Evaluation of disruption of sediment connectivity and herbicide transport across a slope by grass strips using a magnetic iron oxide tracer

Abstract

The use of cover crops has been prescribed as a mitigation measure for offsite contamination, as they reduce sediment and agrochemical loads, thus reducing the hydrological connectivity of the land. However, there is the need to quantify its effectiveness for specific agrochemicals in broader range of conditions, to validate management for its safe environmental use. The objectives of the present study were: (1) to explore the use of simulated rainfall and magnetic iron oxide to understand the impact of vegetation strips on runoff and soil losses at the plot scale and, (2) to evaluate the effectiveness of vegetation strips in buffering sediment and herbicides coming from bare soil areas. The study encompasses six sets of rainfall simulations (three replications each) under four different soil management scenarios: dry vegetation cover sowed that year, dry vegetation cover two years after sowing, freshly-tilled soil, and tilled soil with a compacted surface due to rainfall and trampling. The experiments involved the use of a magnetic iron oxide as a sediment tracer to obtain a better understanding of the trapping efficiency of the vegetation strips. Three runoff plots were established on a hillslope under a Fluvisol alluvial terrace. Each of the plots contained three bare areas tagged with magnetic iron oxide and three strips with Lolium multiflorum L. The results indicate that by using cover crop strips, runoff and sediment losses were approximately 20% and 4%, respectively, of the losses measured in the bare compacted soil, while losses on the freshly-tilled surface were similar to that of the cover crop. Herbicide losses were greatly reduced by the cover crops, with losses ranging from approximately 0.1%–0.6% of the doses applied the day before on the bare area. Nevertheless, on the compacted bare soil, terbuthylazine, oxyfluorfen and diflufenican losses were 1.63%, 4.35% and 9.67%, respectively, of the dose applied the previous day. This can be explained because the tilled and compacted soil showed the highest cumulative runoff and soil losses values (28 mm and 248.9 g m−2). The formation of micro-relief steps after the first simulation reduced the hillslope connectivity and, thus, the soil losses and runoff. Tracer selectivity from soil textures with fine particle size (clay) was observed, as there was an enrichment of these particles in the collected sediment. This study quantified the impact of cover crop strips on mitigating offsite herbicide contamination and improved our understanding of sediment redistribution at the hillslope scale.