Nitrogen addition reduces soil respiration in a mature tropical forest in southern China.

Response of soil respiration (CO2 emission) to simulated nitrogen (N) deposition in a mature tropical forest in southern China was studied from October 2005 to September 2006. The objective was to test the hypothesis that N addition would reduce soil respiration in N saturated tropical forests. Static chamber and gas chromatography techniques were used to quantify the soil respiration, following four-levels of N treatments (Control, no N addition; Low-N, 5 g N m−2 yr−1; Medium-N, 10 g N m−2 yr−1; and High-N, 15 g N m−2 yr−1 experimental inputs), which had been applied for 26 months before and continued throughout the respiration measurement period. Results showed that soil respiration exhibited a strong seasonal pattern, with the highest rates found in the warm and wet growing season (April–September) and the lowest rates in the dry dormant season (December–February). Soil respiration rates showed a significant positive exponential relationship with soil temperature, whereas soil moisture only affect soil respiration at dry conditions in the dormant season. Annual accumulative soil respiration was 601±30 g CO2-C m−2 yr−1 in the Controls. Annual mean soil respiration rate in the Control, Low-N and Medium-N treatments (69±3, 72±3 and 63±1 mg CO2-C m−2 h−1, respectively) did not differ significantly, whereas it was 14% lower in the High-N treatment (58±3 mg CO2-C m−2 h−1) compared with the Control treatment, also the temperature sensitivity of respiration, Q10 was reduced from 2.6 in the Control with 2.2 in the High-N treatment. The decrease in soil respiration occurred in the warm and wet growing season and were correlated with a decrease in soil microbial activities and in fine root biomass in the N-treated plots. Our results suggest that response of soil respiration to atmospheric N deposition in tropical forests is a decline, but it may vary depending on the rate of N deposition. [ABSTRACT FROM AUTHOR]