Effect of planting and returning Vicia villosa on soil active organic carbon and yield of subsequent maize in coastal saline soils

Fallow in the winter-spring season is becoming a common practice in the Yellow River Delta region, influenced by heavy soil salinization, scarce available water in spring, and reduced precipitation induced by climate change. However, fallow in winter can cause ecological problems such as soil erosion and secondary salinization, which will inevitably lead to environmental degradation once large areas of crop land being fallow. This study investigated the influence of planting and returning Vicia villosa (V. villosa treatment) in the winter-spring season on soil physicochemical properties, especially on active organic carbon and yield of subsequent maize crops compared to fallow, to provide a reference for the application of cover crops in improving saline-alkali land productivity in the Yellow River Delta. The field experiments were conducted from September 2020 to October 2021. For the V. villosa treatment, V. villosa was sown in September 2020 and returned to the soil as green manure during its blooming period in May 2021, and maize was sown in July 2021. For the fallow treatment, the experimental area remained fallow before maize sowing, and maize was sown on the same day under the same cultivation management as for the V. villosa treatment. The results showed that during the growing period of V. villosa, the soil electrical conductivity (EC) decreased, and the readily oxidizable organic carbon content (ROC) increased. When V. villosa was returned to the soil, soil pH decreased, and soil nutrients and active organic carbon contents improved significantly compared with fallow. During the entire experimental period, the average pH of the V. villosa treatment decreased by 0.12, and the average contents of total nitrogen (TN), total phosphorus (TP), organic carbon (SOC), ROC, dissolved organic carbon (DOC), and ROC/SOC of the V. villosa treatment increased by 15.1%, 5.5%, 6.3%, 99.1%, 8.2%, and 89.9%, respectively, compared with those of fallow treatment. However, the average EC values for the two treatments were approximately equal. Compared to the fallow treatment, the V. villosa treatment significantly increased the subsequent maize straw biomass, grain yield, and total aboveground biomass by 25.3%, 15.9%, and 21.4%, respectively, indicating a better yield improvement effect. Principal component analysis showed that maize yield was positively correlated with soil TN, SOC, DOC, and ROC, but negatively correlated with pH and EC. EC and soil organic carbon components were strongly correlated before the return of V. villosa. However, TN had the greatest influence on soil organic carbon components in each maize growing period after V. villosa return, followed by pH. The content of each organic carbon component increased with increasing TN content and decreasing pH. This study indicates that planting and returning V. villosa in the winter and spring seasons could increase soil active organic carbon content by increasing soil TN and decreasing pH, which comprehensively enhanced maize yield. Overall, in the Yellow River Delta, the introduction of V. villosa as a cover crop has prominent advantages in soil amelioration and yield improvement of subsequent crops when compared to fallow in the winter-spring season, which could be considered as the optimal planting pattern for the comprehensive utilization of saline-alkali land.