Effects of Nitrogen Addition on Soil Aggregate Stability and Mycorrhizal Morphological Characteristics: Differential Responses of Arbuscular Mycorrhizal and Ectomycorrhizal Fungi

Soil aggregate stability plays a pivotal role in ecosystem functioning and carbon sequestration. Nitrogen deposition influences aggregate stability and drives differential responses in AM and ECM fungi, yet the underlying mechanisms remain unclear. This study aimed to determine how N addition influences soil aggregation, mycorrhizal morphological characteristics, and soil organic carbon (SOC) across two mycorrhizal types. A temperate forest experiment was conducted in northeastern China using 12 plots subjected to four N treatments (control, low, medium, high). Soil properties, soil aggregate traits, mycorrhizal morphological characteristics, and aggregate distributions were quantified. Relationships were examined via correlation analyses, random forest models, and structural equation modeling. N enrichment substantially increased SOC and soil water content, enhancing the proportion of large soil aggregates (4–8 mm) and elevating mean weight diameter (MWD) and geometric mean diameter (GMD), two critical indicators of soil aggregate stability. AM fungi exhibited a stronger response to N addition than ECM, reflected in greater hyphal development and carbon accumulation. SOC and water content correlated positively with aggregate stability, whereas soil pH showed a negative association. N inputs enhance soil structural stability by promoting SOC and water retention, with AM fungi demonstrating heightened sensitivity to N addition. These findings emphasize the integral role of mycorrhizal dynamics in shaping soil carbon stabilization under increasing N deposition.