Increase in mineral-associated organic carbon does not offset the decrease in particulate organic carbon under long-term nitrogen enrichment in a steppe ecosystem

Nitrogen (N) deposition significantly impacts ecosystem carbon (C) cycling. However, most experimental N deposition studies applied N fertilizers in low-frequency, typically once or twice per year during the growing season. Few studies have been conducted to investigate the effects of high-frequency N deposition at varying rates on the formation and stability of soil organic carbon (SOC). Additionally, the effects of N addition on the two SOC fractions — particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) — and the underlying mechanisms are not well understood. To address these gaps, we conducted a long-term N addition experiment in a typical steppe ecosystem in Inner Mongolia, China, beginning in 2008. The N addition rates ranged from 0 to 50 g N m^-2 yr^-1, with a high frequency of N additions (once a month, 12 additions per year). After a decade of N addition, we observed a consistent decrease in SOC (by 3.9 ± 0.51 %) and POC (by 17.5 ± 2.31 %) and an increase in MAOC (by 5.8 ± 1.68 %) compared to the control treatment (i.e., the treatment without N addition). The decline in POC was attributed to stimulated microbial decomposition due to improved quality of particulate organic matter and increased priming effect from SOC. The increase in MAOC was associated with enhanced mineral protection, resulting from increased solubility of iron/aluminum (Fe/Al) that are reactive in directly adsorbing SOC molecules to form stable metal-SOC complexes. However, this increase in MAOC does not offset the decrease in POC, leading to an overall decrease in SOC under N enrichment. This study reveals the crucial roles of microbial decomposition and mineral protection in determining SOC fractions in N-enriched steppe ecosystems.