Bacterial-fungal interkingdom interactions mediate effects of biodiversity on soil carbon dynamics in wheat fields under long-term fertilization

Bacterial-fungal interkingdom interactions (BFIs) determine the compositions of microbial communities and performance of ecosystem functions. However, it is still unclear whether and how BFIs drive soil organic carbon (SOC) dynamics under field fertilization conditions. Thus, we conducted a field experiment for 7 years using a split-plot design, with chemical nitrogen (N) fertilizer application rates (0, 75, 150, 225, and 300 kg ha- 1) as the main plots and manure application rates (0 and 30,000 kg ha- 1) as the subplots. The results showed that particulate organic C (POC) was more responsive to fertilization than mineral-associated organic C (MAOC), making it a valuable indicator for diagnosing changes in farmland C pools. Moderate N application (150 or 225 kg ha- 1) and manure addition increased microbial metabolic activities, including cumulative C mineralization (CCM) and the metabolic quotient (qCO2), as well as soil enzyme activities, whereas N rates exceeding 225 kg ha- 1 had adverse effects. A significant positive correlation was consistently observed between the total richness of microbial species and complexity of BFIs, suggesting that high richness possibly creates complex symbiotic networks by promoting more interspecies interactions. The ratio of positive and negative associations between bacteria and fungi mediated the effect of microbial richness on network complexity, ultimately influencing soil C mineralization through enzyme activities. These findings suggest that interspecies interactions within complex networks, especially competition and cooperation between bacteria and fungi, are more predictive of microbial community-mediated C processes than microbial diversity, which is measured in simple quantities. This study enhances our understanding of C dynamics mediated by microbial interactions under fertilization inducing in dryland wheat fields.