Short-Term Nitrogen Enrichment Increases Soil Respiration Without Detectable Changes in Bulk Soil Carbon and Nutrient Pools in a Subtropical Karst Forest

Forest-soil CO2 efflux may respond to nutrient enrichment even when treatment effects on bulk soil pools are not detectable, but the separate and interactive effects of nitrogen (N) and phosphorus (P) remain unclear in karst ecosystems. We tested these effects in a 2 × 2 factorial field experiment with N addition (100 kg of N ha−1 yr−1), P addition (50 kg of P ha−1 yr−1), or both. Soil respiration (Rs) was monitored over an annual cycle, and plot-level temperature sensitivity was quantified as Q10 from paired Rs–temperature observations. N addition significantly increased Rs, whereas neither the independent P effect nor the N × P interaction was statistically supported. The annual mean Rs was 9.2% higher under N and 19.9% higher under combined N and P than in the control. In contrast, Q10 showed a significant P main effect (F1,16 = 4.825, p = 0.043), while the N and N × P effects on Q10 were not significant. No significant treatment effects were detected for the measured bulk soil properties or microbial biomass. Selected carbon-degrading gene abundances showed specific treatment contrasts. Short-term N enrichment therefore increased total soil CO2 efflux, while P addition altered the apparent temperature sensitivity of Rs without significantly changing the annual mean Rs.