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

Abstract

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 μmol 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 (Q₁₀) 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.