Differential mechanisms drive changes in soil C pools under N and P enrichment in a subalpine spruce plantation

Nitrogen (N) and phosphorus (P) availabilities can affect soil C cycling by altering both plants and microbial activity. It remains unclear how the changes in plant and microbial processes could regulate C dynamics of different soil fractions, thereby affecting soil C sequestration. Based on a field fertilization experiment in a subalpine spruce plantation, we examined the effects of N, P and combined N and P addition (NP) treatments on plant C inputs, soil C contents and enzyme activities of bulk soil and different aggregate fractions. We aimed to explore the mechanisms by which N and P addition affected soil C sequestration. After 4-year fertilization, the N treatment increased soil C content by 21%, accompanied by suppressed polyphenol oxidase activity ( - 43%). However, the P and NP treatments decreased soil C content by 14% and 7%, respectively, accompanied by reduction in fine root biomass ( - 51% and - 50%, respectively). Fertilization also affected soil aggregate C pools, with decreased C content in the macroaggregates (5000-250 mu m; MaA) under the P and NP treatments, and increased C content in the silt and clay fraction ( < 53 mu m; S&C) under all fertilization treatments. The step-AIC analysis showed that the effects of fertilization on bulk soil C content were best explained by changes in soil polyphenol oxidase activity and fine root biomass. Overall, our results indicate that N-induced soil C accrual, mainly accumulating in the S&C, could be largely driven by the suppressed oxidase activity, and that decreases in soil C content under the P and NP treatments, mostly occurring in the MaA, was likely associated with the reduced C inputs by fine roots.