Effects of Straw Return Approach on Nutrient Losses of Mollisols With Various Degradation Levels Under Natural Rainfall Events

ABSTRACT Straw return has been widely adopted to control nutrient losses from sloping cropland around the world. Nevertheless, the effectiveness of straw return may vary greatly with different cultivation measures and land degradation levels. In this study, crop straws were returned via rotary tillage, deep plowing or strip mulching in non‐degraded, lightly, moderately, and severely degraded croplands in the Mollisol region of China. A control trial with straw removal was also set up for each cropland. In total, 16 treatment blocks, each containing a 9 m × 3.3 m runoff plot, were constructed in 2023. Twelve erosive rainfall events with varying types were observed during the soybean growing season. The results indicated that the annual losses of carbon, nitrogen, and phosphorus in this region were about 163.3, 15.6, and 6.5 kg ha −1 , respectively, with particulate‐bound nutrients (98.9%) overwhelmingly exceeding soluble forms (1.1%) during the water erosion processes. Land degradation led to exponential increases in annual nutrient losses. Straw return by rotary tillage showed the largest nutrient loss reduction efficiencies (72.4%–73.9%) in non‐degraded cropland, whereas strip mulching had the greatest benefits (63.4%–78.3%) in degraded croplands. The straw return approach affected nutrient losses mainly through its impact on soil physical properties, which subsequently regulated straw decomposition and nutrient cycling, crop growth, as well as runoff and sediment fluxes. This study helps interpret the patterns of carbon, nitrogen, and phosphorus losses and optimize straw return approaches in different soil conditions in semi‐humid regions.