Interplanting leguminous shrubs boosts the trophic interactions of soil micro-food web in a karst grassland

Abstract

Nitrogen (N) is an essential nutrient for ecosystem productivity, restoration and succession processes. Biological N fixation and chemical N addition are both important strategies for accessing N nutrients in soil ecosystem. However, it is unclear which N supplement strategies is more effective in restoring ecosystem stability, particularly in soil micro-food web component, which plays a crucial role in nutrient cycling. Here, an in-situ study was conducted in a natural grassland to investigate the effects of N supply on soil microbial and nematode communities by three legume species: Amorpha fruticose (AF), Derris fordii (DF), and Indigofera atropurpurea (IA) at two interplanting densities (1: low density, 1.5 × 2 m; and 2: high density, 1 × 1 m), and by two N fertilization rates (N5: 5 g N m−2 y−1 and N10: 10 g N m−2 y−1). Results showed the microbial biomass of total-PLFA, bacteria, fungi, actinomycetes and chlorophyta increased by DF1 and IA2, and the biomasses of total nematodes, herbivores and omnivores were promoted by AF1, AF2 and DF1, but the biomass of most components was inhibited by N fertilizers. The interactions of soil micro-food web were complicated by DF1 and DF2, but simplified by N5 and N10. Furthermore, different N supplement strategies showed distinct energy flow patterns, such as the bacterial channel was promoted exclusively in AF2, the plant channel was enhanced in AF1 and AF2, and the fungal channel was boosted in DF1 and DF2 but declined in N5 and N10. These results demonstrated that the structure and energy flow of the soil micro-food web might be benefited by legume interplanting but impaired by N fertilizers. In particular, interplanting with D. fordii granted a more sustainable way of N supplementation for promoting the complexity and stability of the soil micro-food web, compared to interplanting with other legume species. Our findings provide better understanding of the interactions between legumes and soil biota and have important implications for sustainable restoration of degraded karst grasslands.