The dual role of organic carbon: Mitigation of nitrogen-induced soil acidification but amplification of greenhouse gas emissions.

Nitrogen (N) fertilization accelerates soil acidification and increases nitrous oxide (N₂O) emissions in agricultural systems. Organic carbon (C) is a critical regulator of microbial-mediated N transformations, but the interactive effects of combined C and N inputs on soil acidification dynamics and greenhouse gas emissions remain poorly understood. Here, a 5-month field experiment was conducted to determine the temporal dynamics of soil pH, inorganic N pools, total C and N contents, microbial biomass C and N, and greenhouse gas (N₂O and CO₂) fluxes under four treatments: urea alone and urea combined with three different C sources (glucose, sucrose, starch). Application of N alone induced significant soil acidification (ΔpH = -0.72), whereas co-application of N with C sources mitigated acidification through increasing pH (by 0.28-0.39 units). Starch had the strongest buffering effect. Carbon supplementation substantially enhanced microbial biomass pools (140 %-212 % increase in microbial biomass C; 15 %-46 % increase in microbial biomass N) and stimulated microbial N assimilation, thereby suppressing nitrification processes. Despite these benefits, C addition paradoxically upregulated N₂O-related functional gene abundance and amplified greenhouse gas emissions, elevating N₂O fluxes by 88 %-163 % and CO₂ emissions by 559 %-992 % compared with N-only treatment. Temporal analyses revealed that labile C forms (glucose/sucrose) had rapid but transient effects, whereas starch induced delayed but sustained responses. These results demonstrate that organic C amendments exert dual regulation functions in N-fertilized soils: alleviating acidification through modified microbial N partitioning, but intensifying C and N biogeochemical cycling that drive greenhouse gas production.