Response of Soil Respiration to Nitrogen Addition in Two Subtropical Forest Types*1

Abstract Anthropogenic activities have increased the quantity of nitrogen (N) deposition in terrestrial ecosystems, which directly and indirectly affected soil biogeochemical processes including soil respiration. However, the effects of increasing N availability on soil respiration are still not fully understood. In this study, soil respiration was measured using an infrared gas analyzer system with soil chambers under four N addition level treatments (0, 5, 15 and 30 g N m −2 yr −1 as control, low N addition (LN), mediate N addition (MN) and high N addition (HN), respectively) in Camphor tree and Slash pine forests in subtropical China. Results showed that soil respiration rates were reduced in the fertilized plots respect to control plots on average by 37% in the Camphor tree forests and 27% in the Slash pine forests on an annual base, respectively. No significant differences were found in soil respiration between the LN, MN and HN treatments in both forest types because these fertilized plots reached an ‘adequate N content zone’. In addition, soil microbial biomass carbon (C) content and fine root biomass were declined in N addition plots compared to the control during the study period. Our results indicated that elevated N deposition might alter the patterns of tree growth, C partitioning and microbial activity, which further affect soil C sequestration through reducing soil respiration in subtropical forests of China.