Effect of climate, intra and inter-annual variability, on nutrients emission (C,N, P) in stream water: lessons from an agricultural long term observatory of the temperate zone

International audienceAgriculture greatly contributes to modify C, N and P cycles, particularly in animal breeding regions due to highinputs. Climatic conditions, intra and inter-annual variabilities, modify nutrient stream water emissions, acting intime on transfer and transformation, accumulation and mobilization processes, connecting and disconnecting intime different compartments (soil, riparian areas, groundwater). In agricultural catchments, nutrient perturbationsare dominated by agricultural land use, and decoupling human activities and climate effects is far from easy.Climate change generally appears as a secondary driver compared to land use. If studied, generally only onenutrient is considered. Only long term, high frequency and multiple element data series can decouple these twodrivers.The Kervidy-Naizin watershed belongs to the AgrHyS environmental research observatory(http://www6.inra.fr/ore_agrhys_eng), itself included in RBV (French catchment network of the CZO). Onthis catchment, 6 years of daily data on DOC, NO3, SRP, TP concentrations allow us to analyze the effect ofseasonal and inter-annual climatic variabilities on water quality (C, N, P). Different papers have been publishedon the effect of climate on nitrate (Molenat et al, 2008), SRP and TP (Dupas et al, 2015) and DOC (Humbert et al,2015). We will present first results comparing the effect of climate on these three major solute forms of C, N andP. While C and P dynamics are very close and controlled by fluctuation of water table downslope, i.e. in riparianareas, mobilizing C and P in time, nitrate dynamics is controlled by GW dynamics upslope acting as the major Nreservoir. As example, the dryness conditions in summer appears a key factor of the C and P emissions in autumn.All the three solute forms interact when anoxic conditions are observed in riparian zones. These basic processesexplain how climatic variability can influence and explain interactions between C, N and P emissions in streamwater.These results underline three major lack in most of our observatories: high frequency data as flood event areimportant for C and P emissions; multiple element approach, as very few observatories have currently C, N andP, their solute and particulate forms; climate but also soil wetness, GW fluctuations explaining biotransformationand connection between reservoirs on catchments, so that linking hydrological and biogeochimical condition isnecessary to explain export. These lacks of observations is a barrier to develop process based models assessingand predicting the effect of climate on water quality.ReferencesDupas R., Gruau G., Sen Gu, Humbert G., Jaffrezic A., Gascuel-Odoux C., 2015. Groundwater control ofbiogeochemical processes causing phosphorus release from riparian wetlands. Water Research 84, 307-314Humbert G., Jaffrezic A., Fovet O., Gruau G., Durand P., 2015. Dry-season length and runoff control annualvariability in stream DOC dynamics in a small, shallow groundwater-dominated agricultural watershed. WaterResources Research.Molenat J., Gascuel-Odoux C., Ruiz L., Gruau G., 2008. Role of water table dynamics on stream nitrate exportand concentration in agricultural headwater. Journal of Hydrology 348, 363– 37