Cropping alfalfa for 18 years in saline-alkali soil area increases the importance of fungi in the multi-kingdom network

Soil salinization has become a global ecological and resource problem. Alfalfa cropping has been considered an efficient method of increasing improving soil fertility and sustainable development of agricultural ecosystem. A18-year time series of alfalfa cultivation plots in Songnen Plain in northeast China was selected to investigate the the important role of microbial network in improving saline-alkali soil after alfalfa cropping was evaluated by constructing single kingdom network and multi-kingdom network. This study found that, compared with the single-kingdom network, the construction of multi-kingdom network improved the complexity and stability of the network, specifically in the increase of the number of nodes and edges. After cropping alfalfa in saline-alkali land, fungal nodes were characterized by significantly higher degree and betweenness than bacteria and archaea in the entire multi-kingdom network. Fungi have high natural connectivity, which is of great significance for maintaining network complexity. However, the degree, betweenness and natural connectivity of bacteria nodes are higher in bare land. Furthermore, this study found that the topological properties of microbial networks, specifically changes in network complexity and stability, are connected to environmental factors, particularly the variation in soil salinity and available nutrients. Overall, our findings suggest that cropping alfalfa improves the stability and complexity of microbial networks by fungi stabilizing the self-organization process of multi-kingdom networks. The findings provides some theoretical evidence for the change mechanism of bacteria, fungi, and archaea communities in enhancing soil quality in saline-alkali areas of Northeast China.