Increasing land-use durations enhance soil microbial deterministic processes and network complexity and stability in an ecotone

Studies of community assembly and species interactions are updating our view of the mechanisms underpinning microbial diversity in single-type ecosystems, yet they are rarely conducted in ecotones, such as the agro-pastoral ecotone of northern China (APEN) undergoing frequent land-use conversions. In this study, soil microbial assembly processes and co-occurrence patterns were examined in different land-use types and durations across the APEN. The interpretation of remote sensing images revealed that current land-use types lasted 1 to 9 years for all samples. None of measured soil properties changed significantly between cropland and grassland, while silt and sand differed between the long (5 to 9 years) and short (1 to 4 years) duration. Soil microbial communities were assembled under consistent processes in both land-use types, but determinism clearly exhibited greater influences in the long duration than in the short one. The β-nearest taxon index (βNTI) decreased significantly with the durations in all samples and was also affected by total nitrogen (TN), soil organic matter (SOM), total phosphorus (TP), and clay. For the co-occurrence networks, nodes, edges, modularity, and the proportion of negative edges were similar between cropland and grassland, but obviously higher in the long duration than in the short one. Simultaneously, these metrics of the sub-networks had strong positive correlations with the durations, and weaker but significant correlations with TN, SOM, silt, and sand. Altogether, this study demonstrated that frequent land-use conversions might alleviate the impact of current land-use types, but highlight the importance of land-use durations on soil microbial community assembly and co-occurrence patterns in the APEN, and the deterministic processes and network complexity and stability increased with the durations, which advance the understanding of the mechanisms maintaining soil microbial diversity in ecotones.