In order to study the effects of acid rain on forest soil respiration, three plots were selected in the conifer/broad-leaved mixed forests in the Three Gorges Reservoir area of the Jinyun Mountain (Chongqing) from January 2016 to April 2017. Two groups of treatments were set up:a trenched treatment and an untrenched treatment. Each group was treated with four simulated acid rain gradients of pH 4.5 (control), 4.0, 3.25 and 2.5. The characteristics of total soil respiration and heterotrophic respiration under the four simulated acid rain treatments were measured. Soil temperature and moisture were measured during the respiration measurements, and soil samples were collected to study the effects of soil pH, the carbon-nitrogen ratio, and fine root biomass on soil respiration. The results indicated that the total soil respiration and heterotrophic respiration of trenched and untrenched plots showed a similar seasonal variation trend. The annual mean soil respiration rates of the CK, T4.0, T3.25, and T2.5 treatments were 1.89, 1.88, 1.75, and 1.74 μmol·(m2·s1)-1, respectively, and the annual mean soil respiration rates of the RCCK, RCT4.0, RCT3.25 and RCT2.5 treatments were 1.37, 1.32, 1.19, 1.08 μmol·(m2·s1)-1, respectively. There was no significant differences between the quarterly average of the total soil respiration and heterotrophic respiration before June 2016 (P>0.05). However, after October 2016, the difference was significant (P<0.01) with CK > pH 4.0 > pH 3.25 > pH 2.5. The cumulative soil CO2 emissions of the T4.0, T3.25, and T2.5 treatments in 2016 decreased by 3.89%, 9.64%, and 11.24% respectively, compared with the control, and the RCT4.0, RCT3.25, and RCT2.5 treatments decreased by 6.79%, 13.23%, and 25.56%, respectively. The simulated acid rain treatments reduced the ratio of heterotrophic respiration in the total soil respiration, and the degree of reduction increased with the pH of the simulated acid rain, indicating that the effect of acid rain on the inhibition of heterotrophic respiration exceeded that of autotrophic respiration. Although the simulated acid rain treatments increased the temperature sensitivity of soil respiration (Q10), it had no significant effect on soil temperature and humidity (P>0.05). The soil carbon-nitrogen ratio and fine root biomass were significantly greater in comparison to the control after October 2016. Soil respiration was significantly positively correlated with fine root biomass and significantly negatively correlated with the soil carbon-nitrogen ratio. Soil temperature and water contributed only slightly to soil respiration under the four simulated acid rain treatments. Fine root biomass and the soil carbon-nitrogen ratio were critical factors for variation of soil respiration under acid rain.
Keywords: Q10(temperature sensitivity); heterotrophic respiration; pH; simulated acid rain; soil carbon-nitrogen ratio; soil respiration.