Evaluating the impacts of SRC willows on phosphorus export from a temperate grassland micro-catchment

Willows (Salix SPP.) are increasingly being grown as a source of biomass which can be harnessed for energy production at a commercial scale as one example of a Short Rotation Coppice (SRC) crop. What is less widely understood is the role of the plants in reducing nutrient loads when planted as a nature-based solution to mitigate diffuse pollution from agricultural lands. Strategically planted woodland and forest can play a major role in Nature Based Solutions (NBS) approaches. SRC willows were planted in a 22-ha grassland micro-catchment located on a research farm in Northern Ireland, which is regularly grazed by cattle and sheep. The micro-catchment comprises a small ditch with upstream and downstream monitoring points where a pair of autosamplers were used to collect water samples for nutrient analysis. It was possible to reconstruct a timeseries of hourly discharge in the ungauged micro-catchment using the SHETRAN hydrological model. Subsequently, the modelled flows were used to estimate daily phosphorus (P) loads at both upstream and downstream monitoring points from the measured P concentrations. The nearly four years (June 2018 – April 2022) of monitoring data allowed P loads to be calculated for pre-planting, juvenile trees and mature trees phases. A baseline scenario was run without any changes to the model; this evaluated the changes in P loads from the catchment. A further Post-Willows scenario "P–W" modified some model parameters and evapotranspiration to represent the land use change due to the willow plantation. The results from the baseline were evaluated and showed that (i) there was a statistically significant reduction in particulate P (PP) and total P (TP) concentrations at the outlet of the micro-catchment; (ii) a statistically significant reduction in PP and TP loads, indicating that the willows were reducing PP export (losses) probably by trapping fine sediment in the ditch. Results from the P–W scenario showed a smaller than expected decrease in discharge from the model results. The overall export of TP from the micro-catchment could be further decreased by additional planting of SRC willows, but the export of soluble forms of P did not decrease by a significant amount post-planting which is a recommendation for further study in mitigation programmes. The use of SRC willows in riparian buffer strips (RBS) which are a common form of NBS, has shown promising results in terms of trapping particulate forms of P. The layout of the RBS should also be carefully designed in terms of maximising the buffer strip width, here the areas planted was optimised to make best use of unproductive farmland. If the RBS can be designed to maximise the interception of runoff flowing downhill from agricultural fields then the removal rate should be even higher.