Nitrogen (N) and phosphorus (P) deposition have largely affected soil respiration (R_s) in forest ecosystems. However, few studies have explored how N and P individually or in combination to influence R_s and its components (autotrophic respiration, R_a; heterotrophic respiration, R_h), especially in highly P-limited subtropical forests. To address this question, we conducted a field manipulation experiment with N and/or P addition in a 50-year-old subtropical secondary forest.

We found that N addition on average reduced R_s, R_a, and R_h by 15.2%, 15%, and 11.7%, respectively during 2-year field study. P addition had an inconsistent effect onR_a, with R_a increasing by 50.5% in the first year but reducing by 26.6% in the second year. Moreover, P addition on average decreased R_h by 8.9%-30.9% and R_s by 6.7%-15.6% across 2 years. In contrast, N and P co-addition on average increased R_s, R_a, and R_h by 1.9%, 7.9%, and 2.1% during the experimental period. Though R_s and R_h were significantly correlated with soil temperature, their temperature sensitivities were not significantly changed by fertilization. R_a was predominantly regulated by soil nitrogen availability (NH₄⁺ and NO₃^-), soil dissolved organic carbon (DOC), and enzyme activities, while the variation in R_h was mainly attributable to changes in soil microbial community composition and soil β-D-Cellubiosidase (CB) and β-Xylosidase (XYL) activities.

Our findings highlight the contrasting responses of R_s and its components to N or P addition against N and P co-addition, which should be differentially considered in biogeochemical models in order to improve prediction of forest carbon dynamics in the context of N and P enrichment in terrestrial ecosystems.