Plant Residue Input Enhances Soil Multifunctionality by Reshaping Microbial Communities in Saline–Alkali Soil of Northeast China

Soil microorganisms and plant residue decomposition are critical drivers of soil nutrient cycling and multifunctionality, yet their regulatory mechanisms in saline–alkali soils are not fully understood. This study selected bare land and forestland (shrub and tree stands) in Daqing, Heilongjiang, to investigate the effects of plant residue input on forest soil properties, microbial communities, keystone taxa, and multifunctionality using high-throughput sequencing and multivariate analysis. Results showed that plant residue cover significantly improved soil nutrients (SOC, TN, TP, TK, AN), enhanced alkaline phosphatase activity, and increased soil multifunctionality compared with bare land. Plant residues also increased bacterial α-diversity and shifted community composition, with elevated relative abundances of Proteobacteria, Bacteroidota, Planctomycetota, Patescibacteria, and key genera (Mycobacterium, Pseudonocardia, Bryobacter, Steroidobacter). Non-metric multidimensional scaling (NMDS) and correlation analysis revealed microbial communities and keystone taxa were closely correlated with soil nutrients and multifunctionality. Overall, plant residues enhance forest soil multifunctionality by improving soil organic matter, optimizing microbial community structure, and stimulating keystone taxa, providing a scientific basis for understanding microbial-driven nutrient cycling and vegetation restoration in degraded saline–alkali soils.