Persistence of fertilization effects on soil organic carbon in degraded alpine wetlands in the Yellow River source region

In the restoration of degraded wetlands, fertilization can improve the vegetation-soil-microorganisms complex, thereby affecting the organic carbon content. However, it is currently unclear whether these effects are sustainable. This study employed Biolog-Eco surveys to investigate the changes in vegetation characteristics, soil physicochemical properties, and soil microbial functional diversity in degraded alpine wetlands of the source region of the Yellow River at 3 and 15 months after the application of nitrogen, phosphorus, and organic mixed fertilizer. The following results were obtained: The addition of nitrogen fertilizer and organic compost significantly affects the soil organic carbon content in degraded wetlands. Three months after fertilization, nitrogen addition increases soil organic carbon in both lightly and severely degraded wetlands, whereas after 15 months, organic compost enhanced the soil organic carbon level in severely degraded wetlands. Structural equation modeling indicates that fertilization decreases the soil pH and directly or indirectly influences the soil organic carbon levels through variations in the soil water content and the aboveground biomass of vegetation. Three months after fertilization, nitrogen fertilizer showed a direct positive effect on soil organic carbon. However, organic mixed fertilizer indirectly reduced soil organic carbon by increasing biomass and decreasing soil moisture. After 15 months, none of the fertilizers significantly affected the soil organic carbon level. In summary, it can be inferred that the addition of nitrogen fertilizer lacks sustainability in positively influencing the organic carbon content.