Plant functional composition and species diversity affect soil C, N, and P during secondary succession of abandoned farmland on the Loess Plateau

The relationships among soil C, N, and P, aboveground plants, and belowground organisms are well-known; however, the impact of the aboveground dynamics of plant communities and their ensuing effects on soil C, N, and P, particularly during secondary succession need clarification. This study explored the effects of plant functional composition and diversity on soil C, N, and P at different soil depths. Soil samples were collected at soil depths ranging from 0 to 60 cm from three grassland sites abandoned for 5, 17, and 30 years in the Yang Qing Chuan catchment. Soil properties and fine root biomass were determined for different soil depths, and the plant community characters at each site were also evaluated. The results revealed that the plant community transitioned from dominance by annual herb to dominance by perennial species during the succession, and the percentage of Leguminosae plants, plant diversity, and species richness increased with succession time. The soil water content, organic carbon, total nitrogen, total phosphorous, C:P ratio, and N:P ratio increased significantly with successional time and decreased significantly with soil depth. In contrast, the soil bulk density and pH decreased significantly with successional time and increased significantly with soil depth. Furthermore, the plant functional group composition and diversity significantly affected soil C, N, and P contents and ratios, but was sensitive to the vertical stratification of soil. Plant functional group composition had a stronger correlation to soil C, N, and P contents and ratios at soil depths of 0-20 cm than that of the 20-60 cm depth primarily due to the fact that the nutrients returned are mostly concentrated on the topsoil and seldom deposited in the deeper soils. However, plant diversity had a stronger impact on soil C, N, and P contents and ratios at soil depths of 20-60 cm than at 0-20 cm depth, because plant diversity exacerbates interspecific competition and forces the roots to grow deeper, which alters nutrients in the deep soil. In addition, the percentage of legumes can be used as a practical indicator of soil quality in the 0-20 cm layer during natural succession of abandoned farmlands. This study provides evidence that plant communities have a strong influence on soil C, N, and P, but that the pathways and degree of influence differ across vertically stratified soil.