Hedgerows reduce nitrate flux at hillslope and catchment scales via root uptake and secondary effects

ISI Document Delivery No.: GR5VETimes Cited: 0Cited Reference Count: 51Thomas, Zahra Abbott, Benjamin W.DHI; European Union's Seventh Framework Program for research, technological development and demonstration [607150]; French EC2CO grant "Caracterisation hydrologique et biogeochimique de la denitrification dans les paysages"The authors thank J.C. Marie and Yannick Hamon for technical support. The fieldwork was carried out with in-house funding. DHI sponsored the license the modeling software FEFLOW. The authors were supported by the European Union's Seventh Framework Program for research, technological development and demonstration under grant agreement no. 607150 (FP7-PEOPLE-2013-ITN - INTERFACES Ecohydrological interfaces as critical hotspots for transformations of ecosystem exchange fluxes and biogeochemical cycling) and by the French EC2CO grant "Caracterisation hydrologique et biogeochimique de la denitrification dans les paysages."Elsevier science bvAmsterdam

English. Agricultural contamination of groundwater with nitrate (NO3-) is one of the most widespread and pressing environmental issues. The preservation and planting of hedgerows around agricultural fields can reduce NO3- flux, but the efficacy of hedgerows depends on the amount of NO3- in soil and groundwater, hydrological flowpath and timing, and biogeochemical conditions surrounding and below roots. Quantifying these parameters is a major challenge, usually requiring involved and destructive fieldwork. Here, we present a new analytical method to characterize NO3- stratification using water chemistry sampled during piezometer slug tests. We tested this method with a network of wells in a hillslope intersected by an oak hedgerow during high- and low-water conditions, respectively spring and autumn. We found that hedgerows had a strong seasonal effect on near-surface NO3- dynamics in the proximity of the root system, reducing annual hillslope-level fluxes by 26 to 63%, comparable to NO3- removal from cover crop techniques. Hedgerow root uptake accounted for two-thirds of this reduction, with the remaining third attributable to secondary effects, potentially hedgerow-induced microbial retention or denitrification due to increased organic carbon and heterogeneous redox conditions in the rooting zone. However, a simple scaling exercise suggested that at the catchment level, hedgerow NO3- removal has a smaller effect (ca 1-10% reduction of annual flux), due to the large legacy of NO3- in the aquifer from past fertilizer application. These results suggest that while hedgerows cannot immediately solve problems of past groundwater contamination, protection and reestablishment of hedgerow networks could substantially accelerate recovery of groundwater quality on decadal timescales.