Effects Of Simulated Acid Rain On The Key Processes Of Soil Carbon And Nitrogen Cycle In A Subtropical Secondary Forest

A four years field experiment was performed from March 2010 to October 2014 in order to investigate the effects of simulated acid rain on the key krocesses of carbon and citrogen cycle in a subtropical secondary forest. Split-plot design was arranged in field. Four blocks were arranged, and there were trenched and un-trenched plots in each block. Trenching, which can exclude roots, was performed around the trenched plots. Each split plot included four simulated acid rain treatments, which are the CK (deionized water), Al (pH4.0), A2 (pH3.0) and A3 (pH2.0). There were a total of 32 micro-plots assigned in field, with each micro-plot area measuring 1 m×1 m. From March 2010 to October 2014, a portable soil CO2 fluxes system (LI-8100) was used to measure soil respiration and heterotrophic respiration in the untrenched and trenched plots, respectively. And soil respiration under different simulated acid rain treatments was measured from November 2012 to October 2014. Soil temperature and soil moisture were simultaneously observed when measuring soil respiration rates. The Barometric Process Separation (BaPS) method was used to measure soil CO2 production rates, nitrification rates and denitrification rates in December 2012, May 2013, March 2014 and September 2014. In 2014, the indoor soil incubation experiment was conducted.5 treatments which were deionized water, pH4.0, pH3.0, pH2.0 and pH1.0 were set up, and each treatment was set with 3 replicates. Simulated acid rain (10mL) was added to the corresponding treatments. Soil microbial respiration rates was measured on 1,2,4,6, 8,11,15,18, and 22 days after adding simulated acid rain. The soil urease, invertase, and catalase activities, and pH value were measured after the incubation experiment,.Results showed that the heterotrophic respiration in the trenched plots and the soil respiration in the un-trenched plots had the same seasonal pattern. Soil respiration in the un-trenched plots was significantly (P<0.001) higher than that in the trenched plots. Mean soil respiration rate in un-trenched plots and mean heterotrophic respiration rate in trenched plots were (2.59±0.48) and (1.74±0.28) μmol·m2·s)-1, respectively, during the 4 years experiment period. There was no significant (P> 0.05) difference in mean soil respiration rate or mean heterotrophic respiration rate between measurement years. The relationship between heterotrophic respiration (y) and soil respiration (x) could be fitted with a proportion function. Heterotrophic and autotrophic respiration contributed 65.9% and 34.1% to the soil respiration, respectively. The main contributor to soil respiration was heterotrophic respiration. The relationship between the ratio of heterotrophic respiration to soil respiration and measurement date could be fitted with a linear function. An exponential function could be used to fit the relationship between heterotrophic respiration and soil temperature, and between autotrophic respiration and soil temperature. The temperature sensitivity coefficient (Q10) for heterotrophic respiration was lower than that for autotrophic respiration.The simulated acid rain experiments showed that, from Novermber 2012 to October 2014, mean soil respiration rates for the untrenched plots were (2.49±0.35)、 (2.89±0.24)、(2.27±0.31)、(2.44±0.22)、μmol·(m2·s)-1 for the CK, A1, A2 and A3 treatments, respectively. ANOVA analysis indicated that there was no significant (P>0.05) difference between simulated acid rain treatments. From Novermber 2012 to October 2014, mean soil respiration rates for the trenched plots were (1.64±0.06)、 (1.58±0.08)、(1.78±0.14)、(1.96±0.14) μmol·(m2·s)-1 for the CK, A1, A2 and A3 treatments, respectively. ANOVA analysis showed that there was significant (P< 0.05) difference in heterotrophic respiration between Al and A3 treatments, indicating that intensive simulated acid rain leaded to higher heterotrophic respiration rates than light simulated acid rain. The relationship between soil respiration and soil temperature, and between heterotrophic respiration and soil temperature for all treatments could be modeled with exponential functions. Soil temperature explained most of the seasonal variances of soil respiration and heterotrophic respiration, indicating that soil temperature was the most important factor influencing soil respiration and heterotrophic respiration. Both in the trenched and un-trenched plots, in general, the simulated acid rain treatments had no significant impact on the soil nitrification and denitrification.Further investigations showed that, for all simulated acid rain treatments, the accumulative soil microbial respiration increased with the increase of incubation days. The accumulative soil microbial respiration was 0.202 mg·g-1 during the 29 days experiment. The accumulative soil microbial respiration for the simulated acid rain treatments ranged from 0.097 to 0.166 mg·g-1. In general, soil microbial respiration decreased with the decline of pH of the simulated acid rain added, indicating that soil microbial respiration was inhibited by the addition of simulated acid rain. Intensive simulated acid rain significantly inhibited the catalase activity and soil pH, while simulated acid rain showed no significant effects on soil urease and invertase activities.