| Litter is an important intermediate that link above-and below-ground systems in forest ecosystem.Litter decomposition is driven by litter quality and decomposer activity,both of which are directly and indirectly impacted by climate.As one of the aboveground soil decomposers,saprophagous macroarthropods,such as woodlice,play important roles during litter decomposition process by regulating the activity and population dynamics of belowground decomposer community-the microorganisms.Sodium as an essential nutrient element for detritivores and microbial decomposers,is very low in concentration in plant litter,thus decomposers are often prone to sodium limitation in inland forests.Presently,little is known about how long-term Na subsidies affect litter decomposition and how soil fauna and microbial decomposers respond to Na subsidies in forest systems.Besides,with the increase of atmospheric nitrogen(N)deposition in recent decades,extensive researches into the consequence of this anthropogenic nitrogen pollution on litter decomposition system has been triggered.However,up to now,the majority of experiments on soil biotic responses to atmospheric N deposition focused only on microbial communities and microfauna,comparably less is known regarding belowground responses at higher trophic level,such as woodlice.As such,researches on the response of both soil macrofauna and microorganisms to atmospheric N deposition and sodium limitation,and exploring the potential feedback mechanisms that could modify the rate of litter decomposition is an important task.The study was conducted in the Zijin Mountain in East China.At first,a microcosm experiment was carried out to assess the function of macroarthropoda by exploring the changes and differences in litter decomposition under isopods(Armadillidium vulgare)and its by-products(three contrasting qualities of faeces)treatments.In the experiment,three different litter types were chosen as food of isopods:Pinus massoniana in a coniferous forest(CF),Quercus acutissima in a broad-leaved forest(BF)and Robinia pseudoacacia trees sparsely distributed in the two forests.Correspondingly,the three types of faeces produced by isopods were:P.massoniana faeces with low quality,Q.acutissima faeces,and R.pseudoacacia faeces with high quality.P.massoniana needles and Q.variabilis leaves were chosen as experimental materials and treated by five treatments(PF,QF,RF,isopods,and deionized water control)for six months.Second,a field experiment was conducted to exploring how soil fauna and soil microbes' decomposers respond to Na subsidies and affect litter decomposition in inland forest systems.In the experiment,two dominant forest types,CF and BF,were chosen from Zijin Mountain.Litterbags with 5 mm and 0.2 mm mesh were used to distinguish the relative influences of the major decomposers(i.e.,the invertebrate macro-and meso-fanua)and micro decomposers(micro-fauna and microbes)on decomposition to different levels of sodium subsides.Tree levels of sodium subsides(0.005%,0.05%and 0.5%water solution of NaCl)were used to fertilize the large and small mesh size litterbags in CF and BF every month for 8 months.At last,another microcosm experiment was performed to study response of both soil macrofauna and microorgniams to atmospheric N deposition and explore the potential feedbacks that could modify rates of litter decomposition in subtropical forests of China.The experiment was designed to have two types of litter(Q.acutissima leaves and P.massoniana leaves).P.massoniana needles form CF and Q.variabilis leaves form BF were incubated in microcosms with original forest soils and treated by four treatments consisting of only adding deionized water(control),adding isopods,adding NH4NO3,and adding both NH4NO3 and isopods.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:(1)The indirect effects of isopods were not significant on broad-leaf forest;all three types of faeces did not significantly increase broad-leaf litter decomposition.However,isopod faeces fed on high-quality legume litter significantly increased needle litter decomposition.The effects of isopod faeces on microbial activities were significant:most of the soil microbial biomass and extracellular enzymatic activities increased significantly compared with non-faeces treatments in the soil of broad-leaved and coniferous forests;and high-quality faeces strengthened the correlation of litter mass loss with peroxidase and phenol oxidase in coniferous forest.The result suggested that faeces with higher concentrations of nitrogen and labile carbon(C)could mediate the decomposition of refractory materials like lignin and phenolic,which might be highly relevant for the litter decomposition process in ecosystems with high macrofauna abundance.This study infers that the indirect effect of isopods(faeces)on litter decomposition depends on the quality of litter they consumed and the type of decomposing litter.Isopods indirectly accelerate the nutrient-poor litter decomposition process by converting high-quality litter into faeces.(2)All levels of sodium subsidy increased litter decomposition of the two forests,with increases ranging from 16.4%to 29.9%over controls,suggesting that sodium is limiting to decomposers in the subtropical forest of East China.Sodium subsidy increased the litter decomposition contribution of invertebrate detritivores,but decreased the contribution of soil microbes.However,low-level Na subsidies(0.005%)elevated the biomass of soil microorganisms by 9.46%,but not detritivore,activity in recalcitrant litter.Despite enhanced litter mass loss from invertebrate detritivores,we found decreased microbial enzyme activities involved in C,N and phosphorus(P)cycling under high(0.5%)Na subsidies.This suggests that at 0.5%of Na,microbes experience Na-toxicity and mineralization of C,N,and P nutrients likely decreased in forest ecosystems.These results suggest that sodium concentration has direct impacts on carbon cycling from subtropical forest litter,neither Na shortage nor Na excess would be unfavorable for carbon-release.Further,this study provides evidence that sodium limitation can enhance terrestrial carbon storage.(3)Inorganic N fertilization accelerated the decomposition rate of broad-leaf litter,but not for that of needle litter.Isopods feeding accelerated litter decomposition while decreased the soil microbial biomass and soil extracellular enzymatic activities.However,N addition decreased this feeding effect,with a mass loss of 8.64%lower than that of only isopods feeding treatment in the broad-leaved forest.Moreover,N fertilization inhibited the activities of microbial decomposers related to lignin and phenolic.This study suggests that N deposition slow down litter decomposition process mediated by soil macrofauna-microbe interaction.This study implies that in the long term,soil carbon may be accumulated because of the slowing decay rate of litter under N deposition.According to the results of this study,soil C in subtropical forests would accumulate as a long-term consequence of continued inorganic N fertilization.This may presumably alter the balance of ecosystem C flux,nutrients cycling,and humus formation,which may,in turn,have multiple effects on forest ecosystems. |
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