Relative role of soil nutrients vs. carbon availability on soil carbon mineralization in grassland receiving long-term N addition

High nitrogen (N) input often induces soil carbon (C) limitation, eutrophication of macronutrients, deficiency of base cations, and accumulation of toxic micronutrients. These changes are perceived to be critical factors in regulating soil C mineralization. Previous studies primarily focused on the individual effects of C, macronutrients, exchangeable base cations, and micronutrients on soil C mineralization. However, the relative importance of those factors in regulating soil C mineralization, especially in N-enriched ecosystems, remains unclear. To disentangle the relative contributions of aforementioned factors, lime and/or glucose were added to soils that were collected from a field experiment with historical N addition (6 years) at seven rates (0-50 g N m- 2 year -1) in a grassland ecosystem. Lime and glucose were added to improve the soil C and key nutrient conditions. The responses of soil C mineralization rate to changes in soil C and macronutrients (N and P), exchangeable base cations (K+, Na+ and Mg2+), and micronutrients (Fe2+, Mn2+, Cu2+ and Zn2+) were examined. We found that lime addition decreased soil micronutrients, while glucose addition improved the soil available P and exchangeable base cations, especially at high historical N addition rates. The soil C mineralization was weakly associated with changes in soil nutrients, including the availability of N, P, exchangeable base cations, and micronutrients, which were conventionally and previously considered as the vital drivers of soil C mineralization. However, soil C mineralization strongly increased with glucose-induced enhancement of C availability and the subsequent enhancement of microbial biomass under increasing N addition rates. Based on the Structural Equation Model, the standardized total effects of C, macronutrients (N and P), base cations and micronutrients on soil C mineralization were 0.86, -0.29, 0.15 and -0.08, respectively. Findings from this study demonstrated that the N-induced significant changes in soil nutrients (e.g., eutrophication of N and P, base cations deficiency and accumulation of toxic macronutrients) mediated soil C mineralization, with C availability being the most critical driver for C mineralization in N-enriched soil. This study provides insight into the mechanistic understanding of the relationship between N input and terrestrial C cycling.