Effects of High Nitrogen and Biochar Addition on the Stability of Soil Organic Carbon Pools in Restored Grassland on the Chinese Loess Plateau

Increased atmospheric nitrogen (N) deposition alters the formation and stability of soil organic carbon (SOC) in fragile ecosystems. While biochar (BC) amendment represents a promising strategy for augmenting soil carbon sequestration, its impact on the stability of the SOC pool under high N deposition remains unclear. In this study, we conducted a two-year field trial with three replicates to investigate the effects of combined N (0 and 9 g N·m−2·yr−1) and BC (0, 20, and 40 t·ha−1) addition on the stability of the SOC pool in restored grasslands on the Loess Plateau. We assessed SOC pool stability by examining the influence of soil microbial carbon utilization efficiency (CUE), metabolic constraints, and community composition on the content of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). The results indicate that in comparison to the control treatment (N0BC0), the addition of both high N (N9BC0) and BC (N0BC20 and N0BC40) significantly promoted the accumulation of POC by 15.78%, 9.87%, and 11.05%, respectively. Conversely, the content of MAOC was suppressed under the N9BC0 (−10.64%) and N0BC40 (−8.29%) treatments. However, the combination of high N and BC treatments resulted in increased levels of SOC, POC, and MAOC, while simultaneously reducing the MAOC/POC ratio, with all parameters reaching their peak under the N9BC40 treatment. Meanwhile, high N and BC additions led to differences in bacterial community structure, increased CUE, and enzyme vector angle. Notably, high N shifted the dominant factor of BC on MAOC/POC from physicochemical properties to biological factors. Microbes drive CUE to influence changes in MAOC by adapting to metabolic limitations and stoichiometric imbalances. In contrast, POC is primarily influenced by physicochemical properties. Overall, high additions of N and BC have been shown to reduce the stability of SOC by promoting the accumulation of POC. However, an addition rate of 40 t·ha−1 of BC was found to be more effective in mitigating the negative impacts of high N addition on MAOC. This strategy can serve as an effective management approach for enhancing SOC sequestration in vulnerable regions of the Loess Plateau.