Responses of Soil Microbial Networks to Nitrogen Deposition and Their Regulatory Effects on Carbon Sink Function in Subtropical Agroforestry Systems

Abstract:

Nitrogen (N) deposition has become a key global change factor affecting soil carbon (C) sink function in subtropical agroforestry systems (AFSs). Soil microbes are the core drivers of soil C cycling, and their community interactions (microbial networks) play critical roles in maintaining microbial functional stability and regulating C transformation processes. However, how N deposition affects soil microbial network complexity, keystone taxa, and their linkages with soil C sink function in subtropical AFSs remains unclear. Here, we conducted a 3-year N deposition manipulation experiment (0, 50, 100, 150 kg N ha⁻¹ yr⁻¹) in three typical subtropical AFSs (tea-oil camellia + clover, chestnut + wheat, and bamboo + peanut). High-throughput sequencing, co-occurrence network analysis, and soil C pool determination were used to explore the responses of bacterial and fungal networks to N deposition and their regulatory effects on soil C sink function. Results showed that: (1) N deposition significantly altered soil microbial network structure: low N deposition (50 kg N ha⁻¹ yr⁻¹) increased network complexity (nodes, edges, average degree) by 12.3%-18.7%, while high N deposition (100, 150 kg N ha⁻¹ yr⁻¹) decreased network complexity by 15.6%-24.5%, with the strongest inhibition in bamboo + peanut AFS; (2) Keystone taxa shifted under N deposition: low N favored copiotrophic taxa (e.g., Proteobacteria, Ascomycota), while high N promoted oligotrophic taxa (e.g., Acidobacteria, Basidiomycota); (3) Soil total organic C (SOC), microbial biomass C (MBC), and recalcitrant C contents first increased and then decreased with increasing N deposition, peaking at 50 kg N ha⁻¹ yr⁻¹ (increased by 8.9%-12.6% compared to control); (4) Microbial network complexity was significantly positively correlated with SOC and recalcitrant C contents (r = 0.76-0.83, p < 0.01), and keystone taxa abundance explained 42.3%-56.8% of the variation in soil C pool components; (5) Tea-oil camellia + clover AFS with high plant diversity had higher microbial network stability and C sequestration capacity under N deposition, which alleviated the negative effects of high N deposition. Our findings indicate that low N deposition enhances soil C sink function by increasing microbial network complexity and optimizing keystone taxa composition, while high N deposition weakens C sink function by simplifying microbial networks. Plant diversity mediates the response of microbial networks to N deposition, thereby regulating soil C sink function. This study provides a new perspective of microbial interactions for understanding the effects of N deposition on soil C cycling in subtropical AFSs and optimizing management practices to enhance C sequestration under global change.