Soil-climate factors have a greater influence on the presence of winter cover crops than regulatory constraints in France

Despite the benefits associated with winter soil cover before sowing a spring crop, this practice is adopted unevenly by farmers. The goal of this paper is to identify and hierarchize the main soil-climate factors that hinder the adoption of cover crops. It is the first study to carry out this analysis at the scale of France, which is a country with a wide variety of soil types and climates. For this country, maps of vegetated winter soil cover prior to the establishment of a spring crop were previously produced for years 2018 and 2019, using Sentinel-2 multispectral images. For approximately half of France's acreage with spring crops, the estimation concerning the presence or absence of a winter cover crop was correlated here with the main soil-climate factors of each field. The inclusion within Nitrate Vulnerable Zones is another potentially important variable to explain cover crop adoption, as regulatory requirements of the Nitrates Directives strongly encourage winter soil cover in these zones to reduce nitrogen losses. Because of the regulatory requirements of the Nitrates Directives regarding winter soil cover, the influence of being located in a Nitrate Vulnerable Zone was also evaluated. But despite these incentives for winter soil cover, inclusion in a Nitrate Vulnerable Zone had little influence on cover crop adoption. Principal component analyses performed for different crop rotations showed that the clay content was the soil-climate factor most negatively correlated with the adoption of cover crops. Cover crops were more frequently established on sandy fields with high organic carbon stocks, which corresponds rather to the characteristics of livestock regions. The development of technical solutions for the establishment and destruction of winter cover crops on clay soils could facilitate the adoption of these crops, and thus improve global water quality.