Twelve years of simulated acid rain does not change soil carbon stock but reduces soil carbon turnover rate in a subtropical forest
As a widespread environmental issue, acid rain has a significant impact on the global carbon (C) cycle. Soil is considered a potential C sink and acts as a buffer against climate change. However, the responses of soil C storage and turnover processes to long-term acid rain remain unclear. In this study, a 12-year simulated acid rain (SAR) experiment was conducted in a monsoon evergreen broad-leaved forest in subtropical China. Four levels of experimental treatments were implemented by irrigating the plots with acidic solutions of different pH values: CK (control treatment; natural rain, pH ≈ 4.5), T1 (pH = 4.0), T2 (pH = 3.5), and T3 (pH = 3.0). The 13C natural adundances along soil profile (0–40 cm) were traced to evaluate the soil C turnover rate in situ. Results showed that both T2 and T3 treatments decreased litter production and soil respiration rate simultaneously, whereas T1 treatment had no significant effects on the soil C input and output. Soil C stocks did not differ among different experimental treatments; however, the soil C turnover rates in T1, T2, and T3 were significantly lower than those in CK. Interestingly, under the SAR treatments, the reduction in soil C turnover rate was correlated with decreased soil pH and enhanced soil exchangeable Al3+ content and fungi/bacteria ratio. Our results demonstrate that twelve years of SAR does not change soil C stock but reduces soil C turnover rate in subtropical forests. This finding helps to elucidate the response mechanism of soil C pool to acid rain, and emphasizes the importance that the effects of acid deposition on soil C turnover should be considered when constructing global climate-carbon models.