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

Abstract

Vegetation plays a key role in riparian area functioning by controlling water and nitrate (N─NO 3− ) transfers to streams. We investigated how spatial heterogeneity modifies the influence of vegetation transpiration on soil water and N─NO 3− balances in the vadose soil of a Mediterranean riparian forest. On the basis of field data, we simulated water flow and N─NO 3− 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─NO 3− 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─NO 3− in the hillslope and intermediate zones. Our climate projections highlight the importance of groundwater availability and indicate that soil N─NO 3− 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─NO 3− removal in the riparian area, increasing the risk of excess N─NO 3− leaching into the stream.