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-level sof N treatments (Control, no N addition; Low-N, 5 g N m^-2yr^-1; Medium-N, 10 g N m^-2yr^-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 CO_2-Cm^-2yr^-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 CO_2- Cm^-2h^-1, respectively) did not differ significantly, whereas it was 14% lower in the High-N treatment (58 ±3 mg CO₂-Cm^-2h^-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.