Sediment detachment by raindrop impact on grassland and arable fields: an investigation of controls

Purpose Modern agricultural practices have increased the losses of fine sediment and associated pollutants to watercourses with associated off-site degradation of water quality and aquatic biodiversity. Despite significant investment into agri-environment initiatives which aim to reduce these losses, limited empirical mechanistic evidence exists for the efficacy of many on-farm interventions. As the most likely mechanism of particle detachment in many landscapes, understanding the controls for rainsplash erosion is key to generating this mechanistic understanding. Methods Soil properties were compared to rainsplash erosion rates on a grassland and an arable field in Southwest England. Soil cores were retrieved and measured for dry density, aggregate stability, vegetation cover, and loss on ignition. A rainsplash erosion trap consisting of a plastic funnel containing filter paper was placed into the hole left by each core, and the mass of sediment trapped over a 1-month and 2-week period was recorded. Results Soil compaction on the grassland field was limited to a very small proportion of the total field area at its margins and close to water troughs and was not often associated with a reduction in aggregate stability. Neither soil dry density nor aggregate stability was associated with an increase in rainsplash erosion rate. On arable land, aggregate stability was significantly lower than on the grassland, and rainsplash erosion rates were higher. However, vegetation cover, rather than aggregate stability, was the major controlling factor. Conclusion The hypothesis that livestock are not causing an increase in erosion by raindrop impact on grassland fields but the same soils, when cultivated, experience a significant increase in erosion rate can be accepted based on the results generated. In lowland grassland landscapes with recommended best grazing management practices currently implemented, further reductions in sediment losses may therefore not be achievable. However, a reduction in elevated soil loss from arable fields can be achieved by ensuring vegetation cover is present during wet winter months.