Zahra Thomas

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

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.

Riparian forest transpiration under the current and projected Mediterranean climate: Effects on soil water and nitrate uptake: Riparian forest transpiration effects on soil water and nitrate uptake

Vegetation plays a key role in riparian area functioning by controlling water and nitrate (N─NO3−) transfers to streams. We investigated how spatial heterogeneity modifies the influence of vegetation transpiration on soil water and N─NO3− balances in the vadose soil of a Mediterranean riparian forest. On the basis of field data, we simulated water flow and N─NO3− transport in three riparian zones (i.e., near‐stream, intermediate, and hillslope) using HYDRUS‐1D model. We investigated spatiotemporal patterns across the riparian area over a 3‐year period and future years using an IPCC/CMIP5 climate projection for the Mediterranean region. Potential evapotranspiration was partitioned between evaporation and transpiration to estimate transpiration rates at the area. Denitrification in the forest was negligible, thus N─NO3− removal was only considered through plant uptake. For the three riparian zones, the model successfully predicted field soil moisture (θ). The near‐stream zone exchanged larger volumes of water and supported higher θ and transpiration rates (666 ± 75 mm) than the other two riparian zones. Total water fluxes, θ, and transpiration rates decreased near the intermediate (536 ± 46 mm transpired) and hillslope zones (406 ± 26 mm transpired), suggesting that water availability was restricted due to deeper groundwater. Transpiration strongly decreased θ and soil N─NO3− in the hillslope and intermediate zones. Our climate projections highlight the importance of groundwater availability and indicate that soil N─NO3− would be expected to increase due to changes in plant‐root uptake. Lower water availability in the hillslope zone may reduce the effectiveness of N─NO3− removal in the riparian area, increasing the risk of excess N─NO3− leaching into the stream.