Nitrogen driving force and pressure relationships at contrasting scales: Implications for catchment management

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Journal of Environmental Management 144 (2014) 125e134

Contents lists avai

Journal of Environmental Management

journal homepage wwwelsevier comlocate jenvman

Curative vs preventive management of nitrogen transfers in ruralareas Lessons from the case of the Orgeval watershed (Seine Riverbasin France)

J Garnier a b G Billen a b G Vilain a M Benoit a P Passy a b G Tallec c J Tournebize cJ Anglade a C Billy c B Mercier a P Ansart c A Azougui a M Sebilo d C Kao e

a CNRS UMR 7619 Metis BP 123 Tour 56 Etage 4 4 Place Jussieu 75005 Paris Franceb UPMC UMR 7619 Metis BP 123 Tour 56 Etage 4 4 Place Jussieu 75005 Paris Francec IRSTEA UR laquoHydrosystemes et Bioprocedesraquo 1 rue Pierre-Gilles de Gennes CS 10030 92761 Antony Cedex Franced UPMC UMR 7618 IEES BP 120 Tour 56 Etage 4 4 Place Jussieu 75005 Paris Francee AgroParisTech Centre de Paris e 19 avenue du Maine 75732 Paris Cedex 15 France

a r t i c l e i n f o

Article historyReceived 12 November 2013Received in revised form27 April 2014Accepted 30 April 2014Available online

KeywordsNO3 pollutionDenitrificationN2O emissionsWatershed management

Corresponding author CNRS UMR 7619 Metis BPPlace Jussieu 75005 Paris France

E-mail address JosetteGarnierupmcfr (J Garnie

httpdxdoiorg101016jjenvman2014040300301-4797copy 2014 The Authors Published by Elsevier

a b s t r a c t

The Orgeval watershed (104 km2) is a long-term experimental observatory and research site repre-sentative of rural areas with intensive cereal farming of the temperate world Since the past few yearswe have been carrying out several studies on nitrate source transformation and transfer of both surfaceand groundwaters in relation with land use and agriculture practices in order to assess nitrate ethNO

3 THORNleaching contamination of aquifers denitrification processes and associated nitrous oxide (N2O) emis-sions A synthesis of these studies is presented to establish a quantitative diagnosis of nitrate contam-ination and N2O emissions at the watershed scale Taking this watershed as a practical example wecompare curative management measures such as pond introduction and preventive measures namelyconversion to organic farming practices using model simulations It is concluded that only preventivemeasures are able to reduce the NO

3 contamination level without further increasing N2O emissions aresult providing new insights for future management bringing together water-agro-ecosystemscopy 2014 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND

license (httpcreativecommonsorglicensesby-nc-nd30)

1 Introduction

In the early 20th century the invention of the Haber-Boshprocess allowing industrial production of mineral nitrogen (N)mostly used as fertilizers after World War II profoundly changedagricultural practices (Davidson et al 2012) Although agriculturalproductivity increased providing food to the growing humanpopulation the nitrogen cyclewaswidely opened leading to severeenvironmental degradation (Sutton et al 2011) The control of ni-trogen pollution is therefore a major challenge in agricultural riverbasins (Billen et al 2007 Grizzeti et al 2012) Continental watermasses (from lentic to lotic and from surface- to groundwater) areoften substantially contaminated by nitrate ethNO

3 THORN causing majorproblems for drinking water supply (Ward et al 2005) as well asfor aquatic biodiversity (James et al 2005) Moreover nitrogen

123 Tour 56-55 Etage 4 4

r)

Ltd This is an open access article u

fluxes mostly originating from diffuse sources are delivered to thecoastal zones in excess with regard to other major nutrients such assilica and phosphorus possibly participating in eutrophicationproblems caused by harmful algal blooms with damage to variouseconomic activities (fisheries tourism etc) (Cugier et al 2005Howarth et al 2011 Lancelot et al 2011 Romero et al 2012)

In many intensive agricultural areas such as the Paris Basininorganic nitrogen applied as fertilizers to arable soil exceeding theamount exported by crop harvesting are leached to surface waterand aquifers NO

3 can also be denitrified in soils and riparian zones(Haycock and Pinay 1993 Billen and Garnier 1999 Burt et al2002 Rassam et al 2008) as well as in river and pond sediments(Garnier et al 2000 Tomaszek and Czerwieniec 2000 David et al2006 Gruca-Rokosz and Tomaszek 2007 Garnier et al 2010Passy et al 2012) before ultimately reaching the coastal zone Theprocess of denitrification at every stage of the nitrogen cascadethus represents a natural mechanism of elimination of NO

3contamination re-injecting nitrogen into the pool of inert atmo-spheric di-nitrogen However during this process nitrous oxide(N2O) is produced as an intermediate which is emitted into the

nder the CC BY-NC-ND license (httpcreativecommonsorglicensesby-nc-nd30)

Fig 1 Location of the Orgeval watershed in the Seine Basin and the two sites studied

J Garnier et al Journal of Environmental Management 144 (2014) 125e134126

atmosphere particularly under suboptimal conditions of carbon (C)and nitrogen substrate concentrations (Knowles 1982 Tallec et al2006 Saggar et al 2012) A budget made at the scale of the SeineBasin showed that agricultural soils are dominant contributors ofthe overall N2O emission budget (Garnier et al 2009) N2O is apowerful greenhouse gas also contributing to the destruction ofthe stratospheric ozone layer and the increase of its emissionpossibly related to increased NO

3 use in agriculture or to remedi-ation actions aimed at eliminating NO

3 from water through deni-trification is a matter of serious concern

Whereas the application of Urban Wastewater Directive(UWWTD1991) andWater Framework Directive (WFD 2000) havealready contributed to a quite significant reduction in phosphorusload much is expected for nitrogen reduction from changes in theCommon Agricultural Policy (CAP) encouraging ldquogreeningrdquo prac-tices (EU 2013)

The small Orgeval watershed (z100 km2) is representative ofthe dominant landscape of the central Seine Basin (z76000 km2 atthe entrance of the estuary) characterized by an intensive cerealcrop belt surrounding the large Paris conurbation which hascompletely shaped its hinterland during historical periods (Billenet al 2009a 2013 Barles 2010)

The Orgeval watershed is a long-term experimental observatoryand research site initiated in the early 1960s by IRSTEA the FrenchNational Research Institute of Science and Technology for theEnvironment and Agriculture Whereas early research was mostlydedicated to the issues of hydrology and agricultural drainage withthe intensification of cereal cropping at the expense of cattlebreeding attention has been progressively paid to water qualityissues especially because the aquifers of the Orgeval watershedcontribute to the production of drinking water for the city of Paris

In this paper we present a synthesis of the long-term field andmodelling research carried out in this watershed with the aim ofmaking a diagnosis of the sources of nitrogen contamination itstransfer and transformation processes at the catchment scale Wethen explore using the GIS-based modelling approach developedfor the Seine basin (Seneque-RiverStrahler Ruelland et al 2007

Billen et al 2009b) several management options for decreasingnitrogen contamination of surface and groundwater with partic-ular emphasis on the risk of pollution swapping between waterNO

3 contamination and increased N2O emissionAlthough we use the Orgeval watershed as a practical well

documented case study in which a fully detailed modeling exercisecan be carried out the scope of the results obtained largely en-compasses this particular study site and the conclusions are ofgeneral relevance for all rural areas with intensive industrial cropfarming

2 Site studied and methods

21 Characteristics of the Orgeval watershed

The Orgeval watershed is located 70 km East of Paris (France)and is a small sub-catchment covering 104 km2 in the Marne sub-basin of the Seine River upstream from Paris (Fig 1)

The climate is semi-oceanic with annual rainfall about 700 mmand a mean annual air temperature around 10 C (varying from 06to 18 seasonally)

The Orgeval watershed is highly homogenous in terms ofpedology climate and topography (mean altitude 148 m with fewslopes except in the valleys) The Orgeval watershed is coveredwitha 10-m loess layer under which two tertiary aquifer formations areseparated by a discontinuous grey clay layer (Megnien 1979) Theshallowest aquifer of the Brie Limestone Oligocene formation withmore interactions with surface waters has a relatively shorterwater residence than the deepest Champigny Limestone Eoceneaquifer The lower layer of the surface loess cover is enriched withclay resulting in waterlogged soils in the winter For this reason upto 90 of the arable soils of the Orgeval watershed have beenartificially tile-drained since the early 1960s Land use is mostlyagricultural land (82) dominated by cereal crops (wheat maizebarley and pea) with conventional practices mainly based onmineral nitrogen fertilization The remaining surface is covered bywoods (17) and urban zones or roads (1 of the surface) (Fig 1)

J Garnier et al Journal of Environmental Management 144 (2014) 125e134 127

22 Sampling and field studies lab experiments and chemicalanalysis

Within the Orgeval watershed series of nitrogenmeasurements(mainly nitrate as well as dissolved N2O) have been carried out atleast since 2005 on surface waters Two specific sites have beenequipped (Site 1 since 2007 Site 2 since 2011) for water table NO

3and N2O dissolved concentration and for N2O emissions fromagricultural soils A farm drainage pond was also sampled

221 Surface waterNO

3 concentrations were weekly measured since 1975 at theMelarchez station (order 1) and since 2005 at the outlet of theAvenelles sub-watershed and the Orgeval one (Le Theil station) inthe framework of IRSTEA routine programme Dissolved N2O insurface water have been measured from 2006 to 2008 at monthlyintervals at the same three sampling stations (partly in Vilain et al2010 2012c) (Fig 1)

222 Water tableOn site 1 (Fig 1) three piezometers were installed along a slope

from the plateau to the riparian zone in January 2007 This 6inclination slope oriented northwestward reaches the AvenellesRiver This site is typical of the whole Orgeval watershed both interms of agricultural practices (grain crop with wheat barley andmaize as the main rotation) and fertilizer applications (from 120 to160 kg N ha1 for wheatbarley to 180 kg N ha1 for maize) Threepiezometers were also installed in July 2011 in site 2 The pie-zometers were sampled for NO

3 and N2O determination in the Brieaquifer since their installation

223 Agricultural soilsSuction ceramic cups were also installed on site 1 (Fig 1) during

twowinter drainage periods (January toMarch 2010 and December2012 to April 2013) to quantify the sub-root NO

3 concentrations fora conventional agricultural system Other datawere obtained at site2 (in the winters 2012 and 2013) for an organic agricultural systemand are used for the characterisation of organic agriculture sce-narios (see below)

On site 1 along the piezometric slope hermetically closedchambers (open bases measuring 50 50 30 cm) allowedquantifying N2O emissions (see Vilain et al 2010) from croppingsoil according to the methodology described by Hutchinson andLivingston (1993) and Livingston and Hutchinson (1995) Mea-surements were taken at different topographical landscape posi-tions from the uphill to the riparian position fromMay 2008 to July2009 a forested soil was investigated for comparison d15N-isotopicmeasurements in the soil organic matter were taken along twotransects at six different locations on one occasion in March 2007(Billy et al 2011) For each transect soil was sampled at 10-cmintervals from the surface to 90 cm deep Air-dried and sieved(2 mm) the soil samples were homogenized prior to organic Nisotopic composition analysis These measurements were used asan integrated estimator of long-term soil denitrification processes

To pursue the determination of the source of N2O emissions ingreater detail soils sampled between 2009 and 2011 at severalperiods of the season from the same site 1 cropped slope wereincubated in batch experiments under optimal laboratory condi-tions (nutrients temperature) Since N2O is known to originatefrom nitrification and denitrification both processes were investi-gated As described in Garnier et al (2010) and Vilain et al (2012b)batch experiments were run and the NO

3 NO2 NH

thorn4 concentra-

tions followed during a short incubation time (4e6 h) to avoid anyconfinement in the flasks in triplicate and in the dark For nitrifi-cation assays ammonium was added and the flasks were flushed

with ambient air to ensure aerobic conditions while for denitrifi-cation assays NO

3 was added and the flask was flushed with N2 inorder to produce anaerobic conditions Production of N2O associ-ated with the processes was also measured

224 Farm drainage pondA drainage farm pond on site 2 (Fig1) was also investigated over

3 years for NO3 concentrations (2007e2010) in order to evaluate

the ponds potential for eliminating nitrogen leached from agri-culture (Passy et al 2012) N2O concentrations dissolved in thewater column were determined seasonally in 2010 allowing toestimate emissions (Garnier et al 2009)

225 Analytical methodsAnalytical methods for NO

3 and N2O concentrations in waterare described in Jones (1984) and Garnier et al (2009) respectively

N2O concentrations in gas sample were analysed by gas chro-matography as described by Vilain et al (2010)

Measurement of organic N isotopic composition of the soil isdescribed by Billy et al (2010)

23 Simulating N reduction measures

The biogeochemical model (RiverStrahler) describing theecological functioning of aquatic systems (Billen et al 1994Garnier et al 2002 currently implemented at the scale of theSeine Basin embedded in the GIS-Seneque interface tool (Ruellandet al 2007 Thieu et al 2009 Passy et al 2013) has been used herefor exploring scenarios of mitigating measures at the scale of theOrgeval watershed The principle of the model is illustrated inFig 2

3 Quantifying the N cascade through the Orgeval watershed

31 N leaching from agricultural soils to sub-root water tile-drainsand aquifers

Wheat maize pea and barley cover around 44 14 6 and 4respectively of the cultivated area in the Orgeval watershed (RGA-Recensement General Agricole 2000) The main crop rotations arewheat-pea-wheat (28) and maizeewinter wheatespring barley(20) with a mean crop yield of about 5500 kg cereal equivalentper ha corresponding to about 100 kg N ha1 yr1 The fertilizerapplication rate ranges from 120 to 180 kgN ha1 yr1 Atmosphericdeposition of N adds around 15 kg N ha1 yr1and atmospheric N2fixation (through non-symbiotic fixation and by legume crops insome rotations) about 10 kg N ha1 yr1 (Billy et al 2010) The soilN balance thus reveals a long-term surplus of about50 kg N ha1 yr1

Sub-root concentrations measured from 2010 to 2013 withsuction cups installed 1 m deep under representative arable plotsaverage 22 mg NO

3 N L1 (SD frac14 15) This value is close to theaverage concentration observed in tile drains in the same area(26 mg NO

3 N L1) (Fig 3) These sub-root concentrations are quitesimilar to those observed elsewhere in the Seine Basin in the 1990sIndeed in the chalky Champagne East of Paris the concentrationsobtained were 272 mg NO

3 N L1 for a 10-year wheatbeet rotationbut significantly less with the introduction of alfalfa in the rotation(208 mg NO

3 N L1) (Beaudoin et al 1992) Similar figures werefound in the Northern or Western sectors of the Seine Basin ierespectively 19 mg NO3eN L1 (Machet and Mary 1990) and29 mg NO3eN L1 (Arlot and Zimmer 1990)

With an average discharge of 036 m3 s1 at the outlet of theOrgeval watershed a yearly N leached flux can be estimated to2400 kg km2 yr1 (50 variation)

Fig 2 Representation of the SenequeRiverStrahler model

J Garnier et al Journal of Environmental Management 144 (2014) 125e134128

NO3 concentrations in the Brie aquifer measured from samples

collected in the piezometers installed uphill are around 132 mgNO3eN L1 Samples collected midslope or below the riparianbuffer strip show 35e40 lower concentration down to86 mg NO3eN L1 (Fig 3) probably because of denitrificationprocesses occurring when the water table reaches the bio-geochemically active upper soil layers In the pond studied theaverage annual concentration was even lower (7 mg NO3eN L1)compared to the average concentration entering the pond(135 mg NO3eN L1) At the outlet of the Orgeval watershed theaverage river water concentration is 11 mg NO3eN L1

32 Denitrification and N2O emissions in soils along a croppedslope

Both nitrification and denitrification in soil are able to producethe greenhouse gas N2O particularly under suboptimal conditions(limitation by substrates oxygen tension pH temperature etc)

Fig 3 Concentrations of nitrate cascading within the Orgeval

(Firestone and Davidson 1989) although several other microbialprocesses are able to consume the N2O emitted (eg nitrifierdenitrification (Wrage et al 2001) dissimilatory NO

3 reduction toammonia (Burgin and Halminton 2007) anammox in specificconditions (Dalsgaard et al 2005 2013)

In the same line as the research onwastewater treatment plants(Tallec et al 2006) the relative magnitude of nitrification ordenitrification in the emission of N2O was experimentally exploredin Orgeval watershed soil samples (Vilain et al 2012b c 2014) Itappeared that potential rates of NO

3 production (nitrification) andNO

3 reduction (denitrification) were on average within the samerange (08e09 mg NO3eN g1 dw h1) but the associated potentialN2O productionwasmuch lower (by a factor of 100) for nitrificationthan denitrification (Table 1) corroborating previous findings byTallec et al (2006) The ratio of N2O production to NO3 reductionwas up to 20 for the denitrification potential while the ratio ofN2O emission to NO3 production by nitrification was only about02

watershed (see text for explanations unit in mg N L1)

Table 1Average potential values for agricultural soils in denitrification and nitrification in experimental conditions (batch experiments at 20 C) and associated N2O production (SD forStandard Deviation 7 experiments) Percentages of N2O production are also given for comparison

Potential NO3 productionreduction rates Potential N2O production rates Ratios of potential N2ONO3 rates

mgNO3 eN g1 dw h1 mgN2OeN g1 dw h1

Denitrification 089 (SD frac14 047) 015 (SD frac14 008) 244 (SD frac14 207)Nitrification 081 (SD frac14 0271) 0002 (SD frac14 0001) 018 (SD frac14 016)

J Garnier et al Journal of Environmental Management 144 (2014) 125e134 129

Direct in situ measurements of N2O emissions by agriculturaland forest soil using closed chambers were taken on 21 dates fromMay 2008 to August 2009 (Vilain et al 2010 2012c) For uphillplateau sites a value equalling 029 mg N2OeN m2 d1 was esti-mated for cropland higher than the average one found for forestedsoils 015 N2OeN m2 d1

Higher values close to 041 mg N2OeN m2 d1 were measuredin downslope sites with the level of the water table closer to thesoil surface N2O emissions averaged for footslope and riparianzone was 061 mg N2OeN m2 d1 (Fig 4a) These results showincreasing transformation of nitrogen (denitrification mainly)along the slope and concomitant increasing N2O emission

d15N fractionation values of soil organic nitrogen along a crop-ped slope and averaged over a 1-m soil profile were higher thanthe primary nitrogen (N) sources fromwhich they are derived suchas mineral nitrogen fertilizers atmospheric deposition and

Fig 4 a Seasonal average of N2O emission from soils in a forested area and an agri-cultural slope redrawn from Vilain et al (2010) b Variations of d15N of nitrogenorganic matter averaged over a 1-m soil profile recalculated from Billy et al (2010) cSeasonal averages of NO3eN concentrations in the water of the Brie aquifer as sampledin the piezometers along the slope modified from Vilain et al (2012a)

symbiotic N2 (all characterized by d15N values close to zero) indi-cate indeed the existence of a long-term denitrification process(Billy et al 2010 Vitousek et al 2013) Based on a modellingapproach of the isotopic composition of the soil N compartmentBilly et al (2010) estimated that a 1permil d15N-Norg increase abovethat of the primary N sources corresponds to a denitrification of~10 kg N ha1 yr1 (ie 27 mg N m2 d1) which confirm theprevalence of denitrification

The distribution of d15N of the bulk soil N pool from the uphillplateau down to the riparian zone of the river shows a regular in-crease from 24permil in plateau forested soils and 58permil in crop soil to74permil in the downslope arable soil and in the buffer strip resultswell in agreement with N2O emission from denitrification (Fig 4b)

N2O concentration in the aquifer was also measured by sam-pling the piezometers The values found were largely over-saturated (20 mg N2OeN L1 on average) taking into account thatN2O saturation in water with respect to the atmospheric level of330 ppb varies from 035 to 05 mg N2OeN L1 depending on thetemperature (Fig 4c) We interpreted these high N2O values in theaquifer as resulting from leaching from the root zone althoughdenitrification and N2O production in the aquifer itself is not fullyexcluded critical oxygenation around 2e3 mg O2 L1 being occa-sionally observed (Vilain et al 2012a) The lower N2O concentra-tions in the downslope sites can be explained by microbialtransformation into N2 ie again corroborating a complete deni-trification along the slope N2O degassing from the aquifer alongthe underground flow ie indirect N2O emissions is not excluded

33 In-stream N elimination processes

Direct measurement with bell-jars allowed estimating the rateof benthic denitrification in river sediments Consumption rates onthe order of 31 (SD frac14 11) mg N m2 h1 were observed(Thouvenot-Korppoo et al 2009 Billy et al 2011) Considering ariver bottom area of about 175830 m2 for the Orgeval watershed asawhole this leads to a maximum estimate of 3000e6000 kg N yr1

for benthic denitrification (30e60 kg N km2 yr1 at the watershedscale) showing that in-stream processes represent a marginalvalue in the nitrogen elimination of the 2400 kg N km2 yr1 foundat the base of the root zone

Accordingly N2O concentrations above saturation observed insmall rivers of the Orgeval watershed are inherited from thegroundwater feeding them instead of being produced through in-stream processes Indeed these concentrations rapidly decreasefrom the spring downwards until reaching saturation (Garnieret al 2009)

34 A synthetic budget of N transfers in the Orgeval watershed

Based on the data summarized in the above paragraphs atentative budget of nitrogen transfer at the scale of the Orgevalwatershed was established (Fig 5) describing the fate of NO

3mostly coming from the surplus nitrogen left by agricultural soilsDenitrification in the soil profile and in the downslope areas (wherea temporarily or permanently shallowwater table comes in contact

Fig 5 Summarizing budget of nitrate transfer and transformation and associatednitrous oxide emissions in the Orgeval watershed Calculations are based on theaverage hydrology from 2006 to 2012 a) Current situation based on measurements b)pond reintroduction scenario c) organic farming scenario

J Garnier et al Journal of Environmental Management 144 (2014) 125e134130

with the upper biogeochemically active layers of the soil) elimi-nates more than 40 of the nitrogen leaving the root zone

The various denitrification figures in this budget are in goodagreement with the values found (i) for soil denitrification (Pinayet al 1993 Hefting et al 2006) (ii) for the riparian zones (Billenand Garnier 1999) and (iii) for in-stream benthic denitrification

at the scale of the whole Seine hydrographic network (Thouvenot-Korppoo et al 2009)

On the basis of (i) the N2O emissions from soils together with afine resolution of the topography and land use in the watershed (ii)the N2O fluxes from rivers and groundwater deduced from con-centration measurements (Garnier et al 2009 Vilain et al 20102012a) the total N2O emissions for the whole Orgeval watershedwere estimated at 142 kg N2OeN km2 yr1 (Vilain et al 2012c)This represents about 10 of the sum of the denitrification ratesoccurring in soils footslopes and riparian zones and in-streamsediments (see Fig 5a) This N2O percentage emission is in agree-ment (within a factor of 2) with the potential values found exper-imentally for denitrification

4 Curative management measures to reduce NO3

contamination

Drainage or irrigation water retention ponds are often seen asbuffer interfaces where N elimination is effective The creation ofsuch systems is often considered within the framework ofcompensatory measures possibly included in the wetland status(Dahl 2011) In addition these waterbodies can be viewed asanthropogenic refuge for biodiversity (Chester and Robson 2013)

41 NO3 and N2O concentrations in an artificial pond

We investigated such a pond established at the outlet of a tiledrain collector draining 35 ha of cultivated land Its surface area is3700 m2 with a volume of 8000 m3 (ie a mean depth of about2 m) The concentrations at the entrance of the pond averaged135 mg NO3eN L1 (Fig 6a) over the period studied close to thevalue found for the concentration in the Brie aquifer (see Fig 3)NO

3 concentrations in the pond show a systematic summerdecrease down to 15 mg NO3eN L1 in late summer (annual mean7 mg NO3eN L1)

These values are accurately reproduced by a simplified model ofstagnant water (Garnier and Billen 1993 Garnier et al 2000 seealso Passy et al 2012) (Fig 6a)

Regarding N2O concentrations the values averaged 38 mgN2OeN L1 ie a tenfold over-saturation (with extreme concen-trations of 84 and 11 mg N2OeN L1 for a data series in 2010n frac14 14) Based on the saturation concentration (Weiss and Price1980) and the gas transfer coefficient of 04 m h1 (Wanninkhof1992 Borges et al 2004) the annual mean N2O emissions at thepond surface can be estimated at 34 mg N2OeN m2 d1 a valuesimilar to the emission at the cropped downslope (see Fig 4)

The observed decrease in NO3 concentrations in the pond

during the period of high biological activity suggests that suchponds could effectively be used as curative management in-frastructures for NO

3 reduction in surface water However theconcomitant outgassing of N2O represents a serious limitation as itcan result in the simple swapping from one type of pollution toanother

42 Simulation of the effect of pond creation at the scale of theOrgeval watershed

Interestingly historical maps of the Orgeval area (eg the so-called Cassini map dating back to the middle of the 18th century)reveal that the traditional landscape of the Brie region was char-acterized by a large number of ponds established on the headwa-ters both for driving mills and for pisciculture In the Orgevalwatershed the number of ponds was in the range of 60 and theirsurface area amounted to 1 of the total surface area of the

Fig 6 a Interannual NO3eN concentrations in a drainage pond in the Orgevalwatershed Dotted line NO3eN concentration at the entrance solid line simulatedNO3eN concentrations in the pond black dots are the measured NO3eN concentra-tions b Simulated N fluxes at the outlet of the Orgeval watershed with a range ofsurface area of ponds (from the reference situation to 10 of the total surface area ofthe Orgeval watershed) c Associated N abatement is shown in comparison (recalcu-lated from Passy et al 2012)

J Garnier et al Journal of Environmental Management 144 (2014) 125e134 131

watershed (Passy et al 2012) Most of these ponds were dried andconverted to cropland during the first half of the 19th century

In order to explore the role of pond implementation in theOrgeval watershed as a measure to reduce the nitric contaminationof surface water the SenequeRiverStrahler model (Ruelland et al2007 Thieu et al 2009 Passy et al 2013) was run and connecteddrainage ponds were virtually introduced at different surface areas(Passy et al 2012) The results showed that a 34 and 47 reduc-tion of the N flux at the outlet of the Orgeval watershed can beexpected with a total surface area of ponds equalling 5 and 10 ofthewatershed respectively compared to 9 abatementwith the 1pond coverage of the Cassini map (Fig 6b c) Reintroducing pondsin the landscape necessarily increases the residence time of thewater masses increases the primary production providing morecarbon for denitrification for example However although possiblya refuge for biodiversity eg for fish to feed and spawn a shift fromlotic to lentic species can be damageable

Whereas the process of denitrification could be used for miti-gation measures in combatting nitric contamination in the hydro-systems by creating or restoring wetlands cautionmust be taken to

Fig 7 Long-term chronicle of observed NO3eN concentrations in the

limit a shift from nitric to N2O pollution Considering the N2Oemitted in the experimental pond studied an increase of the N2Oemission to about 60 kg N2OeN km2 yr1 by the Orgeval catch-ment could be expected in the case of 5 pond area close to theemission by agricultural soils (see Fig 5b) However due to con-tradictory results (cf Welti et al 2012) a comprehensive assess-ment of ecosystem services and disservices in agriculturallandscapes remains a challenge (Burgin et al 2013)

5 Preventive management measures to reduce nitrogencontamination

51 Good Agricultural Practices

Good Agricultural Practices consisting in lowering and frac-tionation of N fertilization return of crop residues to the soil andintroduction of catch crops were promoted in the 1990s Whencorrectly applied these measures are able to significantly reduce Nleaching (Beaudoin et al 2005) The long-term chronicle of NO

3concentrations in a headwater stream of the Orgeval watershedavailable since 1976 from IRSTEA however shows that NO

3 con-centration has only levelled off in the 1990s to 97 mg NO3eN L1

on average and reached 109 mg NO3eN L1 in the 2000s (Fig 7)No trend toward a reduction is in fact observed for the Orgevalcatchment It appears that the current agricultural practicesalthough they involve careful calculation of the nitrogen fertiliza-tion with respect to the requirement of crop growth during thevegetative period are not able to further reduce the nitrogen sur-plus which is leached during the winter period Alternative agri-cultural systems are therefore probably required for reducing NO

3leaching

52 Organic farming

A few farms in the Orgeval watershed have been converted toorganic farming practices These farms use long crop rotations(8 yrs) established on small plots (lt10 ha) starting with 2 or 3years of alfalfa then alternating cereals and legumes (peas or horsebean) External inputs of organic nitrogen partly in the form ofcomposted manure are extremely limited Although the cerealyield of these exploitations is about 15e20 lower than the con-ventional yield their overall nitrogen surplus is much lower Pre-liminary measurements (Benoit et al unpublished) of sub-rootNO

3 concentrations measured with suction cups under thedifferent plots of one such farm (site 2 Fig 1) shows values of about134 mg NO3eN L1 (SD frac14 48) ie about half the value found forconventional farming Note that the value found is higher than therange of the values reported by Thieu et al (2011) for organicfarming based on literature data

Melarchez River a headwater stream in the Orgeval watershed

Fig 8 Seasonal variations of NO3eN concentrations at the outlet of the Orgevalwatershed the year 2006 taken as an example Rather good agreement is obtainedbetween the observations and the simulation for 2006 Compared to the referencesimulation the organic agricultural scenario shows a 45 decrease in annual meannitrate concentrations (Org Agri mean) The amplitude of the response is shownwiththe exploration of the SD range (Org Agri min and max)

J Garnier et al Journal of Environmental Management 144 (2014) 125e134132

53 Modelling NO3 contamination resulting from GAP and

generalized organic farming

The SenequeRiverStrahler model has been run for exploring theeffect of changes in agricultural practices at the scale of the Orgevalwatershed The current situation modelled by considering a meansub-root water concentration of 22 mg NO3eN L1 under arableland was compared with that corresponding to a concentration of134 mg NO3eN L1 (SD frac14 48) (organic farm see above) Anaverage decrease of 45 (25e68) of the annual nitrogen concen-trations at the outlet of the watershed is obtained (Fig 8) Such apreventive measure would not increase N2O emissions a resultcorroborated by our own experimental measurements in theOrgeval watershed (Benoit et al unpublished) and could evenreduce them (Aguilera et al 2013) Fig 5c compares the implicationof this preventive scenario to the curative one (Fig 5b) and thecurrent situation (Fig 5a)

6 Discussion and Conclusions

The introduction of reactive nitrogen into the biosphere bymodern agriculture has drastically increased and the sequence ofeffects it causes in the atmosphere in terrestrial ecosystems infreshwater and marine systems and on human health is known asthe nitrogen cascade (Galloway et al 2003) In a river networkwitha continuous unidirectional transport of water and elements the Ncascade superimposed on the N spiraling a concept defined as thetravel distance of a water N atom before returning to the waterdownstream (Howard-Williams 1985)

A front-line question for the near future is Can we changeagricultural practices to re-equilibrate the nutrient stoichiometry ofsurface water preventing eutrophication and still satisfy the needsof the population (in food and drinking water) with sustainableagriculture Considering that more than 50 of terrestrial reactivenitrogen is now from Haber-Bosch mineral nitrogen lsquoindustrialproductionrsquo (mostly in the food system or a consequence of it) toovercome environmental problems of N pollution in the next 50years suggestions for future research should focus on new ap-proaches for analysing water-agro-food systems (Billen et al 2013)based on the concepts of socio-ecological trajectory (Fischer-Kowalski and Rotmans 2009) and territorial ecology (Barles2013) The territorial watershed scale would be a suitable scale toinitiate new directions in agricultural systems Many discussions

are converging to request a tightening of the feedback loop be-tween production and consumption so as to achieve sustainability(Sundkvist et al 2001 Davis et al 2012) A political consensus onthis matter is very difficult to achieve (Leridon and De Marsily2011 Swinnen and Squicciarini 2012) but the regional scale al-lows a good level of coherence for decision and management ie alevel at which implementation of measures appears relativelypossible

The Orgeval watershed is nowadays one of the long-surveyedwatershed case study areas that has been subjected to biogeo-chemical investigations in addition to the 50 years of study in hy-drology The facilities offered for monitoring have made it possibleto determine a comprehensive budget of nitrogen transfer andtransformations at the scale of this territory Specific nitrogenfluxes delivered at the outlet of the Orgeval watershed has beenestimated at 1130 kg N km2 yr1 and is on the order of thatdelivered at the outlet of the Seine Basin as a whole(1600 kg N km2 yr1 for the 2002e2007 period see Passy et al2013) A similar observation can be made for the N2O emissionz140 kg N2OeN km2 yr1 for the Orgeval watershed compared tothe 180 kg N2OeN km2 yr1 obtained at the scale of the Seinewatershed (Garnier et al 2009)

The studies conducted in the Orgeval watershed reveal thatdenitrification mostly in waterlogged soils in slope shoulders andriparian zones is a major process for nitrogen elimination along itscascade from agricultural soil to the river outlet already reducingthe fluxes of leached nitrogen between the base of the root zoneand their discharge into the river system by 40e50 (see Fig 3)Globally at least 10 of the total denitrification flux ends asgreenhouse gas N2O emissions

Among the measures which can be envisaged to further reducenitrogen contamination of surface water the creation of shallowponds can be valuable especially in many traditional landscapeswhich were once characterized by numerous ponds Historical landuse situations are indeed recognised useful for planning measuresto achieve environmental targets (Glavan et al 2013) Many au-thors have stressed the value of such landscape managementespecially when other ecological functions can be associated suchas conservation of the biodiversity connectivity in the landscapeetc (Ruggerio et al 2008 Le Viol et al 2012 Armitage et al 2012)However ponds often promoted as compensation measures oreven for wastewater management (Howard-Williams 1985)should not be implemented excessively or inconsistently theconnectivity of pond networks should be considered at the terri-torial landscape scale so that they remain favorable to biodiversityBronner et al (2013) for instance report that in the US the policy ofenvironmental compensation measures has led to a strongdecrease of high-quality forested wetlands at the expense of low-quality wetland area such as many isolated freshwater pondsUsing the SenequeRiverStrahler model we have shown that a30e40 reduction of NO

3 at the outlet of the watershed could beobtained by introducing drainage ponds up to 5 of the total sur-face area of the watershed However this would increase N2Oemissions by about 50

A more effective preventive reduction measure would be theconversion of agriculture to organic farming practices with lowfertilization which has been shown to allow significant reductionof NO

3 concentration at the base of the root zone with respect tocurrent conventional practices This type of measure not only re-duces nitrogen contamination at the source thus also acting ongroundwater contamination but is the only one which allowsreducing instead of increasing overall N2O emissions by thewatershed The generalization of organic farming which requireslocal supply in organic manure as well as an outlet for its fodderproduction would be facilitated by the reintroduction of livestock

J Garnier et al Journal of Environmental Management 144 (2014) 125e134 133

farming in this specialized cereal cropping area Clearly meetingthe objectives of the Water Framework Directive requires deepstructural changes in the agriculture towards more sustainable andefficient systems (EU 2013) rather than simple adjustments offarming practices (Volk et al 2009 Glavan et al 2012)

The combination of local studies together with an adaptedmodelling tool has proved here to be a relevant approach forquantifying nitrogen transformations and transfers at the water-shed scale even allowing the exploration of mitigation measuresprior to field applications of ecological engineering investigationsAlthough several other process-based models might have beenused (eg SWAT Arnold et al 1998 Neitsch et al 2005 INCAWhitehead et al 1998 Wade et al 2002) SenequeRiverStrahlerwas preferably used here especially because it is currently used bythe Seine Water Agency for WFD reporting Other models based onregression approaches (eg GREEN Grizzetti et al 2005 MONERISBehrendt et al 2002 NEWS-DIN Dumont et al 2005) would nothave been able to explore scenarios like those tested here becausethey would be too far from the calibrating data sets

Acknowledgements

The FIRE-FR3020 research federation is greatly acknowledgedfor its interdisciplinary research framework and for funding thesites equipment We extend our thanks to the PIREN-Seine pro-gram for providing funding for the analysis Franccedilois Gilloots andEric Gobard are sincerely acknowledged for having allowed us toconduct this research in their fields Thanks are due to the IRSTEAresearch institution for opening their experimental watershed(Orgeval watershed) to other scientific communities This workwaspartly carried out in the scope of the DIM-ASTREA amp AESN-ABACANR-ESCAPADE and ADEME-EFEMAIR projects

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