The increasing nitrogen (N) deposition has greatly enhanced N input to terrestrial ecosystems and even resulted in an imbalance of N and phosphorus (P) availability. N and P are primary nutrients affecting carbon (C) cycling in terrestrial ecosystems, but the effects of N and P availability on soil C pool, the largest C pool in terrestrial ecosystems, are not well studied, especially the underlying mechanisms. Bulk soil C pool consists of heterogeneously functional pools, which can be affected by N and/or P differentially. To better evaluate the effects of N and P on soil C pool, there is a need to identify the soil fractions that are affected and the extent to which they are affected. We investigated dynamics of soil enzyme activities and C in bulk soil and three aggregate fractions, i.e. macroaggregates (5000-250 μm; MaA), microaggregates (250-53 μm) and silt and clay (<53 μm; S&C), following two growing seasons of N addition (0, 5 and 20 g N m-2 yr-1) without and with P addition (15 g P m-2 yr-1) in a subalpine spruce plantation.
Results/Conclusions
We found distinct effects of N and P on enzyme activities and C content across bulk soil and aggregate fractions. The activities of two lignolysis enzymes (catalase and polyphenol oxidase) in bulk soil increased after N and P addition, while β-glucosidase activity was suppressed by N in S&C but increased by P in MaA and P addition increased catalase activity of S&C, consistent with the common decomposition sequence that polysaccharides and lignin deplete in particle and mineral fractions, respectively. Bulk soil C content showed no response to nutrient treatments, whereas C distribution in aggregates was affected, with increased relative C content of S&C in N-only addition plots and increased and decreased relative C content of MaA and finer aggregates (<250 μm), respectively, after P addition. We attributed the distinct effects of N and P on C distribution in aggregates to differential responses of aggregates, in addition to the effect of increased fine root biomass induced by P. Furthermore, our results indicate N addition has the potential to increase soil C storage while P would cause soil C losses, and the dynamics of soil C and enzyme activities in aggregates are better predictors of long-term soil C sequestration than those in bulk soil.