Forest management as an intervention: responses of soil organic nitrogen and the microbially-mediated pathways in moso bamboo plantations

Bamboo forests provide critical ecological and economic benefits in subtropical regions, yet their productivity is often limited by nitrogen (N) availability. Intensive management can significantly increase bamboo productivity, but its impact on the sustainability of soil N cycling remains unclear. This study used metagenomic sequencing to investigate alterations in microbial N cycling across Moso bamboo plantations under contrasting management intensities: intensive (M2), extensive (M1), and unmanaged conditions (M0). Both acid-hydrolyzable N (AHN) and non-AHN were significantly higher ( p ≤ 0.05) in M0 than in M1 and M2, whereas amino acid N, ammonia N, and amino sugar N levels declined ( p ≤ 0.05) as management intensity increased. Concomitantly, the microbial functional genes shifted from a balanced N-cycling profile in M0 to a denitrification-dominated regime in M2. Specifically, M2 showed suppressed gene abundance for N fixation ( nifH ) and nitrification ( amoC ) but a marked increase in denitrification genes ( narG , nirK , and nosZ ). This transition was mediated by distinct microbial communities: M0 was associated with oligotrophic Chloroflexi, M1 with versatile Proteobacteria and Nitrospirae, and M2 with disturbance-adapted Actinobacteria. Distance-based redundancy analysis and Mantel tests confirmed that soil N fractions were significantly ( p ≤ 0.05) correlated with total organic carbon, total phosphorus (P), available P, N-cycling genes, and associated microbial communities. Partial least squares path analysis identified soil P availability as the strongest driver of soil organic N dynamics, highlighting tight N–P coupling. These findings demonstrate that intensive bamboo management shifts the microbial N cycle toward denitrification, converting soil organic N from a conserved pool into a more loss-prone state, with soil P availability as the primary driver. Maintaining long-term soil fertility in bamboo plantations therefore necessitates integrated N-P management strategies.