Effects Of Acid Rain On Soil And Fine Root Of Typical Plantation In Yangtze River Delta Region

Acid rain is one of the major environmental problems that affect human survival at present and in the future.Southern China has become the third largest region with heavy acid deposition after Europe and North America,and the Yangtze River Delta region?YRD?is one of the heaviest acid rain regions.Acid rain had negative effects on forest ecosystem,such as decreased the growth rate of trees,improve the sensitivity of forest ecosystems to climate change,and change the composition of species,which caused forest degradation.To solve the problem of acid rain,implementation of existing policy in China will lead to reductions in SO₂.However,those of NO_x will continue to rise due to the rapidly growing number of motor vehicles,even under tentatively proposed control measures,which lead to acid rain is still a serious problem.The type of acid rain will change from sulfuric acid rain?SAR?to sulfuric and nitric mixture acid rain?MAR?or nitric acid rain?NAR?.Hence,it is particularly urgent to understand the difference between the effects of SAR?MAR and NAR on the forest ecosystem in YRD region.In this study,1-year-old saplings of Quercus acutissima and Cunninghamia lanceolata,mature plantations of Q.acutissima and C.lanceolata were treated with different SO₄^2-/NO₃^-?S/N?ratios of acid rain from 2015 to 2016 over a period of 12 months.The aim was to research the effects of acid rain acidities?pH 4.5,3.5 and2.5?and types?SAR?MAR and NAR?on physiological properties of sapling,litter decomposition and the fine-root growth,and combined with the analysis of soil properties and microbial community,and then explore the comprehensive response mechanism of forest trees to acid rain and the key factors that lead to the decline of forest ecosystem under the stress of acid rain.We found that:?1?The annual average pH values of precipitation and the annual average frequency of acid rain were 4.87±0.28 and 57.12±18.67%in YRD region.The content of SO₂ in the atmosphere significantly decreased year by year.However,there were difference for the change trendency of NO₂ content among different cities,such as the content of NO₂ decreased slowly in Shanghai and Nanjing,and obviously increased in Suzhou and Jiaxing.The precipitation pH values were still influenced by SO₂ in YRD region,and the S/N ratio in precipitation has fallen from 7.5 in the1990s to 2.0.The type of acid rain has been changer from SAR to MAR in YRD region.?2?Acid rain S/N ratio significantly influenced the Chlorophyll a?Chla?and Chlorophyll b?Chlb?contents of Q.acutissima sapling,and acid rain pH significantly decreased the Chla and Chlb contents of C.lanceolata sapling,and Chla and Chlb contents with acid rain S/N 1:5 were significantly lower than those with S/N 1:0 at pH 2.5.The chlorophyll fluorescence parameter,maximal efficiency of Photosystem II photochemistry?Fv/Fm?,and catalase?CAT?activity were significantly affected by acid rain,and the tolerability of Q.acutissima leaves to acid rain NO₃^-was lower than that of C.lanceolata leaves.In addition,acid rain NO₃^-had effects of fertilization on the growth of sapling roots,especially under the effects of acid rain with pH 3.5,and the fertilization effects of NO₃^-on Q.acutissima roots were higher than those on C.lanceolata roots.However,the fertilization effects of NO₃^-under stronger acid rain would reduce.?3?Fine root growth was more sensitive to the effects of acid rain in the short-term.Both soil pH and fine root biomass?FRB?significantly decreased as acid rain pH decreased and decreased with the percentage of NO₃^-increased in acid rain.In addition,weaker acid rain would promote the growth of fine root,and the effects of promotion on Q.acutissima fine root were higher than those on C.lanceolata fine root.MAR and NAR significantly increased fine-root superoxide dismutase?SOD?activity in spring,and catalase activity was more sensitive to acid rain stress than SOD and peroxidase?POD?activities.The contents of potassium?K?,calcium?Ca?,aluminum?Al?and the ratios of Ca/Al and Mg/Al in fine roots of Q.acutissima and C.lanceolata were more sensitive to acid rain acidity and S/N ratio.The structural equation modelling results showed that acid rain S/N ratio and pH had stronger direct effects on FRB than indirect effects via changed soil and fine root properties.Overall,fine-root element contents and antioxidant enzymes activities were significantly affected by acid rain S/N ratio and pH during most seasons.Fine-root Al ion content,Ca/Al,Mg/Al ratios and catalase activity could be used as better indicators than soil parameters for evaluating the effects of different acid rain S/N ratios and pH on forests.?4?The effects of acid rain on litter decomposition and soil microbial in Q.acutissima plantation were positive in the early period of the experiment,except for SAR on soil microbes.Soil pH with NAR decreased more rapidly with the amount of acid rain increased in summer than with SAR treatments.Only stronger acid rain?both SAR and NAR?was capable of depressing litter decomposition and its inhibitory effect was stronger on leaf than on fine root litter of Q.acutissima.Meanwhile,NAR had a higher inhibitory effect on litter decomposition than SAR.In summer,autumn and winter,total microbial biomasses were negatively impacted by the increased acidity level resulting from both SAR and NAR.However,higher acidity level of NAR?pH=2.5?had the strongest inhibitory impact on soil microbial activity,especially in summer.In addition,Gram-negative bacteria?cy19:0?and fungi?18:1?9?were more sensitive to both SAR and NAR,and actinomycetes was more sensitive to SAR intensity.Finally,soil total carbon,total nitrogen and pH were the most important soil property factors affecting soil microbial activity in Q.acutissima plantation,and high microbial indices?fungi/bacteria?with high soil pH.?5?The cy19:0,18:1?9,i15:0,10Mel16:0 and i16:0 were the main microbial biomarkers that caused the difference of microbial community between Q.acutissima and C.lanceolata plantations,while acid rain with lower acidity increased this difference.In addition,the inhibitory effect of SAR on fine-root decomposition of Q.acutissima was higher than that of C.lanceolata.Fungi had stronger effects on fine-root decomposition both of Q.acutissima and C.lanceolata.However,fine-root decomposition of Q.acutissima was also sensitive to the changes of other soil microbial,such as Gram-negative bacteria and actinomycetes.Acid rain?SAR?significantly reduced the C,N,K,Ca and Mg loss during decomposition,and the residual mass of fine-root litters was closely related to the residual C and K both of Q.acutissima and C.lanceolata.Overall,Yangtze River Delta region is suffering the stress of acid rain,and the SO₄^2-/NO₃^-ratio of acid rain decreased year by year.The effects of acid rain on sapling growth,soil microbial community,litter decomposition and fine-root growth were difference due to species,acid rain acidity and S/N ratio.In the early period of the experiment,NO₃^-had fertilization effects.However,NO₃^-would increase the inhibitory effects of acid rain on forest ecosystem with acidity and stress time increase.Fine-root growth was more sensitive to the changing of acid rain type and acidity,could be used as superior indicators for evaluating the effects of acid rain on forest ecosystems.In future,the change of acid rain types from SAR to MAR or NAR might complicates the ongoing challenge of forest ecosystem stability and increase the risks to the forest ecosystem functioning in YRD region.