Effects of simulated acid rain on soil CO2 emission in a secondary forest in subtropical China

Acid rain, which is caused mainly by dissolution of sulfur dioxide (SO2) and nitrogen oxides (NOx) in the atmosphere, has been reported to have negative effects on ecosystems. However, few investigations have focused on the impacts of acid rain on soil CO2 emission in forest. In this study, the effects of simulated acid rain (SAR) on soil respiration (R-s) and its heterotrophic component (R-h) in a secondary forest in subtropical China were investigated. Soil CO2 efflux was measured by using a Li-8100 infrared gas analyzer with attached chamber. Measurements were generally made once a week from 21 March 2010 to 16 May 2011 in order to investigate the seasonal variations of R-s and R-h under different SAR treatments. Soil temperature and moisture at the depth of 5 cm were measured at the time of soil CO2 efflux measurements. Results indicated that different SAR treatments exhibited similar seasonal patterns of R-s and R-h. Seasonal mean R-s rates for the CK (deionized water), A1 (pH 4.0), A2 (pH 3.0) and A3 (pH 2.0) treatments were 2.63, 1.92, 1.89 and 2.16 mu mol m(-2) s(-1), respectively, while mean R-h rates for the four treatments were 1.80, 1.64, 1.76 and 1.79 mu mol m(-2) s(-1), respectively. Two-factor analysis (respiration components and SAR) of variance implied that SAR had significant (p = 0.031) effects on soil CO2 emissions, but this was contingent on the specific respiration components. SAR showed significant inhibition effects on R-s (autotrophic + heterotrophic components) rather than R-h. The ratio of R-h to R-s was significantly higher in the CK than in the acid rain treatments (A1, A2 and A3). Soil temperature and moisture were two controlling factors regulating the seasonal patterns of R-s and R-h for each of the SAR treatment. Soil temperature and moisture accounted for more than 80% of the seasonal variations observed in R-s and R-h. This work highlights that the effects of SAR are important to consider in assessing the annual soil CO2 emission, particularly under the scenario of increasing acid rain pollution. (C) 2012 Elsevier B.V. All rights reserved.