| In order to study the effects of simulated warming and acid rain on the key processes of soil carbon and nitrogen cycle in a winter wheat-soybean rotation cropland, a randomized block experiment containing 4 treatments of control (CK), warming (W), acid rain (A), and warming (W) and acid rain were set up. A portable soil CO2 fluxes system (LI-8100) was used to measure soil repatriation rates for all treatments. Soil temperature and moisture were simultaneously measured when soil respiration rates were measured. A barometric process separation (BaPS) technology was used to measured rates of soil CO2 production, nitrification, and denitrification. In order to study the mechanisms of the effects of simulated acid rain on soil microbial respiration, an in-door incubation experiment was performed. Five treatments of pH 1.0, pH 2.0, pH 3.0, pH 4.0 and control were set up. Soil microbial respiration, enzyme (urease, catalase, and invertase) activities, and pH of different treatments were measued.Results showed that different treatments had similar seasonal variation patterns of soil respiration, which were consistent with the variability in soil temperature. Seasonal mean soil respiration rates for CK, W, A, and WA treatments were 1.65 ± 0.10,1.99±0.24,1.65±0.03 and 1.54±0.12 μmol·(m2·s)-1, respectively, during the winter wheat growth season in 2012-2013 rotation year. Soil respiration rates during the soybean growth season were higher than that during the winter wheat growth season. Seasonal mean soil respiration rates for CK, W, A, and WA treatments were 4.13±0.19,4.85±0.42,3.90±0.14 and 5.01±0.36 μmol·(m2·s)-1, respectively, during the soybean growth season in 2012-2013 rotation year. Paired t-test showed that there was a difference (p= 0.065) in soil respiration between CK and WA treatments during the soybean growth season. There was a significant difference (p< 0.05)in soil respiration between W and A treatments. There was a highly significant difference (p< 0.01)in soil respiration between A and WA treatments. During the winter wheat-soybean rotation period from 2012 to 2013, there was a difference (p= 0.054) in soil respiration between CK and W treatments, and there was a highly significant difference (p< 0.01)in soil respiration between W and A treatments. Mean soil respiration rate of the CK, W, A, WA treatments were 2.04±0.22,2.60± 0.21,1.84±0.03 and 2.45±0.16 μmol·(m2·s)-1 during the 2013-2014 rotation year. W treatment had the significantly higher soil respiration rate than A treatment. Seasonal mean soil respiration rate for the CK, W, A, and WA treatments were higher than that for winter wheat wheat growth season. Mean soil respiration rates for the CK, W, A, and WA treatments during the soybean growth season were 3.31± 0.05,3.64±0.29,3.70±0.11 and 3.46±0.04 μmol·(m2·s)-1, respectively, and there was no significant difference (p> 0.1) between treatments. Soil temperature was the most important factor influencing the seasonal variability in soil respiration during the winter wheat-soybean rotation year. The relationship between soil respiration and temperature could be explained by an exponential function. The R2 for the exponential function for different treatments during the 2012-2013 rotation year was above 0.5; R2 for CK, W, A, WA were 0.593,0.630,0.609, and 0.708, which indicated that soil temperature could explain 59.3%,63.0%,60.9%, and 70.8% variations of soil respiration for the CK, W, A, and WA treatments, respectively. The R2 for the exponential function for different treatments during the 2013-2014 rotation year was above 0.276; R2 for CK, W, A, WA were 0.386,0.469,0.440, and 0.276, which indicated that soil temperature could explain 38.6%,46.9%,44%, and 27.6% variations of soil respiration for the CK, W, A, and WA treatments, respectively. A function including two factors of soil temperature and moisture for the A treatment performed better in modeling the soil respiration than the exponential function during the 2012-2013 and 2013-2014 rotation years.Further investigations based on BaPS indicated that different treatments had no significant effects on the soil CO2 production rates for the winter wheat-planted soil. Soil CO2 production rates were not significantly different between treatments. Soil CO2 production rate for the soybean growth season was close to that for the winter wheat growth season in 2013, while soil CO2 production rate for the soybean growth season was higher than that for the winter wheat growth season in 2014. This phenomena may be due to the difference in soil temperature and the different crop growth stages. A marginal significant difference could be found between warming (W) and CK treatments, and between simulated acid rain (A) and CK treatments on April 13,2013. Simulated acid rain (A) treatment increased significantly (p< 0.05) soil denitrification rate compared to the CK treatment on May 18,2014. On the other measurement dates, there were no significant differences in soil nitrification and denitrification rates between treatments. Moreover, soil nitrification was significantly correlated to soil denitrification. Soil denitrification rate increased with the increase of nitrification rate, which indicated that there was a coupling relationship between nitrification and denitrification. The substrates and products of nitrification were the products and substrates of denitrification.The results relevant to the effects of simulated acid rain on soil microbial respiration and enzyme activities in the indoor incubation experiment showed that there was no significant difference in soil microbial respiration between CK and pH 4.0 treatments. However, there were significant difference between CK and simulated acid rain treatments except for the pH 4.0 treatment. The soil respiration rates for different simulated acid rain treatments ranged as follows:pH 4.0> pH 3.0> pH 2.0> pH 1.0. These results indicated that acid rain was the important controlling factor influencing the variance of soil microbial respiration. The inhibition effects due to acid rain increased with the decrease of pH of aicd rain. In addition, there was no significant difference in soil urease and invertase activities between simulated acid rain treatments. Intensive acid rain (pH 1.0) significantly inhibited the soil catalase activity compared to other treatments, which indicated that the catalase was damaged under the intensive acid rain condition, and the inhibition effects increased with the decrease of pH of acid rain. |
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