Nitrogen Sources Reshape Microbial Community Assembly and Interaction Networks to Regulate Straw Decomposition and Carbon Release in Cropland Soils of the North China Plain

The decomposition of crop residue is indispensable for carbon (C) cycling in agricultural ecosystems, and nitrogen (N) application has a pronounced effect on this by regulating the C/N ratio and microbial growth strategy. This study investigated how different inorganic and organic nitrogen sources interact with soil properties to regulate straw decomposition, nutrient release, microbial community assembly, and microbial interaction networks across typical cropland soils of the North China Plain. Five treatments, i.e., no N addition, urea, calcium nitrate, yeast derivatives, and glutamate, were conducted in fluvo-aquic soil, lime concretion black soil, cinnamon soil, and yellow-cinnamon soil. We found that soil type strongly regulated straw decomposition dynamics, with cumulative decomposition rates varying greatly among the four soil types after 180 days, ranging from 55.78% to 68.75%. In particular, the lime concretion black soil exhibited the highest straw decomposition and C release rates, which harbored higher relative abundances of potential straw-decomposing taxa, including Luteibacter, Chitinophaga, Dothideomycetes, and Leotiomycetes. Bacterial community assembly shifted from stochastic dominance at early stages to deterministic selection at later stages, whereas fungal community assembly remained predominantly stochastic with increasing dispersal limitation during late decomposition. The dominant bacterial phyla were Proteobacteria and Bacteroidota, and the fungal community composition succeeded from Alternaria, Rhizopus, Cladosporium and Meyerozyma to Chaetomium, Schizophecium and Apodus. Compared with no N addition, yeast derivatives significantly increased the straw decomposition rate in fluvo-aquic soil, lime concretion black soil, and cinnamon soil by 15.00, 10.66, and 12.19 percentage points, respectively, within 14 days. This is related to the enhanced complexity of the bacterial co-occurrence network, among which Altererythrobacter, Sphingomonas, Candidatus_Xiphinematobacter, Devosia, and Chitinophaga were identified as key microbial groups. Glutamate stimulated later decomposition by modifying the fungal community structure and strengthening fungal interactions centered on Alternaria and Schizothechium. These findings reveal how N forms regulate straw decomposition through soil-dependent microbial assembly and interaction networks, providing a mechanistic basis for optimizing nitrogen management strategies for efficient straw nutrient recycling.