Effects Of Long-term Simulated Acid Rain On Soil Respiration And Enzyme Activity In A Subtropical Forest

In order to intestigate the effects of long-term simulated acid rain on soil respiration and enzyme activity in the north subtropical secondary forest,taking the forest in the northern suburb of Nanjing as an example,the field observation experiment was conducted from 2015-10 to 2017-9.The split plot experiment was set up,in which the root treatment and continuous root treatment were set up,and the trenches were dug around the root-cut quarter to prevent the root enter the root-cut area.Each split plot contains 4 simulated acid rain treatments,contral CK,pH4.0(A1),pH3.0(A2)and pH2.0(A3).Each split plotwas lm*lm.In 2015-10 to 2017-09,The Li-8100 portable soil carbon fluxes were used to measure the soil respiration rate and the constant cell heterotrophic respiration rate in the root plot of the experiment field.The soil temperature and humidity were observed at the same time.In addition,the soil enzyme activities(invertase,urease and catalase)between different experimental plots were observed.ANOVA showed that the heterotrophic respiration rate under different simulated acid rain treatments in root-cut plot and the soil respiration rate under different simulated acid rain treatments in the continuous root areahad obvious seasonal variation regularity,which were high in summer and low in winter.During the period from 2015 to 2017,the average soil respiration rate of CK,A1,A2 and A3 in the continuous root zone were2.16±0.29,2.40±0.33,2.17±0.13,2.34±0.35?mol(m2 s)-1.The average soil respiration of simulated acid rain treatment was significantly higher than that of the control group.The average heterotrophic respiration rates of CK,Al,A2 and A3 treatments in 2015-2017 werel.55±0.09,1.43±0.09,1.65±0.07,1.72±0.24?mol.(m2 s)-1.The average heterotrophic respiration of simulated acid rain treatment was significantly higher than that of the control group except A1.ANOVA showed that there was no significant difference in heterotrophic respiration between different treatments(P>0.05)from 2015-11 to 2017-10.The variation of soil moisture with seasons was similar between different simulated acid rain treatments in root-cut plot and different simulated acid rain treatments in continuous root plot.Whether in the untrenchingor trenching,There was a significant exponential regression relationship between soil respiration and heterotrophic respiration and soil temperature in different acid rain treatments.Soil temperature was the most important factor that determines the seasonal variations of soil respiration and heterotrophic respiration in continuous root plot and root-cut plot.There was no significant relationship between soil respiration and heterotrophic respiration and soil moisture among different acid rain treatments.According to the regression analysis,there was a very significant relationship between heterotrophic respiration and soil respiration(P<0.001),andthe ratios of soil respiration to heterotrophic respiration of CK,A1,A2,A3 in different acid rain treatments were 0.671,0.550,0.737 and 0.777,respectively.From 2015-12 to 2017-09,with the seasonal change and the rising of temperature,the activities of soil urease,invertase and catalase in root-cut plot and continuous root plot showed the same trend as temperature after different simulated acid rain CK,A1,A2,A3 treatments.Meanwhile,with the decrease of pH value,the enzyme activity decreased in varying degrees.The relationship between soil heterotrophic respiration and soil temperature was studied under different simulated acid rain treatments with 5 adjacent temperature points(5?,10?,15 ?,20 ?,25 ?),and the exponential equation was used for fitting analysis.The results showed that cumulative soil heterotrophic respiration increased with the increase of temperature,and there was a significant(P<0.05)positive proportional relationship between soil heterotrophic respiration and soil temperature.The determination coefficient R2 of the regression equation was above 0.81,and the fittingdegree of the equation was better.The relationship between soil heterotrophic respiration and soil urease,invertase and catalase activities at different temperatures and simulated acid rain was studied.Experimental studies have shown that there was a significant(P<0.001)linear regression relationship between soil heterotrophic respiration and these three enzymes.Considering all soil conditions such as simulated acid rain pH and temperature,the R2 of the activities of urease invertase and catalase was 78%,92%and 92%,respectively.There was a significant(P<0.001)linear regression relationship between heterotrophic respiration and DOC under different temperatures and simulated acid rain conditions.And DOC can account for 56.8%variability of cumulative soil heterotrophic respiration.