Abstract: Biochar has aroused great attention due to its stable chemical properties and the potential benefits for soil carbon balance in terrestrial ecosystems. A large amount of biochar derived from fires accumulated in forest soils. However, little is known about the priming effect induced by biochar on soil organic carbon (SOC) decomposition, and how such effect is regulated by nitrogen availability. Here, we added biochar (B, 5% of SOC) and ammonium nitrate (0, 5%, 10% of soil total nitrogen, coded as N0, N5, N10 respectively) to the soil in a subtropical evergreen broadleaved forest to investigate the responses of priming effects induced by biochar to nitrogen addition. Results showed that nitrogen addition had little effect on biochar decomposition, with about 1.0%, 1.0%, and 1.1% of biochar being decomposed in the treatments BN0, BN5 and BN10, respectively. In addition, the priming effects induced by biochar were 9.0±2.1, 8.3±2.4, 5.9±0.8 g C·kg^-1 SOC in BN0, BN5 and BN10, respectively. Biochar significantly accelerated SOC decomposition, thus caused strong positive priming effect (47.2%). There was no effect of nitrogen addition on SOC decomposition during incubation (P>0.05), whereas the nitrogen effect on priming effect was related to incubation stage. The extent of priming effect increased from 40.3% to 63.1% at the early stage, but decreased from 51.1% to 17.4% at the late stage due to nitrogen addition. The net carbon balance induced by biochar addition was generally positive regardless of nitrogen addition. Our results suggested that biochar might play an important role in soil carbon sequestration in subtropical forests and may alleviate climatewarming in future.

Key words: biochar, nitrogen addition, soil organic carbon decomposition, priming effect, carbon balance.