Spatial-Temporal Pattern And Microbiological Mechanisms Of Forest And Farmland Litter Decomposition At Home And Away Sites In Subtropical China

Litter decomposition is a key biochemical process critical to both carbon（C）and nutrient cycling in terrestrial ecosystems,and plays a crucial role in determining the global C balance,maintaining the material cycle of ecosystems,and securing the stability of agricultural and forestry ecosystem productivity.Microbial communities are the engines of decomposition,but they are often considered as inert receptors that are controlled by climate and litter quality.Therefore,existing decomposition models largely ignore their active participation in the regulation of organic matter（OM）decomposition,which limits our in-depth understanding of biogeochemical cycles under global environmental change.In order to better understand the environmental adaptation strategies of microorganisms in the context of future environmental changes and their impact on litter decomposition and soil ecosystem function.Here,we collected four litter species（forest:Schima superba and Castanopsis fargesii,farmland:Zea mays and Eleusine indica）and soils（0～10 cm soil layer）from the evergreen broadleaved forest and the arable fields with Z.mays located at the Jiulianshan National Observation and Research Station of Chinese Forest Ecosystem in subtropical China.The two study sites represent the two important ecosystems in China,i.e.,the subtropical forest and farmland ecosystems,respectively.We designed a reciprocal transplant field experiment with litter and soil from two ecosystems with contrasting climatic conditions.In each ecosystem,a completely randomized experiment was constructed with 4 replicates of 8 treatments,including[forest soil+forest litter],[forest soil+farmland litter],[farmland soil+forest litter]and[farmland soil+farmland litter],to explore the dynamics of litter decomposition patterns,microbial characteristics,and soil ecological processes and functions at different times.On this basis,the potential mechanism of‘home-field advantage（HFA）’of litter decomposition mediated by microorganisms can be revealed,and the hypothesis of‘Greedy species release（GSR）’is proposed to explain the phenomenon（negative or zero effect of HFA）contradictory to the HFA theory,in order to provide a common explanation mechanism.This study helps to reveal the process and control mechanism of litter decomposition,and to improve the ability to predict the response of terrestrial ecosystems to environmental changes.The main results of this work are as follows:（1）The average mass loss rate of litters decomposed at home sites was 19.5%higher than that of decomposed at away sites.The HFA of litter decomposition was confirmed,and the HFA of S.superba and C.fargesii litter decomposition were higher than that of Z.mays and E.indica litter decomposition.The average mass loss rate of litter decreased by23.61%after the forest soil was transplanted from the forest study site with suitable temperature and humidity to the farmland study site with high temperature and dryness,and the average mass loss rate of litter increased by 29.28%after the farmland soil was transplanted from farmland to forest study site.GSR existed in litter decomposition,and GSR decreased HFA index（HFA_i）,where HFA_iin the farmland study site<HFA_iin the forest study site HFA_i.Litter N and P contents were significantly positively correlated with litter mass loss rate（p<0.05）and showed an enrichment-release process to a certain extent with decomposition time,which are the key indicators to predict the decomposition rate of litter.（2）The decomposition of S.superba at away sties reduced theα-diversity of bacteria and fungi,and the relative abundance of bacterial taxa such as Acidobacteria,Chloroflexi and fungal taxa such as Basidiomycota,Glomeromycota increased.The decomposition of Z.mays at away sties increased theα-diversity of bacteria and fungi,and the relative abundance of bacterial taxa such as Proteobacteria,Actinobacteria and fungal taxa such as Ascomycota,Mucoromycota increased.The accumulation of specific microbiomes by litter is the fundamental reason driving HFA.Theα-diversity of bacteria and fungi decreased after the forest soil was transplanted from forest to farmland study site.The high temperature and dryness environment inhibited the growth of the bacterial taxa such as Acidobacteria and Proteobacteria,as well as dominant fungal taxa such as Basidiomycota and Mortierellomycota.On the contrary,the number of bacterial ASVs（Amplicon sequence variants）and the relative abundance of dominant bacterial and fungal groups（phyla）increased after the farmland soil was transplanted from farmland to forest study site,which led to the acceleration of substrate utilization rate by‘hungry’microorganisms.The GSR hypothesis was supported by a decrease in the positive correlation between the relative abundance HFA_iof dominant bacterial and fungal taxa and litter decomposition HFA_i,after soil reciprocal transplant of forest and farmland study site soils.（3）The molecular ecological network（MEN）topological parameters of the soil decomposed by S.superba litter,such as total nodes,total links,and average connectivity,were lower than those of the same type of soil decomposed by Z.mays.Similarly,the MEN robustness,compositional stability and node persistence of the soil decomposed by S.superba litter were lower than those of the same type of soil decomposed by Z.mays.There was a positive correlation between the MEN topological parameters and stability indexes in different soil treatments during the decomposition of litter.Network complexity and stability had stronger positive correlations in soils decomposed by Z.mays compared with decomposed by S.superba.In addition,the MEN topological parameters of different soils treatment were positively correlated with soil multifunctionality parameters such as litter decomposition,soil C and nutrient contents,and soil enzyme activities.Network complexity and soil multifunctionality parameters have a stronger positive correlation in soils decomposed by Z.mays compared with decomposed by S.superba litter.（4）The SOC increment（△SOC）of litter decomposition in home site was lower than that in away site as a whole,and the△SOC in the early stage of litter decomposition was significantly lower than that in the late stage of litter decomposition.The△SOC of forest soil decomposed by S.superba and C.fargesii litter decreased after forest soil was transplanted from forest to farmland study site,while the△SOC of forest soil decomposed by Z.mays and E.indica litter increased after forest soil was transplanted from forest to farmland study site.The△SOC of farmland soil decreased after farmland soil was transplanted from farmland to forest study site regardless of litter species.Soil total nitrogen（TN）and total phosphorus（TP）showed zero or negative growth patterns due to the low amount of litter addition and leaching.The soil TN and TP increment（△TN and△TP）of litter decomposition at home site was lower than that of litter decomposition at away site.The soil△TN and△TP in forest study site were lower than those in farmland study site.Soil C-,N-and P-cycle enzyme activities of litter decomposition at home site were higher than those of litter decomposition at away site.Soil enzyme activities in forest ecosystem were higher than that in farmland ecosystem.Soil enzyme activities were positively correlated with soil moisture（SM）,NH₄⁺-N,SOC and TN,and negatively correlated with soil p H and NO₃^–-N.In summary,the litter decomposition is controlled by both HFA and GSR,the ecological effect of their relative intensity is ultimately reflected in the actual response strategies of soil microorganisms.Soil microorganisms are very sensitive to litter species and environmental changes,and show a variety of environmental response strategies,such as adaptation,dormancy and death.The relative abundance of major bacterial and fungal taxa（phyla）were increased in suitable temperature and humidity in forest ecosystem,resulting in enhanced microbial decomposition capacity.Conversely,bacterial and fungalα-diversity and their relative abundances were reduced in high temperature and dryness in farmland ecosystem,which caused to the microbes to become‘inactive and inefficient’.Nutrient availability changes the ecological interactions of taxa to form a new association network.High-quality litter decomposition makes a more complex and stable MEN,due to an equilibrium of stable co-existence and niche partitioning of dominant taxa arising from competitive exclusion,and vice versa.Network complexity can beget more stable network and stronger ecological function.Litter decomposition at home site and suitable temperature and humidity environments promoted C mineralization and nutrient release,while high temperature and dryness environments reduced C and nutrient turnover in the ecosystem.This study highlights the important role of microorganisms independent of climate and litter quality and their response strategies to environmental changes.The results enhance the understanding of the spatial-temporal pattern and microbiological mechanisms of forest and farmland litter decomposition,and provide theoretical support for terrestrial ecosystem management under environmental changes.