Effects Of Warming And Simulated Acid Rain On Soil Respiration And Enzyme Activities In A Cropland

In order to investigate the effects of warming and simulated acid rain on soil respiration and enzyme activities, randomized block experiments were performed in field. There were six treatments, which were control (CK), warming (W), simulated acid rain with pH 4.0 (CK-4.0), warming and simulated acid rain with pH 4.0 (W-4.0), simulated acid rain with pH 2.0 (CK-2.0), and warming and simulated acid rain with pH 2.0 (W-2.0). There were four replicates for each treatment. 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. Soil urease, invertase, and catalase activities were measured during different crop development stages. In order to investigate the effects of simulated acid rain and straw amendments on microbial respiration of farmland soil, an indoor incubation experiment was performed. Four simulated acid rain treatments, which were CK (pH=6.3), A1 (pH=4.0), A2 (pH=3.0), and A3 (pH=2.0) were set up. There were four straw amendments treatments for each simulated acid rain treatment. There were three replicates for each treatment. Soil microbial respiration, invertase activity, and catalase activity were measured.Results indicated that soil respiration rates for different treatments had the similar seasonal variability patterns, which was in accordance with the variability in soil temperature. During the 2013～2014 winter wheat-soybean rotation season, soil respiration rates for CK, W, CK-4.0, W-4.0, CK-2.0, and W-2.0 treatments were 3.3±0.05,3.40±0.06,3.20±0.13,3.06±0.11,2.83±0.03, and 2.79±0.05μmol·(m2·s)-1, respectively. ANOVA showed that there were significant (p<0.05) differences in soil respiration rates between CK and W-4.0, between CK and CK-2.0, and between CK and W-2.0 treatments. Soil respiration rate in the W treatment was significantly (p< 0.05) higher than that in the CK treatment. However, soil respiration rate in the simulated acid rain and simulated acid rain and warming treatments was significantly (p<0.05) lower than that in the CK treatment. During the 2014～2015 winter wheat-soybean growth season, soil respiration rates for CK, W, CK-4.0, W-4.0, CK-2.0, and W-2.0 treatments were 3.06±0.06,2.98±0.04,2.88±0.06,3.05±0.08, 2.89±0.05,2.70±0.03μmol·(m2·s)-1, respectively. There were significant (p<0.05) differences in soil respiration rates between CK-4.0 and CK, between CK-2.0 and CK, and between W-2.0 and CK. The variation patterns of soil respiration in the 2014～2015 growth season was similar with that in the 2013～2014 growth season. Overall, warming enhanced soil respiration. Intensive simulated acid rain (pH 2.0) inhibited soil respiration rates, while simulated acid rain treatment with pH 4.0 had no significant effects on soil respiration. In both of the two rotation years, the seasonal variability in soil respiration rate was affected by soil temperature. The exponential regression functions can be used to explain the relationship between soil respiration and soil temperature.During the 2014 soybean growth season, there were no significant (p>0.05) differences in urease and invertase activities between treatments. There was significant (p<0.05) difference in catalase activity between CK and other treatments. Simulated acid rain inhibited urease and invertase activities and enhanced catalase activity. At the turning green stage in the 2014～2015 winter wheat growth season, warming treatment enhanced soil urease activity. Simulated acid rain had no significant (p<0.05) effects on urease activity. Warming enhanced the invertase activity at the overwintering and turning green stages, while simulated acid rain inhibited the invertase activity at these two stages. Simulated acid rain inhibited the catalase activity at the overwintering stage. Warming enhanced the urease activity at the trefoil stage in the 2015 soybean grow season. Simulated acid rain had no significant (p<0.05) effects on urease activity. Warming inhibited the invertase activity at the trefoil and branching stages, but enhanced the catalase activity at the trefoil stage. The relationship between seasonal mean soil respiration rate and urease activity in the 2014～2015 winter wheat grow season can be explained (P=0.046) by an linear regression fucntion. The relationship between seasonal mean soil respiration rate and invertase activity during the same time period can also be explained (P=0.072) by an linear regression fucntion. Urease and invertase activities were two important factors to explained the variations in soil respiration among treatments.Further investigations showed that soil microbial respiration for different simulated acid rain and straw amendment treatments had highly significant (p< 0.001) difference. Simulated acid rain and straw amendment treatments had no significant (p=0.892) coupling effects on soil microbial respiration. Further investigations indicated that simulated acid rain could inhibit soil microbial respiration, and the inhibition effects increased with the increase of simulated acid rain concentration. Straw amendments can promote soil microbial respiration, and the promotion effects were more obvious for the treatments with high amount of straw added. The relationship between soil microbial respiration and invertase activity can be explained by a significant (p=0.028) linear function, The relationship between soil microbial respiration and catalase activity can also be fitted by highly a significant (p<0.001) linear function. This results indicated that invertase and catalase were good indicators of the magnitude of soil microbial respiration.