Effects of elevated CO2 and nitrogen addition on soil organic carbon fractions in a subtropical forest

Aims The aim of this study was to investigate the effects of elevated CO 2 concentration and nitrogen addition on soil organic carbon fractions in subtropical forests where the ambient N deposition was high. Methods Seedlings of typical subtropical forest ecosystems were transplanted in ten open-top chambers and grown under CO 2 and nitrogen treatments. The treatments included: 1) elevated CO 2 (700 μmol mol -1 ); 2) N addition of 100 kg NH 4 NO 3 ha -1 yr -1 ; 3) combined elevated CO 2 and N addition; and 4) control. We measured soil total organic carbon (TOC), particulate organic carbon (POC), readily oxidizable organic carbon (ROC), and microbial biomass carbon (MBC). Results Results showed that elevated CO 2 alone did not significantly affect soil TOC, POC and ROC after 4 years of treatment, but increased soil MBC and soil respiration compared to the control. N addition alone had no significant effect neither on soil TOC, POC and ROC, but decreased MBC and soil respiration over time. However, the elevated CO 2 and N addition together significantly increased soil POC and ROC, and had no significant effect on soil MBC. Conclusions This study indicated that even in N-rich subtropical forest ecosystems, inputs of N are still needed in order to sustain soil C accumulation under elevated CO 2 .