Response Of Soil Environment And Soil Respiration To Acid Rain In Typical Forest Stands In Jinyun Mountain

Acid rain is a global problem,especially in southern China.In forest ecosystem,acid rain can cause soil acidification,destroys nutrient cycling process,and has a significant impact on forest vegetation,soil chemical properties,enzyme activity,microorganisms,and soil respiration.In this study,a simulated acid rain experiment was carried out in mixed coniferous-broadleaved forest and evergreen broad-leaved forest in a forest ecological station of Jinyun Mountain,Chongqing in the Three Gorges Reservoir area,which is a typical acid rain area in southwestern China.The method of combining field monitoring with indoor index determination was used to observe biological and abiotic indicators such as soil chemical properties,enzyme activity,microbial activity and community structure,and soil respiration.The purpose is to understand the variation characteristics of soil environment under acid rain and explore the response mechanism of soil respiration and its components to a series of biological and abiotic factors.The results are as follows:(1)The influence of acid rain on soil temperature and humidity was not significant.(2)The Soil pH and hydrolytic nitrogen were significantly reduced by simulated acid rain in the later stage of the experiment,while organic matter content and C/N ratio were significantly increased,but the effects on total nitrogen and total phosphorus were not obvious.Soil fine root biomass under different acid rain treatments was promoted in the early stage of the experiment and inhibited in the later stage.(3)The activities of soil sucrase,urease and acid phosphatase in evergreen broad-leaved forest were higher than those in mixed coniferous-broadleaved forest,and cellulase activity was lower than that in mixed coniferous-broadleaved forest.Soil sucrase,urease and acid phosphatase activities were significantly inhibited by acid rain at the later stage of the experiment,and cellulase activity was significantly promoted.(4)There was no significant difference in soil microbial biomass carbon and biomass nitrogen under simulated acid rain treatment.There were significant differences in microbial community structure between different forest stands and different simulated acid rain treatments.The relative abundance of fungi PLFAs in mixed coniferous-broadleaved forest was significantly increased by acid rain.Total bacteria were significantly reduced by acid rain.Actinomycetes PLFAs had no significant difference.The PLFAs markers in evergreen broad-leaved forest was similar to that in mixed coniferous-broadleaved forest,but there was no significant difference between simulated acid rain in total bacteria PLFAs.Redundancy analysis(RDA)showed that only soil C/N ratio and organic matter content contributed to the variation of soil microbial community structure.(5)The effect of simulated acid rain on soil respiration and heterotrophic respiration were significant,and this effect was cumulative.The effects of acid rain treatment on monthly mean soil respiration and heterotrophic respiration were not significant in the early stage(January-March 2016),but gradually inhibited in the later stage.The annual mean and cumulative soil respiration responsed to simulated acid rain in the two typical stands was comparatively consistent.There was no significant difference between control and pH 4.0 treatment in 2016.The pH 3.25 and pH 2.5 treatments were significantly inhibited,and all acid rain treatments were inhibited in 2017.The response of annual mean and cumulative heterotrophic respiration to simulated acid rain was different between the two forest stands.There were significant differences among different simulated acid rain treatments in 2016 and 2017 in mixed coniferous-broadleaved forest.But there was no significant difference among control and pH 4.0 treatment in evergreen broad-leaved forests,pH 3.25 and pH 2.5 treatment were significantly inhibited.With the increase of acid rain acidity,the proportion of heterotrophic respiration to soil respiration gradually decreased in the two typical forest stands.Soil temperature in two typical forest stands was the main factor affecting seasonal variation of soil respiration and heterotrophic respiration.The Q10 value of soil respiration(temperature sensitivity)increased with the decrease of acidity.Soil respiration and heterotrophic respiration were positively correlated with soil pH,fine root biomass,hydrolytic nitrogen,urease,and sucrase activity,and negatively correlated with soil organic carbon,carbon-nitrogen ratio,fungi and the ratio of fungi to bacteria.The correlation analysis showed that soil acidification caused by acid rain had a negative effect on soil carbon and nitrogen cycle,and changed the decomposition and utilization efficiency of microorganisms for organic matter and nitrogen.Soil microbial community structure adjusted to the change of soil environment,and soil enzyme activity changed synchronously.These factors together drove the viriation of soil respiration under simulated acid rain.(6)Principal component analysis,grey correlation analysis and entropy-weighting DTOPSIS method were used to evaluate the effect of acid rain on soil environment.Comprehensive analysis showed that the simulated acid rain treatments of pH 2.5 and pH 3.25 had the greatest negative effect and the most significant damage to soil environment.