Research Progress of Soil Microbiology

Lide WANG，Tuo YAO，Chunxiu GUO，Fugui HAN，Fangling WANG1，，3，4，5，Tao SUN，Yinhua ZHANG

1.Gansu Desert Control Research Institute，Lanzhou 730070，China；

2.Pratacultural College of Gansu Agricultural University，Lanzhou 730070，China；

3.Minqin National Station for Desert Steppe Ecosystem Studies，Mingqin 733300，China；

4.State Key Laboratory Breeding Base of Desertification and Aeolian Sand Disaster Combating，Wuwei 733000，China；

5.Gansu Hexi Corridor Forest Ecosystem National Research Station，Wuwei 733000，China

Research Progress of Soil Microbiology

Lide WANG1，2，3，4，5*，Tuo YAO2，Chunxiu GUO1，3，4，5，Fugui HAN1，3，4，5，Fangling WANG1，2，3，4，5，Tao SUN1，3，4，5，Yinhua ZHANG1，3，4，5

1.Gansu Desert Control Research Institute，Lanzhou 730070，China；

2.Pratacultural College of Gansu Agricultural University，Lanzhou 730070，China；

3.Minqin National Station for Desert Steppe Ecosystem Studies，Mingqin 733300，China；

4.State Key Laboratory Breeding Base of Desertification and Aeolian Sand Disaster Combating，Wuwei 733000，China；

5.Gansu Hexi Corridor Forest Ecosystem National Research Station，Wuwei 733000，China

This paper aims at summarized the research progress of soil microbes，in amount of soil microbes including bacteria，fungi and actinomycetes，soil microbial biomass，including microbial biomass carbon，microbial biomass nitrogen and microbial biomass phosphorus，function of microbial and screening and application of beneficial microorganisms etc.，and future research are discussed combined with our project team for many years of work.

Soil Microbiology；Soil microbes；Microbial biomass；Research progress

S oil microorganism is an important part of ecological system，the core of the soil ecosystem，mainly playing the role of extensive decomposition，involved in the regulation of soil nutrient cycling directly or indirectly［1］，the energy flow［2］，organic mat ter conversion［3］，the formation of soil fertility［4］，degradation of pollutants and environmental purification［5］et al.，especially playing a leading role in the transformation of material and energy flow in ecosystem process，with great biological chemical activity［6］.The soil is microbial stronghold，and soil microbial composition is complex in large quantity.According to Whitman research estimates，there are about 2.6×1029prokaryotic cells the global soil［7］.In addition，soil microbial functional diversity，composition of the earth's most abundant biological resources，is also the most important gene resource library and metabolite Library［8］.Much attention is paid to the distribution of soil microbial community diversity，and pattern，which are important elements in circulation.It has become a hotspot of current research in ecology and microbiology fields as well as others.

Study on soil microbiology generally includes soil microbial classification and soil microbial biomass.The classification of soil microorganism in general refers to soil bacteria，fungi and actinomycetes，and includes everything else that is less than 5×103um3biology.Soil microbial biomass is the total soil microbial biomass，mainly including the soil microbial biomass carbon，soil microbial biomass nitrogen and soil microbial biomass phosphorus，excluding the large animal and plant body such as root［9］.Soil micro bial biomass is the power of soil organic matter and soil nutrients （C，N，P，S）transformation and cycle，the formation and decomposition of humus，and participate in the regulation of organic matter，the energy and nutrient cycling in soil biochemical process［10］，is an important plant nutrient reserves.

The soil microorganism is the regulation of soil nutrient cycle and energy flow in soil ecosystem，a part of organic matter and available nutrients，which is the most active part of the soil，and plays an important role in the metabolic process of soil.Therefore，it is very necessary to find out the number of microorganisms in soil，to explore the function of soil microbes，to mine and screen soil microbial resources for understanding soil microorganisms and its relationship with environment.

Study on Soil Microbiology History

It is reported that in the literature，the microorganism not only plays a leading role in soil microbial transformation of material［1，3］and energy flow［2］，but also regulate soil nutrient availability and other effects of crop growth［11］and evolution process［12］in direct or indirect way.

Researches available，however，focused on bacteria and fungi in terms of microbial process.In recent years，the archaea that life in the extreme environment microorganism has been identified by researchers，and widely exists in agricultural and natural ecosystems［13］.Recently，researchers explore the microorganisms in the rhizosphere soil biological processe using fluorescence in situ monitoring（FISH）and fluorescence quantitative PCR analysis confirmation，forexample，ammonia，methane formation in rice rhizosphere plays an important role［14］. So far，it is not clear for people to realize the ancient bacteria in the rhizosphere soil，and the potential function and the real situation also need further study.

According to the domestic theses，soil microbial ecological distribution，quantity and biomass of soil microorganism tend to be volatile upon regions，involving sandy grassland［15］andtypical degraded grassland in Hulun Buir［16］，Gurbantunggut desert［17］and desert grassland plain［18］in Xinjiang，the East Qilian Mountains Alpine shrubl and［19］，Changbai Mountain tundra ecosystem［20］，Helan Mountain altitudes typical vegetation zone［21］，desert grassland［22］and sand field［23］in Gansu，vegetation types in Loess Hilly Region［24］and purple soil slope farmland in the south［25］.Different regions，different land types，distribution of soil microorganism，quantity and biomass were studied systematically.

Bacteria，actinomycetes and fungi are the three main groups in grassland of soil microorganism，and the quantity of bacteria is the most，accounting for more than 90%，followed by actinomycetes and fungi.But the biomass of all of groups is the larger than fungi，accounting for about 50%of the microbial biomass，and bacteria and actinomycetes are smaller.Anthropogenic interference can stimulate to increase microbial diversity within a certain range，more than the environment tolerance［26］.In addition，the effects of different plant community on soil microorganisms are also different. Soil microbial quantity of desert steppe ecosystem under different plant communities in Jinchang，Gansu，China was studied in this paper by Ma Wengweng.The results showed that the change tendency of soil microbial quantity of community dominated by Kalidium foliatum was much higher than that of the community by Alhagi sparsifolia.The number of soil microbial，such as bacteria and actinomycetes，decreased with the soil depth in the same plant community，and maximum fungus in the subsurface layer of soil in community of A.sparsifolia［22］.

Microbial biomass has been used for sensitive index of soil change and in recent years studies in China and abroad focus on the soil［25，27-30］，proposing that the microbial biomass turnover time is less than one year［30］or 1-3 years［31］.

Soil microbial biomass carbon is of higher activity in the soil organic matter，as an important source of soil nutrients.What's more，much research has been conducted on soil microbial biomass carbon as a biological indicator at home and abroad［28，31-32］.Soil microbial biomass carbon with high sensitivity can reflect the change of soil micro before changes of total soil carbon，soil microbial biomass carbon amplitude is 42-2 046 kg/hm2in China，representing 2%-4%of soil organic carbon［33］.

Soil microbial biomass nitrogen is an important store of soil nitrogen，and the most active component of soil organic nitrogen，very sensitive to environmental conditions.Fertilization，tillage，cultivation and other technical measures will affect the amount of soil microbial biomass nitrogen［27，34-35］.It is playing an important role in soil nitrogen cycling and transformation process.

The present of soil microbial biomass nitrogen is 2%-6%of soil total nitrogen［33］，which is quite lower.But the nitrogen by microbial transformation is larger than that of fertilizer nitrogen in soil，also greater than the nitrogen plant away，showing that the soil microbial biomass nitrogen is source and sink of the soil nutrient［36］.The turnover rate of microbial biomass nitrogen is 10 times faster than that of plant residue nitrogen.The nitrogen of microbial biomass is much more than other libraries' by volume in the grassland soil［37］.Joergensen［38］believes that when the microbial biomass nitrogen turnover rate is less than one year，inorganic nitrogen mineralization quantity can meet the need of plant growth.

Soil microbial biomass phosphorus content varies greatly in the range of 10-100 kg/hm2，with extreme cases of exceed 100 kg/hm2.Usually，it accounts for 2.4%-23.3%of the total phosphorus content，occupied 5%-47% of organophosphorus，and higher than that of available phosphorus soil extractable［33］.Soil microbial biomass phosphorus is a part of soil organic phosphorus，very important in the soil，compared with the soil organic phosphorus compounds，microbial biomass phosphorus more easily available phosphorus mineralization into plant available.It is a part of the high activity of organic phosphorus，important not only for supply source of soil available phosphorus，but also the direct phase equilibrium available phosphorus in soil.Crop straw decomposition of soil microbial biomass phosphorus during the years flow flux in the number of equivalent to plant uptake of 3-5 times［39］.Crop takes in about 5-20 kg/hm2phosphorus from soil yearly，including only 10%-50%soil microbial biomass phosphorus［40］.Therefore，microbial biomass phosphorus is of important significance in the availability of the regulation of soil phosphorus for plants and the ecological cycle of phosphorus.

Function and Selection of Soil Microorganism

Function of soil microorganism

Soil microorganism plays a very important role in the transformation of material and energy flow of ecosystem.The present researches in this field are as follows:abundance of microbial genes associated with nitrogen cycling as indexes of biogeochemical process rates across a vegetation gradient in Alaska［1］，the effect of Ammonia oxidation coupled to CO2fixation by Archaea and bacteria in an agricultural soil［2］，soil microbial-root and microbial-rhizosphere processes to increase nitrogen availability and retention in agroecosystems［3］，reduction of rare soil microbes modifies plant-herbivore interactions［11］，evolutionary ecology of plant-microbe interactions:soil microbial structure alters selection on characteristics of plant［12］，effects of green manure application combined with chemical fertilizers on soil microbial biomass［27］，artificial reconstruction of degraded red soil forest microbial carbon source metabolism function in Jiangxi［41］，responses of soil microbial community structure to the leaf litter composition［42］，Analysis，regulation and high-throughput of soil microbial community in mono-cropping system［43］，population and ecological characteristics of soil fungal in larch forest［44］etc.

Research result shows that microbe will decompose as plant degrades.In initial stage of decomposition，microbial populations of different plant residues surface would greatly increase，indicating that bacteria and fungi are the most significant.Decomposition of plant litter in different phenological phase has been obvious difference，and the plant is closely relat-ed to contained chemical components especially the C/N value.In the process of plant residue decomposition，fungi are alternation in each group. Initially，Mucor and Trichoderma have the role of pioneer species，and then the number of Chaetomium and Trichothecene are increasing.Fungal taxa composition is complicated，and most kinds are the cellulose bacterium.With the decomposition process，the number of actinomycetes increased rapidly，mainly involved in refractory material decomposition［44］.

For some plants，there may be some deleterious substances for the growth of itself in plant root exudates. For example，on the conditions of soilless culture，cucumber root can produce secretion of cinnamic acid.For example，it will be a certain degree of inhibition for allelochemicals growth. However，it doesn't exist this kind of phenomenon in natural or farmland ecosystem.Because allelochemicals is decomposed by microorganisms as carbon source and nitrogen source，or microbial biofilms"blocked"or microorganisms"weakened"the toxicity of harmful substances on plant，the result is not toxic to plant their own［45］.

In recent years，Kuzyakov and Berendsen prove that the soil microorganisms and soil biological directly or indirectly involved in the regulation of various biogeochemical processes in soil，moisture and nutrient to coordinate for crop，at the same time，providing good aeration condition，and could control pollution and purify the environment［5，45］.

Selection and application of beneficial microorganism groups research

Microbial agriculture to the microbial fertilizer，microbial pesticide，microbial food and microbial feed as the core，namely"white agriculture"，will become an important direction of the sustainable development of agriculture in the future.In the ecological system，microorganisms play an important part on soil fertility.Therefore，soil microorganisms，especially the beneficial microbial groups，such as nitrogen fixing bacteria，phosphate solubilizing bacteria，are great significance to study on selection and application.

At present，the soil beneficial microorganism selection and application research are as follows:Isolation，identification and soil remediation of fomesafen-degrading strain FB8［46］；interactions of four PGPRs isolated from pasture rhizosphere［47］；isolation and identification of Sulfobacillus sp. strains and their application in pyrite bioleaching［48］；isolation and identification of associative nitrogen fixing bacteria in rhizosphere of oat in alpine region［49］. Plant growth promoting rhizobacteria and its application in phytoremediation of contaminated soils［50］；phylogenetic diversity characteristics of soil bacteria producing nematode-attracting volatiles and identification of their active compounds［51］et al.

Zhang Ying researched that four PGPRs were isolated from the rhizospheres of Medicago sativa，Trifolium repens and Trifolium pratense.The relativity of isolated bacterial strains was analyzed.The results showed that there was no antagonistic reaction between different single strains so that compound strains could be utilized.In a bio-phosphorus culture solution，the Hsg+lhs11 compound strain had the highest increment of effective-phosphorus.Effective-phosphorus increments gave additional effects of"1+ 1＞2".There were linear correlations between effective-phosphorus increment and pH value，effective phosphorus increment and total content of organic acid，pH value and total content of organic acid.Single strains and compound strains both had the ability to produce IAA.The IAA content in the Hsg+lhs11 compound treatment was higher than that of the single strain Hsg or lhs11 treatments，both of which presented a"1+1＞2"additional effect.Consider the growth-promoting effects of different treatments，the Hsg+lhs11 compound strains and Hsg+ls1-3+lhs11 compound strains had better interaction effects［47］.

Yao Tuo used the acetylene reduction method of Avena Sativa to associate nitrogen fixation bacteria，and isolate from selection and test.The results showed that oats associated nitrogen fixation strain distribution in the root surface，and root surface soil is far from root surface，located away from the root soil and root in the least［44］.

In recent years，there are a few reports on effects of effective microbial inoculants on plant growth character［52-53］and resources of cellulose decomposition function microbes and its utilization potential in grassland system interfaces［54］.

Research Outlook of Soil Microbiology

From the current progress of study on soil microbiology，although there are many reports，less attention is paid to the aspect from a comprehensive viewpoint，and the related research of soil microbiology may also need further exploration from the following aspects:

（1）Soil microbiology in the past mainly in agriculture，grassland，and forest.The study on soil microbial secondary succession process is relatively few，especially on soil microorganism of arid secondary grassland research reported.In fact，the use and rebuilding of vegetation in the work involves secondary succession question.Therefore，it is necessary to explore dynamic mechanism and regulation of soil microbial secondary succession of different climate zones，different plant communities，soil type and depth.

（2）In order to accelerate the decomposition of the mineralization of organic matter and release more nutrient supply for plant growth，for the decomposition of organic matter of beneficial microorganisms，such as cellulose decomposing bacteria，azotobacter，phosphobacteria，it is made into bacterium agent，applied in soil，can improve the physicochemical properties of the soil，improve soil microbes increased，and enhance soil microbial activity，with economic and ecological significance.Therefore，the selection and development of beneficial microorganisms should be further explored［47，49，52-53，55-56］.

（3）Although the research on soil microbiology of China has made significant progress，it is not enough in terms of the overall depth globally. Most researches focus on artificial management effects on soil microbiological factor understanding，and the lack of in-depth study on soil ecological service function.In addition，thereis a very large ascendant space in terms of research methods and theoretical innovation in China［45］.

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Responsible proofreader：Xiaoyan WU

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Should the article be accepted and published by Agricultural Science&Technology，the author hereby grants exclusively to the editorial department of Agricultural Science&Technology the digital reproduction，distribution，compilation and information network transmission rights.

Supported by Scientific and Technological Innovation Project of Gansu Agriculture and Animal Husbandry Bureaus（GNXC-2012-45）；National Natural Science Foundation of China（41161049）.

*Corresponding author.E-mail:wld69@tom.com

Received:September 3，2015 Accepted:October 13，2015