Effects Of Nitrogen And Acid Deposition On Litter Decomposition In Subtropical Forest Ecosystem

With the frequent human activities and the rapid economic development in recent decades,there are induced climate and environment change in global scale,such as the elevated atmospheric nitrogen(N)and acid deposition,which are expanding problems that affect the functioning and composition of forest ecosystems,particularly the decomposition system of forest litter.To date,numerous studies have investigated the effects of single type of N or acid rain on litter decomposition in forest ecosystems.However,natural atmospheric N and acid deposition contain a set of complex compounds.Atmospheric N deposition includes not only inorganic N(e.g.,NH4+ and NO3-)deposition but also organic N(e.g.,urea and amino acid)deposition.Natural acid rain includes not only sulfuric acid rain(SAR)but also nitric acid rain(NAR).Thus,single type of N or acid rain addition may not accurately reflect their ecological effects on the litter decomposition in forest ecosystems.Studying and identifying the effects of different types of N and acid deposition on litter decomposition is an important task.The study was conducted in the Zijin Mountain in Nanjing,China.At first,a microcosm experiment was carried out to assess the changes and differences in litter decomposition under different forms of N addition between leguminous and non-leguminous forests.In the experiment,two litter types were chosen as experimental materials:Robinia pseudoacacia leaves from a leguminous forest(LF)and Liquidambar formosana leaves from a non-leguminous forest(NF).The litter samples were incubated with original forest soils and treated by five forms of N addition(NH4+,NO3-,urea,glycine,and a mixture of all four)for six months.Second,a field experiment was conducted to determine the responses of soil microbial biomass and enzyme activities to different forms of organic N fertilization between coniferous forest(CF)and broad-leaved forest(BF).In the experiment,two dominant forest types,CF(dominated by Pinus massoniana)and BF(dominated by Quercus variabilis),were chosen from Zijin Mountain.Five forms of organic N(urea,glycine,serine,nonylamine,and a mixture of all four)were used to fertilize the soils in CF and BF every month for 1 year.At last,another microcosm experiment was performed to study the effects of sulfuric,nitric,and mixed acid rain on litter decomposition,soil microbial biomass,and enzyme activities in subtropical forests of China.Pinus massoniana needles form CF and Quercus variabilis leaves form BF were incubated in microcosms with original forest soils and treated by five types of acid rain with different SO42-to NO3-ratios(1:0,5:1,1:1,1:5,and 0:1)for six months.During the incubation period,litter mass losses,soil microbial biomass,soil pH,and enzyme activities were investigated.The results of this study are as follows:(I)All forms of N addition accelerated the litter decomposition rates of LF and NF,and the litter decomposition rates under mixed N addition were significantly higher than those under single form of N additions.Mixed N and NO3--N addition significantly accelerated the litter decomposition rates of LF leaves,while mixed N,glycine-N,and urea-N addition significantly accelerated the litter decomposition rates of NF leaves.Soil enzyme activities under mixed N addition were higher than those under single form of N additions in the two forest types.N addition had no significant effects on soil pH and soil microbial biomass.The results indicate that N addition may alter microbial allocation to extracellular enzyme production without affecting soil microbial biomass,and then affected litter decomposition process.The results further reveal that mixed N is a more important factor in controlling litter decomposition process than single form of N,and the mixed N-induced acceleration of litter decomposition rates may induce carbon(C)stored belowground to be transferred to the atmosphere as greenhouse gas carbon dioxide(CO2),which may alter the balance between C uptake and release.(2)The microbial biomass and enzyme activities of soils fertilized with urea and amino acids increased significantly,whereas those fertilized with nonylamine and mixed N decreased significantly in CF and BF.Urea and amino acid fertilizations had more positive influence on enzyme activities of BF than on those of CF.Nonylamine fertilization had more negative influence on enzyme activities of CF than on those of BF.Organic N fertilization shifted soil microbial biomass away from the excretion of N-degrading enzymes and toward the excretion of C-degrading enzymes in the two forest types.These results suggest that organic N type is an important factor that affects soil microbial biomass,soil enzyme activities,and their relationship.Organic N deposition may seriously affect soil C and N cycling,as well as CO2 releasing from the soils by influencing microbial activities and biomass.This study thereby provides evidence that soil microorganisms have strong feedbacks to different forms of organic N deposition.(3)Different types of acid rain inhibited litter decomposition,soil microbial biomass,and most enzyme activities of CF and BF,and the inhibitory effects of NAR were more significant than those of SAR and MAR.The resistance to external acid of microbial communities in BF was higher than that in CF.NAR and MAR treatments slowed down soil C,N,and phosphorus(P)mineralization by attenuating the correlations between litter mass losses and the enzymes involved in C,N,and P cycling.Results reveal that the ratio of SO424 to NO3-in acid rain is an important factor which profoundly influences litter decomposition process.In the future,a decreasing ratio of SO2-to NO3-in acid rain will be observed with the rapid economic growth in subtropical forests.Thus,soil C would accumulate as a consequence of future acid precipitation.This accumulation may seriously alter the balance of ecosystem C flux,nutrients cycling,and humus formation,which may,in turn,have multiple effects on forest ecosystems.