Soil organic carbon (SOC) is a crucial indicator of soil quality and fertility. However, excessive nitrogen (N) application, while increasing Moso bamboo yield, may reduce SOC content, potentially leading to soil quality issues. The impact of N on SOC and its active fraction in Moso bamboo forests remains underexplored. Investigating these effects will elucidate the causes of soil quality decline and inform effective N management strategies. Four N application gradients were set: no nitrogen (0 kg·hm⁻²·yr⁻¹, N0), low nitrogen (242 kg·hm⁻²·yr⁻¹, N1), medium nitrogen (484 kg·hm⁻²·yr⁻¹, N2), and high nitrogen (726 kg·hm⁻²·yr⁻¹, N3), with no fertilizer application as the control (CK). We analyzed the changes in SOC, active organic carbon components, and the Carbon Pool Management Index (CPMI) under different N treatments. The results showed that SOC and its active organic carbon components in the 0~10 cm soil layer were more susceptible to N treatments. The N0 treatment significantly increased microbial biomass carbon (MBC) content but had no significant effect on SOC, particulate organic carbon (POC), dissolved organic carbon (DOC), and readily oxidizable organic carbon (ROC) contents. The N1, N2, and N3 treatments reduced SOC content by 29.36%, 21.85%, and 8.67%, respectively. Except for POC, N1,N2 and N3 treatments reduced MBC, DOC, and ROC contents by 46.29% to 71.69%, 13.98% to 40.4%, and 18.64% to 48.55%, respectively. The MBC/SOC ratio can reflect the turnover rate of SOC, and N treatments lowered the MBC/SOC ratio, with N1 < N2 < N3, indicating the slowest SOC turnover under the N1 treatment. Changes in the Carbon Pool Management Index (CPMI) illustrate the impact of N treatments on soil quality and SOC sequestration capacity. The N1 treatment increased the CPMI, indicating an improvement in soil quality and SOC sequestration capacity. The comprehensive evaluation index of carbon sequestration capacity showed N3 (−0.69) < N0 (−0.13) < CK (−0.05) < N2 (0.24) < N1 (0.63), with the highest carbon sequestration capacity under the N1 treatment and a gradual decrease with increasing N fertilizer concentration. In summary, although the N1 treatment reduced the SOC content, it increased the soil CPMI and decreased the SOC turnover rate, benefiting soil quality and SOC sequestration capacity. Therefore, the reasonable control of N fertilizer application is key to improving soil quality and organic carbon storage in Moso bamboo forests.

Moso bamboo forests are ecologically and economically important in China, but the effects of nitrogen addition on soil carbon dynamics and vegetation are underexplored. This study applied six nitrogen treatment levels in a monoculture Moso bamboo forest, collecting soil samples from 0 to 30 cm depth and understory vegetation data. The results showed that nitrogen addition significantly reduced stable carbon components, while particulate organic carbon (POC) increased, suggesting that excessive nitrogen may destabilize the soil carbon pool by accelerating SOC decomposition or reducing accumulation. Vertical variations in soil carbon components were more significant in the 20–30 cm layer, indicating stronger impacts on deep SOC cycling. Nitrogen reshaped the community structure, notably affecting dominant species such as Viola davidii and Rubus buergeri. However, species diversity indices showed no significant changes, likely due to the vegetation’s high adaptability to nitrogen. Correlation analysis indicated that stable carbon components positively influenced vegetation diversity, while POC had a negative effect. Partial least squares path modeling (PLS-PM) explained 95.12% of the variance in the relationship between nitrogen, soil carbon components, and vegetation diversity. Nitrogen negatively affected soil carbon components but positively influenced vegetation diversity, while soil carbon components negatively impacted diversity. Nitrogen may promote certain carbon component accumulation but could weaken the forest’s carbon sink function.

This study aimed to investigate the accumulation of aboveground biomass, organ allocation patterns, and their driving mechanisms in Moso bamboo (Phyllostachys edulis) forests along different elevational gradients and to compare regional differences in growth processes. A total of 54 sample plots were established along an elevational gradient from 50 to 550 m across three different regions, with 100 m elevational intervals. Two-way ANOVA, regression analysis, Tukey’s HSD multiple comparisons, and generalized additive models (GAMs) were used to examine distribution patterns. (1) Individual bamboo biomass followed a unimodal pattern with increasing elevation, peaking at 150 m (16.20 ± 3.88 kg culm⁻¹), which was significantly higher than at other elevations (p = 0.048). Allometric covariance analysis showed that the b value did not differ significantly among elevations (p = 0.882), indicating a stable diameter at breast height (DBH)-biomass relationship. (2) Stand biomass was highest at 50 m (34.75 ± 10.97 t·ha⁻¹) and declined with elevation to 18.54 ± 7.13 t·ha⁻¹ at 550 m, revealing a decoupling from the elevational trend of individual biomass. (3) Branch and leaf dry mass allocation exhibited a “higher at both ends, lower in the middle” pattern. Culm allocation was highest at 150 m (80.3%), though differences among elevations were not statistically significant (p = 0.591). (4) Stand density decreased with elevation, while mean DBH first increased and then decreased, reaching a maximum at 450 m (9.77 cm). Differences in stand density and DBH among elevations were highly significant (p < 0.001). (5) ANCOVA showed that after controlling for mean DBH, the effect of elevation on individual biomass was substantially weakened (p = 0.051), with partial η² declining from 0.48 to 0.21 (a 56% reduction), indicating that DBH accounted for a substantial portion of the elevation effect on individual biomass.The individual biomass of Moso bamboo in central-southern Jiangxi peaked at approximately 150 m elevation. Elevation was associated with biomass mainly through its association with DBH (a size effect), rather than through changes in allocation ratios or allometric relationships. The pathway “elevation→DBH→individual biomass” appears to be the primary mediating pathway explaining the decoupling, although a causal interpretation requires further experimental validation. These findings provide a theoretical basis for elevation-differentiated management of Moso bamboo forests.