Biochar Amendment Effects On Microbial Abundance,Activity And Functioning In Croplands

The role of biochars produced via pyrolysis of agro-wastes in soil and management,particularly in remediation of polluted soils,has been widely and increasingly concerned.Amendment of biochars has been known to directly and indirectly affect soil microbial abundance and communitity composition,and inturn biological performance and functioning for nutrient cycling in agriculture.Among these,biochar's effects on manipulating soil organic matter decomposition and terrestrial C cycling has been widely addressed so far.However,the responses of biochar in soil microbes varied due to the differences of factors such as soil conditions,biochar types,biochar amendment rates and experiment types and length.Moreover,although being successful in pollution remediation,the effects of biochar on soils microbes in heavy metal polluted agricultural soils were still unknown and few attention was paid to the effects on mechanisms of these effects.Therefore,this paper firstly using a meta-analysis assessed the effects of soil microbial activities,and analyzed factors influencing microbial response to biochar,including biochar production and soil condition and experiment condition.And then,a field study was added to study the effects of biochar on soil microbial activities when agricultural soils were under heavy metal stress.It is well known that rhizosphere microbes play important roles in stimulating plant growth,nutrient uptake and alleviating heavy metal phytotoxicity.In order to elucidate the comprehensive effect of biochar on soil environment quality and provide scientific basis of biochar application in situ remediation for heavy metal contaminated soils,in the field studies,soil microbial community activity and structure in rhizosphere and bulk soil were characterized,combined with soil enzyme assay.The main results were as follows:(1)Using a meta-analysis of experiment data retrieved from literature published up to March 1,2015,changes were examined in microbial biomass and soil respiration in agricultural soils with biochar addition.Microbial responses to biochar addition were quantified in soil respiration quotient(RQ),microbial quotient(MQ)and metabolic quotient(qCO2)and their differences were evaluated between with and without biochar addition,and among groups of biochar production conditions and experiment conditions.There was an overall increase by 25%in soil microbial biomass carbon(SMBC)and nitrogen(SMBN)but a decrease by 13%in qCO2,under biochar compared to the control.Whereas,microbial biomass carbon was increased by 26%but total soil CO2 production unchanged,across all short term experiments up to 6 months following a single biochar addition.Microbial responses to biochar addition to agricultural soils were much uncertain with respect to both biochar and experiment condition.A significant reduction(by<20%)in qCO2 was found under crop residue and manure biochars in term of feedstock,and biochars pyrolyzed at high temperature over 500? in term of pyrolysis temperature.Whereas,the reduction was great(by over 30%)both in clay soils and in neutral soils but moderate(by 15%)in soil organic carbon(SOC)depleted soils,respectively in terms of soil texture,reaction and SOC level.Thus,soil conditions exerted great impacts on microbial metabolic quotient changes compared to biochar conditions.(2)The two-year field study was conducted from Octorber,2013 to May,2015 in a barren and alkaline cropland contaminated with Cd and Pb in Jiyuan,Henan province.The soil is a typical calcareous soil derived from paleo-deposits from the Yellow River and is classified as a calcic Aqui-alluvic Primisol in Chinese Soil Taxonomy.The crops were winter wheat and maize in rotation and winter wheat was sown in late October and harvested early June the following year.Biochar amendment was conducted in a maize field in October,2013,after maize was harvested and two weeks before wheat sowing.The treatment was biochar application of 0,20 and 40 t ha-1(C0,C1 and C2)respectively,using the zero amendment as the control.Two-year experiment showed that biochar significantly and persistently increased soil pH and DOC concentration and decreased CaC12 extractable Cd and Pb in rhizosphere.Meanwhile,biochar at 40 t ha-1 had much better effects than 20 t ha-1.Comparatively,the trends in the bulk soil were similar to the results in the rhizosphere under biochar addition in the first year,but biochar did not change soil pH,DOC,CaC12 extractable Cd and Pb concentration at the second wheat maturity stage.Biochar could persistently enhance soil SMBC and SR in rhizosphere and bulk soil.At the first wheat maturity stage,both rhizosphere and bulk soil qCO2 were decreased by 26%and 24%respectively in this alkaline cropland contaminated with Cd and Pb under C2,which was differently to the results of meta-analysis.Thus,heavy metal pollution was also an important factor which exerted great impacts on qCO2 changes.At the second wheat maturity stage,there was no change of qCO2 in bulk soil under biochar addition but rhizosphere qCO2 was still decreased by 17%under C2.Responses of soil enzymes were different between rhizosphere and bulk soil under biochar addition.At the first wheat maturity stage,C1 significantly increased the activity of a-Glucosidase,?-Glucosidase,xylosidase and ?-Cellobiosidase related to soil C cycling,activity of p-Acetyl-glucosaminidase related to soil N cycling,activity of sulfatase related to soil S cycling and activity of phosphoesterase related to soil P cycling in bulk soil but not change soil enzymes in rhizosphere.Meanwhile,C2 did not change soil enzymes in bulk soil but significantly decreased the activity of soil enzymes related to soil C,N,P and S cycling.At the second wheat maturity stage,C2 significantly increased the activity of P-Glucosidase,P-Acetyl-glucosaminidase,sulfatase and phosphoesterase in rhizosphere and increased the activity of a-Glucosidase and phosphoesterase in bulk soil.This study indicated that the adsoption of enzymes by biochar may underestimation of enzymes activities when biochar was present in soil.Moreover,this study found that the pH value in rhizosphere was lower than in bulk soil and the DOC concentration and CaC12 extractable Cd and Pb were higher in rhizosphere than in bulk soil.All of the biological parameters including SMBC,SR and enzyme activities were higher in rhizosphere than bulk soil,indicating a rhizosphere effect.(3)Results from our two-year experiment showed that the bacterial and fungal abundances were higher in rhizosphere than in bulk soil while the relative bacterial-to-fungal ratio was lower in rhizosphere than in bulk soil.Biochar significantly and persistently increased the bacterial and fungal abundances both in rhizosphere and bulk soil,and also affected relative the bacterial-to-fungal ratio.At the first wheat maturity stage,the bacterial-to-fungal ratio in rhizosphere was significantly decreased under C1 and C2 while biochar did not change bacterial-to-fungal ratio in bulk soil.The results by Illumina high-throughput sequencing showed that the bacterial and fungal community structure were significantly differently between rhizosphere and bulk soil,and the bacterial and fungal diversity were lower in rhizosphere than in bulk soil.The dominant phyla for bacterial community across the samples were Proteobacteria,Acidobacteria,Chloroflexi,Actinobacteria,Planctomycetes and Chloroflexi.The relative abundances of Bacteroidetes,Proteobacteria and Actinobacteria were higher in rhizosphere than in bulk soil,which were dominant bacteria in wheat rhizosphere in this alkaline cropland contaminated with Cd and Pb.There were opposite responses between bacterial diversity and fungal diversity under biochar addition.Biochar significantly increased bacterial diversity both in rhizosphere and bulk soil but decreased fungal diversity in rhizosphere.In addition,the responses of bacterial community structure showed different pictures between rhizosphere and bulk soil under biochar addition,and the effects of C2 on soil bacterial community structure were stronger than C1.Biochar significantly and persistently decreased the relative abundance of Acidobacteria and C2 persistently increased(by 40%on average)the relative abundance of Actinobacteria in rhizosphere.At the second wheat maturity stage,C2 siginificantly decreased(by 25%)the relative abundance of Bacteroidetes,C1 and C2 significantly increased(9%,85%and 66%,respectively)the relative abundance of Proteobacteria,Chloroflexi and Planctomycetes in rhizosphere.Futhermore,biochar affected some bactertial taxa which only accounts for 1%of total bacterial amount in rhizosphere.Biochar significantly increased the relative abundance of Firmicutes and Cyanobacteria at the first wheat maturity stage,and Firmicutes,Gemmatimonadetes and Armatinobactia at the second wheat maturity stage in rhizosphere.Comparatively,in bulk soil,biochar significantly decreased the relative abundance of Acidobacteria,Chloroflexi and Planctomycetes,increased the relative abundance of Actinobacteria and Proteobacteria at the first wheat maturity stage.However,the effect of biochar on bulk soil bacterial community structure was weakened and it only significantly decreased the relative abundance of Acidobacteria in bulk soil.The dominant phyla for fungi community across the samples were Ascomycota,Zygomycota and Basidiomycota.Biochar amendment influenced the fungal community in rhizosphere more intensively than in bulk soil.Generally,Ascomycota was promoted by biochar while Zygomycota and Basidiomycota were inhibited by biochar.(4)Results from our two-year experiment showed that the application of biochar can significantly increase soil inorganic nitrogen concentrations and potential nitrification activities.With the addition of biochar,the abundance of ammonia-oxidizing bacteria(AOB)increased more intensively than the abundance of ammonia-oxidizing archaea(AOA).Biochar also strongly increased potential N2O reduction activities,however N2O production activities in the rhizosphere only showed a slight increase,a finding which corresponds to the higher ratio of nosZ/(nirS+nirK)identified with the addition of biochar compared to a control sample.Furthermore,the potential of biochar to reduce N2O emissions in the rhizosphere was identified,a result which may be attributed to biochar having stronger effects on reducing heavy metal bioavailability in this layer;results from the bulk soil samples did not record a similar reduction in N2O emissions.Our results suggest that biochar can significantly affect rhizosphere functional microbial communities of wheat,increase N transformation rates,decrease N loss and reduce environment pollution in a heavy metal contaminated soil.However,the effect of biochar on soil N cycling was shown to exist persistently in the rhizosphere but weaken in the bulk soil after the first year of the experiment.Therefore,more long-term studies are needed to systematically understand the durability of effects of biochar on N cycling.In summary,biochar could promote microbial growth and activity and thus their ecological functioning in agricultural soils.The altitude of these changes are subject to an assemblage of material added(type and properties of biochar),the amended soil(soil property and fertility level)and the experiment condition(biochar application rates,experiment type and the duration).In the studied alkaline metal contaminated dry cropland poor in organic carbon,the mitigation of mciorbial stress by heavy metals seemed weaker than in acid rice paddies with Cd contamination from South China for most of the significant changes in microbial growth and N transforming functioning were found following an application of biochar at high dosage of 40 t ha-1,visible in rhizosphere throughout the consective 2 wheat seasons.This study suggests a further deep understanding of thebiochar-root-microbes interaction for revealing the biochars role in croplands.Furthermore,the changes in mutual dynamic of soil pH-labile OC and metal mobility should be extensively elucidated for explore the mechanism of pollution remediation success by biochar in the long run.Yet,it is still a great knowledge gap that microbial communities could differ in their response of activity and the N cycling functions to a biochar amendment to soil with varying properties and indigenous mciorbial community.This could be a research priority in the study of using biochar for remediating polluted soil and improving the ecosystem nutrient cycling in future global agriculture..