Distinct effects of N and P addition on soil enzyme activities and C distribution in aggregates in a subalpine spruce plantation

Nitrogen (N) and phosphorus (P) have a critical role in soil carbon (C) cycling, but the mechanisms underlying the responses of soil C dynamics to N and P availability remain unclear because both N and P addition have divergent effects on different soil C pools. Here, we studied enzyme activities and C dynamics in bulk soil and three aggregate fractions, i.e. macroaggregates (5000–250 µm; MaA), microaggregates (250–53 µm; MiA) and silt and clay (< 53 µm; S&C), following two growing seasons of N addition (0, 5 and 20 g N m⁻² y⁻¹) fully crossed with P addition (15 g P m⁻² y⁻¹) in a subalpine spruce plantation. We found increased activities of two oxidases (catalase and polyphenol oxidase) in bulk soil following N and P addition. β-glucosidase activity was suppressed by N in MiA (– 34%) and S&C (– 23%) but increased by P in MaA (+ 35%). P addition increased catalase activity of S&C (+ 20%). Bulk soil C content showed no significant response to N or P addition, whereas C distribution in soil aggregates was affected, with increased relative C content of S&C in N-only addition plots (+ 53%) and increased relative C content of MaA after P addition (+ 11%). Our results demonstrated divergent sensitivities of both C pools and enzyme activities in different soil aggregates to fertilization, which can enhance our understanding of the responses of bulk soil C pool and enzyme activities to fertilization.