Phosphorus addition alters the response of soil organic carbon decomposition to nitrogen deposition in a subtropical forest

The continuous increase of nitrogen (N) deposition may exacerbate phosphorus (P) deficiency, which affects soil organic carbon (SOC) decomposition by changing microbial community characteristics in subtropical forests with highly weathered soils. However, there is currently little information about the role of P and the N × P interaction in SOC dynamics. Here, a field nutrient manipulation experiment was established in a subtropical plantation forest in China. Soils collected from simulated N deposition and P addition treatments for 5 years were incubated at 25 °C for 130 days. Soil microbial composition was measured using the phospholipid fatty acid method and the enzyme activities related to SOC hydrolysis were measured. The SOC concentration and δ13C in bulk soil and three particle-size fracfractions were also determined. The cumulative CO2 respired over 9 days, representing the utilization of carbon sources under field conditions, increased with N deposition levels under the without-P treatment, while no significant differences were found among the three N deposition levels in the with-P treatment. Meanwhile, P addition generally suppressed the SOC decomposition during 130 days incubation. Similarly, P addition decreased the potential organic carbon decomposition (C0) and C0/SOC ratio. In contrast, C0 increased with N deposition in the without-P treatment, while was unaffected by N deposition under the with-P treatment, suggesting the response of SOC decomposition to N deposition was affected following P addition by alteration of SOC quality. Moreover, N deposition tended to deplete the δ13C of the SOC and P addition enriched the δ13C of the macro-particulate organic carbon. Addition of P increased total microbial, fungal and bacterial biomass values by 41.6%, 90.0% and 46.9%, respectively, whereas N deposition had no significant effect. Soil fungi/bacteria ratio significantly increased by N deposition and P addition, which partly explained the reduction of SOC decomposition after P addition. The cellobioside activity significantly decreased by 48.3% after P addition, while cellobioside and β-xylosidase activities increased with N deposition, suggesting that N deposition and P addition had opposite roles in the SOC stability. These results indicate that the positive effect of N deposition on SOC decomposition was suppressed when P was added by changing microbial community and enzyme activity and enhanced P availability may result in increased SOC accumulation under N deposition scenarios in subtropical forests.