Abstract: Soil respiration in forest is a key process that underlies our understanding of the terrestrial carbon cycle, and therefore, its response to acid rain is an increasing concern. Here, we investigated this issue in three subtropical forests of different successional stages \[i.e. a young pine forest (PF), a transitional mixed conifer and broadleaf forest (MF) and an oldgrowth broadleaved forest (BF)\] at the Dinghushan Nature Reserve in southern China. Simulated acid rain was applied semimonthly from June 2009. Soil respiration was measured under four simulated acid rain (SAR) treatments (CK, the local lake water, pH 4.5; T1, pH 4.0; T2, pH 3.5; and T3, pH 3.0) from April 2012 to March 2013. Generally, SAR reduced soil respiration in the forests, but this reduction varied with forest type, SAR application level, and measurement season. The repeated measures ANOVA showed that SAR did not affect soil respiration in the PF (P>0.05), but it significantly reduced soil respiration in the BF (P<0.05) and the reduction was marginally significant in the MF (P=0.10). Compared with the CK, mean annual soil respiration in treatments T1, T2, and T3 was reduced by 0.1%, 10.5%, and 17.1% in the BF, -1.7%, 8.1%, and 13.9% in the MF, and 1.1%, 1.9%, and 8.1% in the PF, respectively. The sensitivity of the response of soil respiration to SAR showed an increasing trend with the progressive succession of forests. In the BF, there were no significant differences among the CK, T1, and T2 treatments, while the T3 treatment was significantly lower than the CK and T1 treatments (P<0.05). These negative effects were evident in the warmwet season (P<0.05), but not in the cooldry one. The depression of soil respiration in the MF and the BF was related to the reduction of microbial activity caused by soil acidification under the SAR. Moreover, consistent with the situation of soil respiration, the response sensitivity of soil acidification and the reduction of microbial activity showed an increasing trend with the progressive succession of forests.

Key words: climate change., statistical experiment, Lancang River, abrupt change, hydrological time series