Effects of simulated acid rain on soil respiration and its components in a subtropical mixed conifer and broadleaf forest in southern China

Soil respiration is a major pathway in the global carbon cycle and its response to environmental changes is an increasing concern. Here we explored how total soil respiration (R-T) and its components respond to elevated acid rain in a mixed conifer and broadleaf forest, one of the major forest types in southern China. R-T was measured twice a month in the first year under four treatment levels of simulated acid rain (SAR: CK, the local lake water, pH 4.7; T1, water pH 4.0; T2, water pH 3.25; and T3, water pH 2.5), and in the second year, R-T, litter-free soil respiration (R-S), and litter respiration (R-L) were measured simultaneously. The results indicated that the mean rate of R-T was 2.84 +/- 0.20 mmol CO2 m(-2) s(-1) in the CK plots, and R-S and R-L contributed 60.7% and 39.3% to R-T, respectively. SAR marginally reduced (P = 0.08) R-T in the first year, but significantly reduced R-T and its two components in the second year (P < 0.05). The negative effects were correlated with the decrease in soil microbial biomass and fine root biomass due to soil acidification under the SAR. The temperature coefficients (Q(10)) of R-T and its two components generally decreased with increasing levels of the SAR, but only the decrease of R-T and R-L was significant (P < 0.05). In addition, the contribution of R-L to R-T decreased significantly under the SAR, indicating that R-L was more sensitive to the SAR than R-S. In the context of elevated acid rain, the decline trend of R-T in the forests in southern China appears to be attributable to the decline of soil respiration in the litter layer.