Effects Of Simulated Acid Rain On The Roots And Soil Ecological Processes Of A Subtropical Broadleaf Phyllostachys Edulis Mixed Stand

Acid rain,one of the global ecological crises caused by human activities,has a significant impact on forest plants,soil and ecosystem structure and functions.However,the effects of acid rain on the belowground ecological processes of forest ecosystems,especially on the structure and metabolism of soil microbial communities,are poorly understood.The subtropical region is one of the largest regions affected by acid rain in China,where broadleaved moso-bamboo(Phyllostachys edulis)mixed forest is a widespread forest type with relative stability;however,the effect of acid rain on the roots and soil ecological processes and their driving mechanisms are not clear.In this study,I conducted an acid rain simulation experiment in the National Natural Reserve of Tianmu Mountains in subtropical China(30°18′30″N～30°21′37″N,119°24′11″E～119°27′11″ E)during the period of 2016-2018.The experiment included 3-level acid rain treatments,i.e.,T1(pH=4.0),T2(pH=2.5)and CK(pH=5.5).Investigated the root morphologies and biomass distribution patterns of bamboo and the broad-leaved trees with the root coring method,soil microbial biomass and community structure with the fumigation method and the high-throughput sequencing method,respectively,and soil respiration with the infrared gas exchange method(Li-8100 IRGA).Using these data,together with the soil chemical and biological properties,I aimed at reveal the effect of acid rain on the belowground ecological processes of the broadleaf bamboo mixed forest and the underlying mechanisms.The main results were as follows:1.The rain-acid treatments significantly affected both horizontal and vertical distributions of the forest trees.Acid rain significantly reduced the total root length,the total root surface area,and the total root volume of the broad-leaved tree species by 40-60%,39-63%,and 42-62%,respectively,while it less reduced these traits of the bamboo trees by 4-23%,-5%(i.e.,increased)-16%,and 2-30%,respectively.Horizontally,the root systems of the broadleaved trees under the acid rain treatments were gradually decreasing with increasing the distance away from the sample tree,while those of the bamboo trees were not.Vertically,the root distribution of the bamboo trees tended to increase in the surface layer under the acid rain treatments,while that of the broad-leaved trees did not.In addition,T1 treatment had no significant effect on the specific fine-root length for the bamboo trees,but T2 treatment significantly reduced it for both bamboo and broad-leaved trees,indicating that the inhibition of acid rain on fine roots increases with the intensity of acid rain.The acid rain treatments significantly decreased the root-node distances of both the first-and second-order lateral roots of the bamboo trees,but it increased the number of their lateral roots.2.The acid-rain treatments had a significant influence on both soil chemical and biological properties of the broad-leaved bamboo mixed forest.The acid rain treatments reduced soil pH,but increased the total soil nitrogen concentration,soil carbon to nitrogen ratio,soil organic carbon concentration,dissolved soil organic carbon concentration,alkali-hydrolyzable soil nitrogen concentration;these effects in T2 treatment was greater than those in T1 treatment.The treatments reduced the soil microbial biomass carbon and microbial biomass nitrogen by 18-42%and 16-32%,respectively,but they increased the activities of soil catalase,sucrase,and urease by 24-59%,18-51%,and 32-63%,respectively.T1 and T2 treatments reduced the soil cellulose activity by 12%and 28%,respectively.3.The acid-rain treatments reduced the α-diversity of soil bacterial communities,but increased the α-diversity of soil fungal communities.A total of 455,392 bacterial sequences were obtained and classified into 34 known bacterial phyla,among which Proteobacteria,Acidobacteria,Chloroflexi and Actinobacteria were the dominant phyla(relative abundance>1%);the relative abundances of these four phyla differed significantly(P<0.01)among the treatments.Arthrobacter and Elsterales were very sensitive in their relative abundances to acid rain and thus could serve as indicators for changes in the soil bacterial community structure.A total of 601,287 fungal sequences were obtained and classified into 13 known fungal phyla,among which Ascomycota,Basidiomycota,Mortierellomycota and Mucoromycota were the dominant phyla(relative abundance>1%);the relative abundances of the four phyla differed significantly(P<0.05)among the treatments.Similarly,Bifiguratu,Geminibasidium,Purpureocillium and Oidiodendron could serve as indicators for changes in soil fungal community structure under acid-rain stress largely due to the sensitivity in their relative abundances.Soil pH and the total nitrogen concentration significantly influenced the soil bacterial and fungal diversity and their community structure.4.Soil respiration was increasing at the early stage and decreasing afterwards,as the simulated acid-rain experiment proceeded.The respiration rates were not significant among the treatments at the initial stage of the experiment in February,2017;and then increased.With the cumulative effect of acid rain,soil respiration significantly decreased in March,2018 by 13%and 26%for T1 and T2 treatments,respectively.Additionally,the acid rain treatments increased the temperature sensitivity of soil respiration.All these effects of acid rain on soil respiration were attributed to acid-rain induced changes in the root biomass,soil chemical properties,enzyme activity,microbial biomass and community diversity,rather than those in soil temperature and moisture.In conclusion,the simulated acid rain treatments had profound effects on the root morphology,root distribution pattern,soil chemical properties,enzyme activity,microbial biomass,microbial community composition and structure of the broad-leaved bamboo mixed forest,which consequently affectedsoil respiration,the major soil carbon cycling process.These effects tended increased with the increase of simulated acid-rain intensity.These findings not only deepen the understanding of the biogeochemical cycling processesin forest ecosystems under global change scenarios,but they also are of significance for maintaining the stability of subtropical forest ecosystems and assessing their ecological and environmental risks.