes) and rivers of wetlands. A decrease innitrate concentrations in the groundwater that flows through a forest ecosystem was measured by Postmaet al. (1991). However, the riparian forest can also be a source of ammonium nitrogen and phosphorus, oracts as a phorphorus sink, depending on the dissolved oxygen concentrations in groundwater (Mulholland,1992). A low dissolved oxygen concentration induces low redox potential and the solubilization of phosphatein the form of ferrous phosphate.Several studies have focused on the processes that explain nutrient retention or elimination from riparian

systems. In wetland soils receiving large amounts of N, denitrification appears to be the most importantprocess for the elimination of nitrate (Lowrance et al., 1984; Groman and Tiedje, 1989; Zak and Grigal,1991; Haycock and Burt, 1993). However, uptake by root absorption or immobilization in soils is anotherway of removing nitrate from groundwater and soils (Sanchez-Perez et al., 1991a; Groman et al., 1992). Therole of vegetation in the protection of groundwater quality was demonstrated by Sanchez-Perez et al. (1991b)in relation to seasonal changes and stages of vegetation of the alluvial forest succession. The reduction ofphosphorus concentrations in groundwater by riparian forests has usually been attributed to root absorptionand chemical precipitation and sorption processes (Reddy and Rao, 1983; Patrick, 1990; Sanchez-Perez et al.,1991a). Some other studies have documented the eects of factors that regulate the transfer of nutrients tothe groundwater, for example geomorphology (McDowell et al., 1992; Pinay et al., 1995) or catchmenthydrology (Cooper, 1990), but few studies have taken into account interactions between several factors, suchas geomorphology (type of substrate), hydrology (role of floods) and biology (vegetation type).In this paper we report the spatio-temporal variations of nitrogen and phosphorus concentrations of the

groundwater for several water years in three ecosystems of a floodplain diering in the type of soil andvegetation. We try to specify in each case the dierent processes and parameters that could control andregulate the transfer of nutrients into groundwater through the soilroot system. We investigate moreparticularly how flooding influences N and P variations in dierent conditions of soil, with high and low(510% organic matter) levels of organic matter and vegetation, meadow and forest.

STUDY AREA

The Illwald forest (Figure 1) covers 1400 ha and is located in the Ill floodplain 40 km south of Strasbourg(eastern France). The Ill River (218 km long, slope 2.2%, watershed area about 4765 km2 and meandischarge 45 m3 s1) is the main tributary of the Rhine River in the French Rhine Plain. This river collectswaters from the streams of the Vosges mountains characterized by an acidic geochemistry. During theQuaternary, the Rhine deposited an heterogeneous coarse, calcareous material in the rift valley between theVosges and the Black Forest. The Ill River carries acidic sediments above the Rhine deposits and conse-quently the substrate in the Ill floodplain is rich in calcium but decarbonated at the surface.The Ill hydrological regime, and consequently the groundwater regime, is characterized by high water in

late winter and the beginning of spring (DecemberMarch) and low water in summer. The Ill floodplaincovers about 10 000 ha of forests, meadows and crops. About 7000 ha of the floodplain area are flooded atleast once every three years. Floods result from two mechanisms: rising groundwater and overflowing ofthe river at discharge exceeding 66 m3 s1. When the water level decreases, a part of the flood waterinfiltrates through the root layer. Mean annual precipitation for the years 19941996 was 546 mm(SE mean 15 mm); rainfall inputs during the low water period (MaySeptember) provide up to 50% ofthe total annual input.The water of the Ill River is highly polluted with organic matter, phosphate, ammonium from euents of

waste water treatment plants or organic euents from villages and towns (Tremolie`res et al., 1994) and heavymetals coming from industry (e.g. mercury; Roeck et al., 1993). Nitrate concentrations are not very high in

Copyright # 1999 John Wiley & Sons, Ltd. HYDROLOGICAL PROCESSES, VOL. 13, 15111526 (1999)

1512 N. TAKATERT, J. M. SANCHEZ-PEREZ AND M. TREMOLIE`RES

the Ill River (2.45 mg l1 NO3-N, annual mean). Despite contamination of the river water, the groundwateris not contaminated by these pollutants because the Ill does not infiltrate through its bed or banks into thegroundwater in the study sector (Tremolie`res et al., 1994). The studied sector is surrounded by highlyfertilized crops (especially maize) (about 200 kg N ha1 year1), which provide a high concentration ofnitrate, around 5.5 mg l1 NO3-N, in groundwater under the cultivated fields. However, the meadowsstudied are neither directly fertilized, nor grazed. Soils are gleys characterized by the presence of a hydro-morphic horizon (gley or pseudogley). These soils are subjected to permanent reducing conditions.Three sites were studied (Figure 1): a meadow area (parcel 24) and two forested areas (parcels 183 and 96).

The two forest sectors correspond to a terminal stage of the alluvial forest succession with alder, ash(dominant tree species), oak and elm. The meadow (parcel 24) and forest (parcel 183, western sector) are

Figure 1. Location of the study site


GROUNDWATER NUTRIENT CONCENTRATION 1513

characterized by clayey-silty soils with an anoxic horizon, called pseudogley, at 1.5 m depth (eutric gley soil),whereas the second forest (parcel 96, eastern sector) is located on clayey-silty soils rich in organic matter witha gley at 1.5 m depth (calcaric gley soil) (Table I, Figure 2). During the low water period, groundwater level islocated at 1 m depth in the western sector and at 0.7 m depth in the eastern sector.

Table I. Texture and organic matter content of soils in the three parcels of the study site

Parcel no. Soil depth(m)

Particle size distribution (%) Organic carbon(%)

Vegetation

Sand Silt Clay

183 00.2 15.4 41.5 43.1 6.8 Alder, ash with oak, and elm0.20.4 8.9 39.6 51.5 8.20.40.8 8.0 22.6 69.4 8.80.82.2 21.1 27.2 51.7

96 00.2 36.0 16.1 47.8 21.0 Alder, ash with oak, and elm0.20.8 61.7 2.7 35.6 23.00.82.2 55.0 19.4 25.5

24 00.2 19.2 36.8 43.8 7.9 Meadow0.20.4 19.8 42.5 37.7 5.60.40.8 8.6 46.5 44.7 4.90.82.0 9.9 40.3 49.8

Figure 2. Schema of location of the three ecosystems and soil texture along a transect, westeast, from the Ill River



MATERIALS AND METHODS

Groundwater sampling

Groundwater samples were collected in a network of piezometers installed at two levels (Figure 2): oneseries was located above the gravel layer in the soil root zone (soilroot piezometer shallow groundwater,2 m deep) and another in the gravel layer 4.5 m deep (deep piezometer deep groundwater). Soilrootpiezometers were slotted from the gravel to within 20 cm of the soil surface (in this type of alluvial sector, themaximum rooting is located between 20 and 80 cm depth) and deep piezometers in the two last meters fromthe bottom. Soilroot piezometers were made of PVC tubes 100 mm in diameter, and the deep piezometersof stainless steel tubing 60 mm in diameter. They were sealed at the bottom. In November 1992, fivepiezometers were installed in the forest parcel 183 in the soilroot zone on a westeast axis from the Ill Riverand one at the deeper level. In January 1994, three soilroot piezometers were installed in parcel 96 (organicsubstrate) and two in the meadow (parcel 24). In November 1995, a series of two piezometers were installedat a depth of 4.5 m in the three sectors.Monthly (daily to weekly during flood events) monitoring of the chemical composition of the groundwater

was done, including periods of high and low water. The periodicity used for sampling is highly representativeof the variations occurring during a water year (Sanchez-Perez, 1992). Daily sampling during flood eventsallows us to follow the variations in chemical content when the water infiltrates through the soil. Wecompared the groundwater quality in areas diering in types of substrate and vegetation. Shallow ground-water was sampled from December 1994 to June 1996 and deep groundwater from November 1995 to June1996. In the forest parcel 183, shallow and deep groundwater has been sampled since 1992 and we cancompare the results in this parcel for the periods from January to June in 1993, 1994, 1995 and 1996.Prior to pumping, the depth to the water table was measured in each piezometer. Water samples were

collected after removal of 10 times the water volume of the piezometer by a motorized pumping. Collectionwas made using an electrical pump. Water was pumped from each piezometer for approximately 1 min priorto collecting the sample, in order to clear the tubing of the previous sample. The water samples were collectedin polyethylene bottles after pumping, filtered in the laboratory through a 0.45 mm sieve and stored in thedark at 4 8C until chemical analyses were performed (less than 4 days).

Water chemistry

We measured the temperature, pH, conductivity and dissolved oxygen in situ after pumping. In thelaboratory, we analysed NO3-N, NH4-N and PO4-P by colorimetry with a microflux automated analyser(Alliance instruments, Integral 4). The procedures used are specified in APHA (1985): ammonium wasdetermined by the indophenol blue method, nitrate by the cadmium reduction method and phosphate by theascorbic acid method. Standard chemical analysis procedures were followed, including use of replicates andblanks. Each analysis was controlled through a series of standards, which are selected according to theconcentrations measured. Data were checked and replicates with a dierence of 5% were re-analysed.

River discharge and groundwater level monitoring

The data concerning discharge of the Ill River were provided by the Service des Eaux et des MilieuxAquatiques (SEMA), the French oce for waters and the aquatic environment. Groundwater levels weremonitored with a pressure probe (PDCR 850 Druck) and recorded by a Campbell CR10 datalogger every30 min.

Statistics

Most of the data did not follow the normal distribution, thus non-parametric statistics were chosen. Wecalculated the medians and quartiles to compare data between the dierent sectors studied. We used themean of values of piezometers in each study sector at each sampling date to identify significant temporal



variations. Results were compared by the KruskalWallis test using Minitab (Minitab, Inc., USA). Weconsidered dierences significant at p5 0.05.

RESULTS

The mean pH was higher than 7 in the Ill River and the deep groundwater, and a little less in the shallowgroundwater (Table II). The electrical conductivity was found to be 300400 mS cm1 in all areas except inthe groundwater of parcel 96, where it exceeded 900 mS cm1. The variations in electrical conductivity(expressed as standard deviations) were lower in the deep groundwater than in the shallow groundwater. Theoxygen level was lower in the shallow groundwater (3040% saturation) than in the surface water (480%)and still lower in the deep groundwater (2030%).During the study period, the Ill River flooded the study area 15 times such that parcels 24 and 183 (western

part of forest) were submerged. By contrast, in parcel 96, groundwater rose to the soil surface withoutleading to surface flooding. The groundwater level variations followed those of the discharge of the river(Figure 3).

Temporal variations in groundwater P and N in the forested stand

The variations in groundwater concentrations of P and N were monitored in parcel 183 from April 1992to June 1996. We assume that there was no change in environmental conditions, i.e. no trees were harvestedin the plots and no changes in water flow controls were implemented in the study period. Thus, for fouryears, we can compare the mean of nutrient concentrations in groundwater in relation to hydrologicalconditions to verify whether variations in groundwater level produce the same variations in nutrient con-centrations from one year to another. The values given in Table III are the median values for data includingthe same high water period from January to June every year, the period during which flood event occur mostregularly.In the Ill River the annual median of nitrate concentrations remained at the same level during the 4 years

(around 2 mg l1), whereas phosphate and ammonium showed a decrease from 1995. In the shallowgroundwater, the NO3-N, NH4-N and PO4-P concentrations varied significantly over the four-year period;

Table II. Average pH, electrical conductivity (mS cm1) and oxygen saturation (%) in the Ill River, shallow and deepgroundwater in meadow and forests in the study site from December 1994 to June 1996. Numbers in parentheses are

standard deviations

pH Electrical conductivity(mS cm1)

Oxygen(%)

Ill River 7.39 368 86.2(0.49) (137) (19.3)

Shallow groundwaterMeadow (parcel 24) 6.94 286 39.7

(0.47) (92) (19.3)Forest (parcel 183) 6.98 411 29.5

(0.49) (129) (12.4)Forest (parcel 96) 6.98 1169 39.1

(0.49) (562) (13.6)

Deep groundwaterMeadow (parcel 24) 7.13 486 20.1

(0.18) (58) (2.7)Forest (parcel 183) 7.32 428 15.7

(0.25) (43) (5.9)Forest (parcel 96) 7.26 985 26.9

(0.19) (61) (8.2)



Figure 3. Variations in (a) nitrate and (b) phosphate concentrations in the shallow and deep groundwater (parcel 183) and the surfacewater (Ill River), and (c) the hydrological regime of the Ill River and groundwater from April 1992 to June 1996. Mean values+SE



Table III. Median concentrations and quartiles of nitrate, ammonium and phosphate over four half-years (JanuaryJune) 1993, 1994, 1995 and 1996 in aflooded forest (parcel 183)

1993 1994 1995 1996

Shallow Deep Ill River Shallow Deep Ill River Shallow Deep Ill River Shallow Deep Ill River

NO3-N (mg l1)

Median 0.16 2.30 0.18 0.25 1.67 0.39 0.16 1.98 1.59 0.13 2.31Lower quartile 0.11 2.05 0.15 0.18 0.65 0.21 0.10 1.38 0.74 0.08 2.04Upper quartile 0.28 2.47 0.31 0.48 2.97 1.14 0.19 2.31 2.15 0.19 2.46

NH4-N (mg l1)

Median 287 121 677 25 27 584 62 62 127 81 122 291Lower quartile 133 88 391 19 21 512 32 35 70 56 86 266Upper quartile 429 337 1031 40 33 750 111 90 185 127 141 640

PO4-P (mg l1)

Median 29 16 198 18 17 187 78 26 122 76 25 151Lower quartile 17 5 118 16 13 130 52 19 96 53 21 134Upper quartile 71 24 349 23 23 306 91 31 168 123 28 255

N 6 6 4 11 11 10 8 8 10 6 6 7

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we measured lower concentrations of NO3-N and PO4-P in 1993 and 1994 than in the following two years.Concentrations of NH4-N in shallow groundwater were higher in 1993 than for the other three years. In thedeep groundwater, the NO3-N and NH4-N concentrations varied significantly over the four-year period,whereas the concentrations of PO4-P did not vary significantly over the same period. NH4-N concentrationswere higher in 1993 and 1996 than in 19941995.Fluctuations of nitrate and phosphate concentrations were lower in the first period (19921993) than in

the second one (19941996), corresponding to lower fluctuations in the groundwater level, which iscorrelated with river discharge (Figure 3). During the first period there was only one period of overflow,in November 1992, whereas the second period was marked by a succession of rises in groundwater in 1994(FebruaryJune) and overflows (January 1994, JanuaryMarch 1995).

Spatio-temporal variations of nutrients in shallow groundwater

Nitrogen. Over the period December 1994June 1996, the river contained the highest concentrations ofnitrate (median of 2.31 mg l1 NO3-N) in comparison with the groundwater (median of 0.631.46 mg l

1

NO3-N, Table IV). At times other than the flood period, nitrate in groundwater remained less than0.5 mg l1 NO3-N in the shallow groundwater of both forest parcels (183 and 96), which contrasts withconcentrations measured in the shallow groundwater of maize fields (6 mg l1). In the forest on an organicsubstrate (parcel 96), the median of the NO3-N concentrations (1

.01 mg l1) was higher than that on asubstrate relatively poor in organic matter (0.63 mg l1 in parcel 183). Under the meadow, nitrateconcentrations were higher (1.46 mg l1) than those in the forest groundwater. During the high waterperiods, the concentration of nitrate in groundwater in all the studied sectors increased greatly, to a levelsimilar to or higher than those of the Ill River (2.31 mg l1 and 1.8 mg l1 NO3-N in January and February,respectively). This increase was much more pronounced in parcel 96, with its organic substrate, in which theconcentration reaches 4.8 mg l1 NO3-N (Figure 4).Variations in NH4-N showed that the Ill waters contained the highest concentrations over the whole study

period (Figure 4). The groundwater sampled in all piezometers showed the same trends in NH4-N variations(Table IV). We observed a general decrease of ammonium in February and September, corresponding to an

Table IV. Median concentrations and quartiles of nitrate, ammonium and phosphate in the shallow and deepgroundwater in the three study sites from December 1994 to June 1996

Meadow groundwater(parcel 24)

Forest groundwater(parcel 183)

Forest groundwater(parcel 96)

Surfacewater

Shallow Deep Shallow Deep Shallow Deep Ill River

NO3-N (mg l1)

Median 1.46 0.16 0.63 0.18 1.01 5.08 2.31Lower quartile 0.43 0.12 0.26 0.14 0.42 5.02 1.94Upper quartile 1.88 0.23 1.58 0.27 2.64 6.19 3.07

NH4-N (mg l1)

Median 64 176 72 119 57 25 199Lower quartile 30 136 40 88 23 23 141Upper quartile 138 248 110 174 94 39 491

PO4-P (mg l1)

Median 73 63 66 22 42 14 174Lower quartile 53 38 47 16 17 12 121Upper quartile 112 97 88 27 61 17 230N 25 14 63 35 54 7 25



Figure 4. Variations in (a) nitrate, (b) ammonium and (c) phosphate concentrations in groundwater of meadow (parcel 24), forest(parcels 183 and 96) and the Ill River (surface water) over the year 1995. Mean values+SE



increase in nitrate and an increase in the groundwater level. However, there was no dierence between thethree stands (median about 65 mg l1, NH4-N).

Phosphate. There was a large variation in phosphate concentrations in the Ill waters, between 90 mg l1

and 450 mg l1 (median of 174 mg l1 PO4-P). In the groundwater, we observed the same variations but at alower level, PO4-P concentrations varying between 10 mg l

1 and 180 mg l1 (median of 42 mg l1 PO4-P inparcel 96, 66 mg l1 PO4-P in parcel 183 and 73 mg l

1 PO4-P in parcel 24).Phosphate concentrations were lower under forest than under meadow (Table IV). Furthermore, we

measured a lower level of phosphate in the shallow groundwater in parcel 96, which is rich in organic matter,than in the same groundwater layer in the parcel poor in organic matter, except during episodes when thegroundwater level rose.During the period of flooding in late January and in February 1995, the PO4-P concentrations were similar

in the Ill water and the groundwater because of mixing of the waters during the flood (Figure 4).Nevertheless, there was an increase in phosphate concentrations in spring and autumn corresponding to riseof groundwater without overflowing, as in the case of nitrate, although the rise occurred later (SeptemberNovember), after a marked fall in August. These variations were higher in parcel 96 on the organic substrate.

Vertical change in N and P concentration

We compared nutrient concentrations at two levels in the groundwater, the first one in the root layerand the second one in the gravel layer at 3.54.5 m depth over the same period (December 1994June 1996).Even though fewer samples were taken in deep than in shallow groundwater, we assume that we can makethis comparison because variations in nitrate and phosphate concentrations remain very low over a wateryear in the deep groundwater (Figure 4) and median values are very similar from one year to another(Table III).In parcels 24 and 183, concentrations of ammonium nitrogen were higher in the deep groundwater of the

forest and meadow than in the shallow groundwater. In contrast the deep groundwater in the parcel on anorganic substrate (parcel 96) showed concentrations lower than those of the shallow groundwater. Thissituation was reversed for nitrate, 1.01 and 0.63 mg l1 NO3-N in the shallow groundwater and 5.08 and0.18 mg l1 NO3-N in the deep groundwater (parcel 96 and 183, respectively, Table IV). NO3-N inputs byriver flood waters were reduced by 73% in the shallow groundwater of parcel 183, and only by 37% in themeadow. In the deep groundwater of the forest and meadow parcels (183 and 24), nitrate decreased by 93%.The highest level of nitrate and the lowest level of ammonium were observed in the deep groundwater of theforest parcel rich in organic matter. Phosphate concentrations in the deep groundwater under meadow werehigher (median 63 mg l1 PO4-P) than in the other stands, where they were always very low (median 22 mg l

1

PO4-P in parcel 183 and 14 mg l1 in parcel 96) compared with the shallow groundwater (median 66 mg l1

PO4-P in parcel 183 and 42 mg l1 in parcel 96). PO4-P inputs by river flood waters were reduced by 62% in

the shallow groundwater of parcel 183, and by 76% in the meadow. In the deep groundwater, retention wasmore marked in the forest, with a reduction of 87% (parcel 183).

DISCUSSION

Processes controlling nutrient transfer

Phosphorus dynamics. Concentrations of phosphate in groundwater are uniformly low compared withthose in the surface water. They vary with the type of vegetation ( forest or meadow) and substrate (organicor inorganic). When the groundwater level rose, phosphate concentrations increased, except during theoverflowing of the Ill River, when there was a mixing of surface water and groundwater. Moreover, Pconcentrations were lower in parcel 96, where flooding did not occur. These observations could be inter-preted as the result of reducing conditions created by soil saturation, and consequently the solubilization ofmineral nutrients produced by microbial activity (Patrick et al., 1985; Holford et al., 1990; Patrick, 1990).



These authors showed that phosphate solubility increased during flood episodes and decreased when soilsbegan to dry. These alternating hydrological periods corresponded to fluctuations in oxygen level in the soil:a low level of oxygen producing modifications of redox potential and leading to the release of the ferrous ironand, consequently, of phosphate. The more reducing is the soil, the smaller is its phosphorus retentioncapacity (Masscheleyn et al., 1992) owing to the forms of Fe in flooded soils and sediments (Khalid et al.,1977). Moreover, in the alluvial calcareous soils of the study site, the high contents of calcium and claycolloids ensured a high retention of phosphate, which results from the precipitation of hydroxy-apatite forms(Faurie and Fardeau, 1990) or the adsorption of calcium-bound phosphate forms (Mansell et al., 1977;Patrick et al., 1985). Sometimes, a greater phosphorus sorption capacity was also observed when highamounts of organic matter were present in soils (Soon, 1991).The same result could be seen in the case of the Ill floodplain where we observed lower concentrations in

the groundwater of the parcel rich in organic matter, except during periods when the groundwater rose andcreated reducing conditions. However, in deep groundwater, concentrations were lower than in shallowgroundwater, probably as a result of sorption by the deep soil horizon enriched in clay, even though the O2concentrations were lower in the deep groundwater than in the shallow one (around 15% saturation and30%, respectively, Table II). The deep groundwater of parcel 183 was poorer in phosphate than parcel 24,the deep soil horizon of the former being richer in clay (Table I). So the transfer of phosphate into thegroundwater appears to result from a combination of processes, the magnitude of which probably dependson the chemical and physical characteristics of the substrate (content of calcium, organic matter, clay andmoisture). The most eective parameters for predicting variations in P retention might include the level oforganic matter or clay and variations in groundwater level (absence of overflow favours P retention).

Nitrogen removal. Although our study site is surrounded by crops enriched with fertilizers, nitrateconcentrations were low in the groundwater compared with the river water, and lower in deep groundwaterthan in superficial groundwater, except in the parcel rich in organic matter. In parallel, we observed slightlyhigher concentrations of ammonium nitrogen in deep as opposed to shallow groundwater, which suggeststhat dissimilatory reduction seems to be eective in the Illwald forest owing to the presence of an anoxichorizon in flooded soils. This process depends on the type of soil and also the drainage status of the soil, ashas been shown by Groman and Tiedje (1989) and Pinay et al. (1995). These authors showed that nitrateremoval by denitrification and plant uptake were much higher in loamy soils compared with sandy ones (lossof about 70% of the total organic nitrogen deposited during floods on a loamy riparian forest soil and 32%on the sandy one). In our study site, throughout the study period, except during the flood period, nitrateconcentrations in both deep and shallow groundwater were higher in the eastern part, which is richer inorganic matter, than in the western part of the forest. This result could be the consequence of accumulationof organic matter and a higher production of nitrate probably as a result of mineralization and nitrification.The level of dissolved oxygen in parcel 96 was a little higher than in the forest parcel 183 (40% versus 30%saturation, Table II) which could favour nitrification versus nitrate reduction at this unflooded site. Ourresults are in agreement with those of Neill (1995), who measured a higher rate of nitrification in non-floodedecosystems (marshs) but a higher net N mineralization rate (measured by an increase in ammonium) underflooded anoxic conditions, than under non-flooded oxidizing conditions.In shallow groundwater, the highest level of nitrate occurred in autumn and winter when the groundwater

level was high, which probably leads to a nitrate produced by the leaching of decomposed organic matter.Nitrate can be transferred to the groundwater if it is not retained by vegetation or reduced. The temporalvariations in nitrate often reveal a trend opposite to the change in ammonium nitrogen concentrations. Thehighest concentrations of ammonium nitrogen occurred during the summer in association with an enhanceddecomposition of organic matter, followed immediately by the lowest level over the year, subsequent to anincrease in nitrate. The alternating periods of nitrification (oxidation) and denitrification (reduction) inrelation to the hydrological cycle explains this opposing trend of nitrate and ammonium nitrogen concentra-tions (Patrick et al., 1985; Hill and Shackleton, 1989). This relationship could be more accentuated in soils



rich in organic matter with a high microbial decomposition, which provides ammonium, then nitrate, underoxidizing conditions. An increase in the groundwater oxygen level (by a factor of two for example fromAugust to September 1995) favours the nitrate form, while a decrease favours the ammonium form in areducing environment. The presence of the alder, with its capacity to fix atmospheric nitrogen, could be anexplanation for N enrichment in soil but does not explain the dierence in N concentration in both forestedparcels of the same floristic composition. The removal of nitrate is favoured by the rise in the groundwater,which allows contact with the vegetation and the microbial communities in the soil (Simmons et al., 1992).Our results suggest that variation in groundwater level controlling the oxygen level and, consequently, theredox potential, appears to be the most important factor, more so than substrate, that regulates the level ofnitrate and ammonium nitrogen in groundwater, as has also been shown by Neill (1995). Neill suggested thatthe change in the pattern of inundation can lead to rapid and significant changes in the N cycling pattern inmarshes.

Vegetation eect. Vegetation uptake is recognized as another process implicated in nitrogen reduction,even though some authors consider this process as secondary (Haycock and Burt, 1993). In the Ill flood-plain, there is a significant dierence between the nutrient level in the groundwater under a meadow and thatunder a forest on the same substrate. Similar results have been obtained by Osborne and Kovacic (1993) in atest on a riparian vegetated buer strip for use in stream water quality restoration. The forest is more eectivethan the meadow because of the larger eectiveness of the biological form (woody compared with herba-ceous) in nutrient uptake. Sanchez-Perez et al. (1991b, 1993) demonstrated the relationship between nutrientretention and the structure and floristic composition of the alluvial forest ecosystem: a more diversified andhighly structured hardwood forest being more eective than a pioneer softwood forest. Haycock and Pinay(1993) found that a poplar riparian zone retained 99% of the NO3-N input in groundwater, whereas a grassriparian zone retained 84% of the NO3-N inputs during winter, but they suggested that vegetation also hasan indirect eect on denitrification through carbon supply and root distribution patterns.The decrease in phosphorus concentrations during the growing season (July and August) could be

attributed to uptake by the vegetation. We suggest that during the growing season this process would be themajor pathway for removal of nitrate (and also phosphate) from groundwater, given that in the study site itwas the low water period. In nitrate removal, the two processes have to be involved: denitrification ordissimilative reduction in the high water period and plant uptake in the growing season, according toSimmons et al. (1992), who showed that more than 80% of nitrate removal occurs in both the dormant andgrowing season.

Parameters controlling nutrient transfer

Vegetation type and diversity, and the type of substrate, can regulate the processes responsible for nutrientretention and reduced nutrient transfer into groundwater. Even when root uptake is not possible, the rootsupplies carbon-rich material both in solid and dissolved forms to the deeper denitrifying sediments(Haycock and Pinay, 1993) and the denitrification rate is dependent on the carbon level (Schnabel et al.,1996). The level of organic matter in the soil stimulates microbial activity and the release of soluble nutrientssuch as nitrate and phosphate by decomposing organic matter. This is what is observed in the parcel withorganic matter (parcel 96) and then denitrification takes place if nitrate is supplied under reducing conditions(Jansson et al., 1994). In parallel, the soil texture can modify the production of nitrate; e.g. under the samehydrological conditions, nitrate is produced at a higher level in sandy soil than in a clayey soil (Sanchez-Perezand Tremolie`res, 1997). The clay content of soils when topsoils and subsoils are analysed together alsorepresents an important factor for the adsorption of phosphate (Soon, 1991), which probably controls thelevel of phosphate in the groundwater. In fact, the phosphate concentrations are always very low in the deepgroundwater under the reducing horizon that is rich in clay (around 50% in parcels 183 and 24). Thereducing potential would be expected to lead to solubilization of the ferrous phosphate and thus to transferof phosphate to the groundwater, but this does not occur.



The frequency, duration and periodicity of floods, and consequently the variations in groundwater level,appear to be the most important factors that influence the structure and the flooded ecosystem dynamics(Day et al., 1988). The variations in ammonium nitrogen and nitrate over the study period, irrespective of thesector, result in an alternation of processes directly related to groundwater level fluctuations nitrificationand denitrification at the interface between the saturated zone and the unsaturated one in soils (Pinay andDecamp, 1988; Sanchez-Perez and Tremolie`res, 1997) as in the sedimentriver water interface (Reddy et al.,1989). On monitoring of the nutrient cycling in a forest unflooded for 30 years in comparison with a floodedforest, Tremolie`res et al. (1998) showed that the nitrogen cycle was modified as a consequence of floodsuppression: nitrate concentrations were lower in the shallow groundwater and higher in the deepgroundwater in the unflooded forest compared with a flooded forest in the same sector (Rhine alluvialfloodplain), which confirms the importance of flooding on nutrient cycling in an alluvial ecosystem. In thiscase, the relatively short time after the suppression of floods due to the canalization of the Rhine River in19601970, seems to be sucient to give rise to observable changes in the biochemical cycle (Sanchez-Perezet al., 1993; Tremolie`res et al., 1998) as has also been shown by Huston (1980), Mitsch et al. (1991) and Neill(1995). The reducing conditions that occur during high water also control the precipitationadsorption andthe release of soluble phosphate in soils (Holdford et al., 1990). However, under the conditions of our studysite, the clay and organic matter content of the soil seems to be the main factor in regulating P retention orrelease.If we compare hydrological conditions over a period of four consecutive half-years from 1993 to 1996 in

the same ecosystems, the 1993 period is less disturbed by floods than subsequent ones in 1994 and 1995.These fluctuations in water level seem to have a direct bearing on the variations in the concentrations ofnitrate and phosphate. Thus, a rise in the groundwater level leads to the solubilization of nitrate produced inthe soil, followed by a rapid reduction of oxygen in the saturated horizon and, consequently, to theappearance of the ammonium form. During a period of low hydrological disturbance (19931994) phos-phate and nitrate are low in the shallow groundwater, whereas during a year of high disturbance (e.g. 1995)these two entities are high. However, we found the highest levels of nitrate for the whole study period ( from1993 to June 1996) during a year of very low disturbance (1996) and phosphate was at the same level as in1995. We suggest that the levels of N and P in shallow groundwater vary with the degree and duration ofdisturbance and, as a consequence, with the balance between the oxidizing period and the reducing periodand the balance of nutrient inputs and outputs. Thus, a year of very low disturbance (no flood) correspondsto a high level of nitrate and phosphate, a year of high disturbance (flood of long duration and frequentrising of groundwater) to medium N and the highest P level and a year of medium disturbance, withgroundwater rising over a variable duration and overflowing over a very short period (a few days), corres-ponds to the lowest level of P and N; these last conditions could be the best for removal of nutrients.

CONCLUSIONS

Our results suggest that the variations in nutrient concentrations in groundwater could be the result of aseries of processes, including the retention on colloids of soils, denitrificationreduction in the anoxichorizon of the soil and vegetative uptake, the role and importance of which vary with the nutrient. All theseprocesses are dependent on the groundwater level variations related to the flood ( frequency, duration andperiod), which control the eectiveness of these processes in nutrient retention and removal. As a con-sequence, the groundwater in a flooded alluvial plain is less contaminated by eutrophicating nutrients,phosphate and nitrate than are river waters. The flooded forest can protect rivers from eutrophication whenthe floodplain is located between uplands or agricultural fields and river and can be used as buer zone, ashave been shown by Correll (1996), Gilliam et al. (1996) and Uusi-Kamppa et al. (1996). The questionremains as to their eectiveness in the long term with respect to the high quantities of nitrate and phosphateinput in surface water or from adjacent agricultural fields. The results of this research suggest that regular



flooding is important for maintaining the N and P filtering of riparian zones, which provides an importantremedy for the non-point source pollution of surface and groundwaters.

ACKNOWLEDGEMENTS

We are grateful for the help of the Centre dAnalyses et de Recherches CAR, Departement Hydrologie etEnvironnement, Strasbourg, for some chemical analyses and IFARE, Institut Franco-Allemand deRecherche sur lEnvironnement (Alsace Region, CNRS, University of Strasbourg and Ministry for theEnvironment) and PNRH (French National Programme of Research in Hydrology) for financial support.

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