Variations in Soil Nitrogen Mineralization Are Associated with Fungal Communities Across Broad-Leaved Forests in Northeast China

Soil nitrogen (N) mineralization plays a pivotal role in regulating N availability in forest ecosystems, which could not only be closely related to soil nutrient supply capacity but also profoundly affect the forest carbon sequestration. Broad-leaved forests play a key role in terrestrial carbon storage; however, soil net N mineralization rates (Rmin) vary considerably among different forest types and their underlying driving mechanisms remain poorly understood. In this study, three typical broad-leaved forests in Northeast China, namely Populus davidiana Dode forest (PF), Fraxinus mandshurica Rupr forest (FF), and Betula platyphylla Suk. forest (BF), were selected. The soil Rmin, environmental parameters, physicochemical properties, and microbial community characteristics were determined among the three broad-leaved forests to explore forest type differences in soil Rmin and their associated factors. The results showed that soil inorganic N contents differed significantly among the three forest types, with significantly higher values in FF than in PF and BF (p < 0.05). Soil Rmin also differed significantly among forest types, which was highest in FF, followed by PF and BF (2.17, 1.31, and 0.95 mg kg⁻¹ day⁻¹, respectively) (p < 0.05). Soil Rmin was significantly positively correlated with soil water content (SWC), soil temperature (ST), and pH, but there was a negative correlation to soil bulk density (BD) (p < 0.05). In addition, microbial biomass carbon, nitrogen, and phosphorus were significantly higher in FF than in PF and BF (p < 0.05). Variation in soil Rmin among three broad-leaved forests was significantly associated with the abundances of Ascomycota, Basidiomycota, and Mucoromycota, but not with bacterial community, suggesting a closer association between fungi and soil Rmin. Structural equation modeling (SEM) indicated that forest type was associated with soil microbial community structure and biomass through associations with soil environmental and physicochemical properties, in relation to soil Rmin. In conclusion, this study highlights the links between vegetation type and soil Rmin in broad-leaved forests, which deepens the theoretical understanding of forest soil N-limitation in Northeast China.