The Microbial Mechanism Of Quercus Wutaishansea Litter Decomposition On The Loess Plateau,china

Litter,the link between soil ecosystem and plant ecosystem,is an important resource of soil nutrients which can provide nutrients for the growth of soil microbial community and plants via decomposition process.Soil and litter microbes were the main drivers for the decomposition process.Broad-leaved forest ecosystem represents the significant global carbon sinks in the world.Consequently,understanding their ecology is very important for predicting and managing the recycles of C and their impacts on climate change.Organisms play critical roles in the carbon sequestration and carbon cycling,as they are the main drivers of C efflux in forest ecosystem.During the ongoing decomposition process,bacteria and fungi have different functions.Fungi are considered as the most important decomposers,especially in the early decomposition stage.Currently,we lack fundamental knowledge on how microorganisms function during the decomposition of plant litter,especially for the identification of the specific roles of individual taxa and different functional groups.To well understand the microbial processes of litter decomposition,it's essential to address the activity of both bacteria and fungi in litters.In the present study,we setup a litter decomposition experiment to explore the relations between the bacteria and the litter properties at different decay stages.We chose Quercus wutaishanica litter as studied subject to explore the effects of litter positions(on the surface of litters,on the surface of soil humus,on the surface of soils without litters and soil humus)on the variations of soil and litter microbes.We analyzed litter quality,mass loss,microbes in soils and litters and litter organic carbon functional groups to investigate the changes during different decomposition time.These results could provide the scientific basis for element recycling and soil humus formation,better understanding of the geochemical cycles of elements during the decomposition process,which would help to well manage the forests during the vegetation restoration process.The main conclusions are congregated as the following:(1)The results from the field litter decomposition experiment showed that litter layers had significant effects on the litter decomposition process.The mass loss ranged from 18% to 23% with the highest value for the litter on the surface of humus,which indicated that the litter placing on the surface of humus decomposed fastest than others.The litter organic carbon content decreased linearly during the ongoing decomposition process,while the total nitrogen content was enhanced gradually,especially in the early stage.The increasing rate for the litter nitrogen kept stable during the later stage.Litter total P content increased during the early stage and decreased during the later decay stage.There was no significant difference for litter total P content between different layers.Litter total K content showed a similar trend with total P content.The litter mass loss was negatively significantly related with litter organic carbon content,and positively significantly related with litter total N and P contents.The relation between litter mass loss and litter total K content was not significant.Litter microbial biomass showed an increase during the early decay stage and a decline during the later decay stage,suggesting that the microbes enhanced the decomposition with the increase of microbial biomass.(2)We used three equations to simulate the decomposition process including Olson model,Exponential model and Logarithmic model.The correlation coefficients and the significant results showed Logarithmic model was the best one to describe the decomposition process.The decomposition constant showed that the decomposition speed was the fastest for the litter placing on the surface of humus and the litter placing on the soil decomposed slowest.(3)Litter micro elements differed from the decomposition process.Different micro elements showed different changed trends.After 340 days' decomposition,the contents of Ca,Mg,Mn and Zn in plant litter indicated an increase at the early stage and a reduction at the later decay stage.The release rates were in the order of Ca>Mg>Mn>Zn.The contents of litter Fe and Cu were increased in the entire decomposition process.The accumulation of litter Fe was higher than litter Cu.Plant litter Ca,Mg and Mn contents were the main influence factors which controlled the decomposition process.There were significant relations between micro elements(Ca,Mg and Mn)and litter mass loss,especially for litter Mn content.With the decomposition processed,the micro elements gradually released.(4)The litter C chemistry was analyzed using the solid state 13 C CPMAS NMR spectroscopy.The data showed that the 13 C NMR spectra was a better method to evaluate the variations of litter decomposition.In this study alkyl C and O-alkyl C were the main components,which were accounted for 70% of the total abundance.Different C components had different changed trends during the ongoing decomposition process.The relative abundance of carbonyl C and aromatic C were increased,and the highest values were detected in the later decay stage.For the whole decay stage,the relative abundance of alkyl C and O-alkyl C were decreased finally.The decomposition of alkyl C and O-alkyl C caused the increase of the relative of aromatic C which suggested that the litter C components were more complex with much more aromatic components.The increase of aromatic C led a slow decomposition speed in the later decay stage.(5)During the decomposition process,the bacterial diversity continued to increase until the end of the experiment.The species index increased from 378 at summer(after 30 days)to 614(after 120 days)at autumn and then decrease to 486 at the end of the decomposition(340 days,spring).Shannon index was significantly increased during 30 and 120 days and then remained relatively stable(120-340 days).The ANOVA showed that all indices were influenced by the sampling dates,suggesting a remarkable seasonal succession for the litter bacterial diversity.(6)Actinobacteria(16.8-24.7%),Alphaproteobacteria(38.7-48.4%),Betaproteobacteria(18.4-38.9%)and Acidobacteria(2.3-7.8%)were the most abundant bacterial taxa throughout the entire experiment(?1% in all 12 samples).Firmicutes,Saccharibacteria,Armatimonadetes,and Chlamydiae were also identified with lower abundances(<1%)in the present study.Alphaproteobacteria and Gammaproteobacteria showed no significant difference between the sampling time.The relative abundance of Betaproteobacteria showed a significant decline from 38.9%(30 days)to 18.4%(340 days).The Actionbacteris were mainly represented by Frankiales,Kineosporiales,Micrococcales and Propionibacteriales.The clear bacterial communities in plant litter were observed at different decomposition time,suggesting a remarkable succession for litter bacterial structure.As expected,the close clustering of plant litter samples at winter and autumn sampling dates indicated similar bacterial community compositions.The samples at summer were distinctly separated from other sampling dates.With the decomposition progressed,the average Bray-Curtis dissimilarity of bacterial communities increased from 24.35%(after 30 days)to 32.26%(after 340 days).The ANOSIM analysis also showed that the bacterial communities were significantly different between the different sampling dates or seasons(R = 0.645,P = 0.001).These observations were confirmed that the litter decomposition had significant effects on litter bacterial composition.The Dist LM analysis revealed that litter pH(21.0%),TN(14.2%),TC(10.6%)and TP(18.1%)significantly affected litter bacterial communities,with 64.9% of the total explanation in the sequential model.In the sequential model,the C:N ratio,C:P ratio and N:P ratio of the plant litter didn't significantly influence litter bacterial communities.The dbRDA analysis revealed that the explanations for the first two axes were 31.9% and 25.1% for the fitted model,and 27.9% and 21.9% of the total variation.The samples clearly separated on a seasonal gradient indicating that the bacterial decompositions were distinct at different decomposition time.The vectors of the environmental drivers also confirmed that plant litter TN was the most important affecting factor.(7)Litter fungal diversity varied seasonally which significantly increased at the early decomposition stage and then remained stable.Ascomycota and Basidiomycota were the dominant phyla.Cylindrium was the most dominant genus which played important roles in decomposition.The relative abundance of Cylindrium was significant higher at the decomposition time of 120 days than other decomposition time.The results of SEQUENTIAL TESTS showed that litter pH,total N and total P contents were the main influence factors,with the explanations of 17.3%,14.2% and 13.1%.Litter total K,organic carbon and the ratios of C:N,C:P and N:P had no significant effects on litter fungal communities.(8)Plant litter decomposition significantly affected soil C and N fractions,especially for soil available N contents(nitrate N,dissolved N and microbial biomass N).Litter amount was an important affecting factor.Litter amount had no significant effects on soil bacterial diversity,while litter amount significantly influenced soil bacterial communities.There was significant difference between normal treatment and double and control treatments for the bacterial communities.Beta,Gamma,and Deltaproteobacteria were significantly decreased in the normal quantity litter addition treatment,and subsequently increased in the double quantity litter addition treatment.The soil available nutrients and the soil copiotrophic bacterial communities were higher in the control and the double quantity of litter decomposition treatments.These results suggested that litter addition affected the soil bacterial structure and can provide guidance to manage vegetation restoration with the increase of litter quantity.(9)There was a distinct pattern in bacterial phyla distribution between leaf litter and soil in both regions.Soil samples were dominated by Acidobacteria(10.7% on average),Proteobacteria(28.6% on average),Actinobacteria(19.4 % on average),Firmicutes(19.0% on average),and Bacteroidetes(5.3% on average).Leaf litter showed a very high abundance of Proteobacteria(65.5% on average)and Actinobacteria(27.3% on average),followed by Bacteroidetes(2.3% on average)and Acidobacteria(3.6% on average).There was no regional difference in phylum distribution for both leaf litter and soil.The bacterial diversity indices showed that soil samples had higher diversity than litter samples.The bacterial community compositions in outdoor samples were distinct from the indoor samples.Outdoor soil samples had significant higher relative abundance of Firmicutes but lower relative abundance Proteobacteria and Gemmatimonadetes.The differences between outdoor litter samples and indoor litter samples were not more distinct than soil samples.At the phylum level,there was no significant differences for Proteobacteria,Actinobacteria,Planctomycetes,Bacteroidetes,Acidobacteria,Firmicutes,Chloroflexi and Verrucomicrobia.The above results indicated that litter and soil microbes were the main drivers of litter decomposition caused the differences between different litter layers.The increase of microbial biomass and diversity enhanced the rapid decomposition during the early decay stage.The variations of litter microbial communities were main causes leading to the difference of decomposition process along the time.Litter bacterial and fungal communit y had a significant difference between different decomposition seasons.During the ongoing decomposition process,the diversity of litter bacteria and fungi increased gradually with the highest value after 120 days' decomposition.During the later stage(120-340 d),the bacterial and fungal diversity became stable with a small range and the values were not significant between different decomposition dates.The variations of microbial diversity suggested that litter decomposition enhanced the growth of microbes and changes of microbial communities.The DISTLM and RDA analysis showed that litter total N,P and pH were the main factors controlled the succession of microbial community during the litter decomposition process on the Loess Plateau.