Effectiveness of conservation cropping practices in mitigating runoff, soil erosion, and pesticide surface losses in Northwestern Europe by Timothée Clément

Soils of the Western European loess belt are intensively cultivated and particularly prone to runoff and soil erosion. Besides threatening soil resources, overland flows cause muddy floods that damage infrastructure downstream and pollute water bodies by transferring nutrients and pesticides. Numerous conservation farming practices are known to mitigate surface flows, but the magnitude of their effectiveness remains uncertain in Northwestern Europe. Based on a systematic review and meta-analysis, we estimated the runoff and erosion mitigation effects of conservation (i.e., non-inversion) tillage, winter cover crops, and tied-ridging (i.e., micro-basin tillage in potato fields). These conservation practices decreased seasonal soil losses by 66%, 72%, and 92%, respectively, compared to conventional tillage. For conservation tillage, we further identified key factors explaining the variability of the flow mitigation effects, including the time since ploughing was stopped, the type of implemented conservation tillage scheme (e.g., number of operations), and the main crop type (winter or spring crop). Besides its use in potato crops, micro-basin tillage has also recently been developed in Belgium for maize crop, using an innovative agricultural roller designed to shape depressions in-between the maize rows. Based on past plot experiments, micro-basin tillage in maize was found to reduce runoff (-69%), soil erosion (-83%), and associated pesticide transfer (e.g., -65% for the active ingredient flufenacet). Lastly, two uncommon conservation practices in Northwestern Europe were investigated through original plot experiments in maize crops:  undersowing and strip-tillage. Undersowing (simultaneous sowing) red fescue or white clover in maize was very challenging in terms of weed control and resulted in no mitigation effect on surface flows compared to a conventionally tilled sole maize crop. Strip-tillage (i.e., tine-tilling the maize row only) significantly decreased runoff (-31%) and soil erosion (-60%). A hydrological process-based model was further calibrated based on the measured flow data in strip-tilled and conventionnaly-tilled maize plots. Subsequently, a scenario analysis was conducted to assess the conservation potential of the strip-tillage technique under extended climate, soil and slope conditions. Simulations reveal that strip-tillage cultivation would halve both the mean erosion rate as well as the flood risk (maximum daily runoff), as compared to conventional tillage. Unfortunately, available data did not allow to evaluate the impact of conservation practices on pesticide transfer in a conclusive way, neither from the quantitative review nor from our field trials. Meanwhile, the outcomes of this thesis can assist farmers, farm advisors or policy makers in (promoting) the implementation of effective conservation farming practices.