| In terrestrial ecosystem,plant-microbial interactions in the litter layer represent one of the most relevant interactions for biogeochemical cycling as litter decomposition is a key first step in carbon and nitrogen turnover.There is still substantial debate as to the relative importance of biotic interactions on above-ground litter decomposition.The term of home-field advantage(HFA)invoked in litter decomposition studies hypothesizes that local adaptation explains higher decomposition rates of leaf litter on "home"soil communities.Pinpointing a general integrated theory regarding the mechanistic causalities of HFA is still a matter of debate,but of major importance to predict plant-soil interactions that have consequences for biogeochemical cycling.Meanwhile,plant roots constitute an important biotic factor regulating the relevant plant-soil interactions.Input of fresh root exudates can affect microbial mineralization of native soil organic matter,through priming microbial activities to synthesize additional exoenzymes,a phenomenon termed "rhizosphere priming effect"(RPE).Roots growth into the forest floor(litter layer)is common,the spatial co-occurrence of roots,leaf litters and soil microbial decomposers suggests that root mediated priming effect also affects litter decomposition,which may further determine the phenomenon of HFA.However,existing studies generally ignore the influences of plant roots on aboveground litter decomposition.Few studies have investigated the effects of root-mediated biological processes on litter degradation and relevant field-specific decomposer community.It is still unclear what factors would be involved in determining the direction and strength of RPE.,and the potential influences that roots may have in explaining HFA have not been explored by any single field study.Underlying mechanism driving their interactions is still a matter of debate.Comparative studies that integrate these interactions are needed to fill this gap.To this end,focused on the micobially-driven biodegradation process,this thesis performed both laboratory and field experiments by using a serious of microcosms incubating leaf litter of contrasting tree species.Through the experiments,the effect of root exudates on litter degradation,the influence of local root status on HFA,the enzymatic kinetics associated with carbon mineralization and nutrient release during litter decomposition,and microbial community structure of different forest sources,were systematically studied.Firstly,a laboratory pot experiment was conducted,which was the first to simulate the interactive effect of root exudate quality,soil fertility and litter quality on RPE.Three litter qualities(two recalcitrant types Pinus massoniana and Quercus variabilis,and a high quality type Robinia pseudoacacia)and two soil conditions(fertile versus barren)were involved,and the initial microbial communities were of the same in all microcosms.Solutions of chemicals often found in root exudates,i.e."exudate mimics",were added frequently into the microcosms to simulate root exudation.By comparing three exudate treatments(different C:N)with a water control,I sought to test the exudate priming effect on litter mineralization and exoenzyme activity,as well as its consequent influences on detritusphere soils.In barren soils,exudates with the lowest C:N ratio had a negative priming effect on litter C loss for R.pseudoacacia,while high C:N ratios had positive PE on litter N loss for P.massoniana.Meanwhile,low exudate C:N ratio tended to have positive PE on N-related enzymatic function in decomposing P.massoniana and Q.variabilis.In contrast,the C-only exudate promoted N-related function in decomposing R.pseudoacacia in fertile soils.The results suggested a "stoichiometric constraint" on microbial utilization of exudates in decomposing recalcitrant litters in barren soils,where positive priming effect would occur.The data will help inform explorations on the mechanistic causality of variations in root-mediated priming effect in real ecosystems.Secondly,a field experiment conducted in a 24 km2 subtropical forest.Leaf litters of P.massoniana and Q.variabilis were incubated at their conspecific-dominated and heterospecific-dominated forest stands.Root-specific in situ incubation microcosms were manipulated by using a series of root ingrowth cores embedded with different sizes of walls to control the access of living fine roots,i.e.,a 5 mm coarse mesh groups to keep all fine roots growing freely,a 0.5-mm medium mesh group to constrain the fine root biomass,and a 45-?m fine mesh size restrict all roots while permitting microorganisms to pass through.For the transplantation in P.massonforest and Q.variabilis forest,a significant positive HFA in the coarse mesh group was detected,with litters decomposed 11%faster at home than away;the effect of such HFA increased to be 34%faster at home than away in medium mesh group,but disappeared in fine mesh group.Root exclusion decelerated native litter decomposition,while hardly any significant net root effect on decomposition rate was found when litters were incubated away.Roots became more influential on the field effect(home vs.away)on litter C and N loss after 9 months than 3 months of incubation.Although microbial functions and their impact on litter mineralization depended on root status,they were not associated with the field effect within this study.In light of these novel findings,this study suggested that a moderate amount of local roots is essential for HFA in leaf litter decomposition,and taking account of root-mediated pecies-specific bio-interactions will advance our understanding of native litter-field affinity.At last,in order to understand how the vegetation type in the Zijin Mount and species-specific root type would affect microbial community assemblages,metagenomic sequencing technic was used to investigate soil samples originated from P.massoniana and Q.variabilis forest stands were reciprocally transplanted for a 1 month incubation.Root ingrowth cores were set in situ around target trees by using plastic planting baskets with railing barriers,which surrounded with or without 50 ?m nylon mesh to control root status.The results showed that among original soil-receiving combinations,56%of the operational taxonomic units(OTU)of the bacteria were widely dispersed,and 13%of the OTUs were endemic;in the other hand,only 14%of fungal OTUs are dispersed species,but 46%are specialist.Bacterial ?-diversity was higher in the soil samples transplanted away than that incubated at home;?-diversity of fungal community derived in the soil matrix derived from P.massoniana forest was significantly higher than that derived from Q.variabilis forest.Receiving forest was the main factor affecting the variation of bacterial community,while the original soil matrix was the key factor affecting the variation of fungal community.Receiving root type(interaction of receiving forest and root status)and original soil together could explain 40%,33%and 29%of bacterial community variation at phylum,genus and OTU levels respectively;these two factors could account for 40%of community variation of the most abundant fungi,and 24%of the variation of the rare fungi.Planctomyces was identified as the biomarker of P.massoniana forest,while Deltaproteobacteria and its another 5 subclasses were the biomarkers of Q.variabilis forest.Relatively,local bacteria were more dependent on the place that they were incubated,while fungi were more dependent on priority effects(succession of the original habitat).The variation pattern of microbial community in responds to the transplantation was different between different microbiota.Between-group variations showed species-specific(soil or vegetation)influences on both bacteria and fungi.Root of Q.variabilis constrained the infection of native fungi on abiotic soil substrate.The findings of this study would contribute to our further understanding of the mechanism of local soil microbial community assembly and its functions engaged in the organic decomposition. |
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