Annual crops yield, soil quality and gully filling practices at the Campiña in Southern Spain

Gully filling practices are periodically carried out in commercial farms of annual crops in the Guadalquivir Campiña to reduce the extension of gully erosion after erosive events. Large quantities of fertile topsoil are scraped, transported and deposited by tractors and specialized machinery within eroded waterways. Thus, gullies are filled at the expense of soil quality in their vicinity leading to decreasing crop yields.

          The aim of this study is to evaluate the impact of recent filling practices in soil quality and crop production (wheat and sunflower) in four small gullied watersheds (< 5 ha) in a commercial farm near the town of Córdoba (Spain). Soil samples were taken at four transects defining 27 locations in each watershed (108 in total), including scraped, non-scraped and reference areas for analyzing  chemical and physical soil properties (cation exchange capacity CEC, texture, organic matter OM, carbonate content). In addition, at each sampling site, bulk density (BD, clod method), undrained shear strength (USS, vane test), soil colour (Munsell) and soil erodibility (jet test) were measured. At the same locations, yield surveys were conducted in 2 m2 plots at the end of May and July 2021 in watersheds on wheat and sunflower, respectively. Wheat grain and sunflower seeds were extracted, dried at 60ºC and weighted for the determination of crop yield.

          Significant differences were found between sampling sites in chemical and physical soil properties, with scraped areas showing light colours, lower CEC and OM and higher dry BD and USS. Soil colour was found to be a good proxy for soil quality. Topography along with soil quality explained ~ 60% of the crop yield variance, with showed large variations (between 3 and 6 t·ha^-1 for wheat and 100 and 800 kg·ha^-1 for sunflower). These results emphasize the need for quantitative analyses of gully erosion degradation to provide alternative and more efficient and sustainable management approaches for gully control.