Effects and driving mechanisms of short-term fertilization on the carbon pool management index in lightly degraded alpine marsh wetlands

The carbon pool management index (CPMI) is a sensitive indicator for assessing degraded ecosystem restoration, but how fertilization affects CPMI in alpine marsh wetlands remains unclear. We conducted a two-factor, three-level field experiment in a lightly degraded alpine marsh wetland using organic fertilizer (O0, O1800, and O2700, i.e., 0, 1800, and 2700 g m −2 ) and Bacillus subtilis (J0, J5, and J10, i.e., 0, 5, and 10 g m −2 ). Two-way ANOVA, correlation analysis, random forest, and structural equation modeling were used to investigate the effects and driving mechanisms of short-term fertilization on CPMI. The results showed that organic fertilizer and its interaction with bacterial fertilizer significantly affected CPMI, indicating that the effect of bacterial fertilizer depended on the organic fertilizer background. Under O0, J5 and J10 increased CPMI by 64.80% and 42.12%, respectively. In contrast, under O1800 and O2700, the addition of bacterial fertilizer showed a decreasing trend in CPMI. Correlation analysis indicated that easily oxidizable organic carbon, soil organic carbon, alkaline hydrolyzable nitrogen and pH were significantly correlated with CPMI. Random forest analysis showed that fungal Shannon diversity and available phosphorus were the most important predictors of CPMI, highlighting the roles of fungal communities and phosphorus availability. Partial least squares structural equation modeling indicated that fertilization indirectly influenced CPMI mainly through soil properties and labile carbon pools (GOF = 0.42). This study reveals how short-term fertilization regulates carbon pool management in lightly degraded alpine marsh wetlands. It also shows that bacterial fertilizer effects depend strongly on the organic fertilizer background.