Recent Advances in Research on Soil Microbial Community Diversity under Conservation Tillage

Mingcong ZHANG, Feng CHANG

1. College of Agronomy, Heilongjiang Bayi Agricultural University, Daqing 163319, China; 2. Forestry and Grassland Bureau of Daqing, Daqing 163300, China

Abstract The influencing factors of soil microbial community under conservation tillage system were summarized, the changes of soil microbial community under conservation tillage system and the general situation of soil microbial community research under rotation system were analyzed, and the contents of soil microbial community need to be future studied were prospected.

Key words Conservation tillage, Crop rotation, Soil microorganisms, Community, Diversity

1 Introduction

Soil tillage is an important management measure in agricultural production, including both traditional farming methods and conservation tillage based on less tillage and no-tillage. Conservation tillage increases the content of organic matter in the soil surface layer and weakens soil respiration through surface residual stubble cover[1], which is beneficial to increasing the richness of soil microbial community. Most soil microorganisms are sensitive to tillage measures and exhibit different responses. Studies have shown that conservation tillage is beneficial to increasing microbial diversity in soil compared with traditional farming methods[2]and to some extent improving the overall stability of soil microenvironment. Soil microbial diversity is a key index to evaluate soil ecological function and is easily affected by human disturbance[3]. Soil microbial activity is an important indicator for assessing soil health status[4], as well as an important factor affecting nutrient element cycling and crop growth[5]. Because soil microbial community is very sensitive to soil disturbance and amount of carbon source, the author summarizes the research situation of conservation tillage based on soil microbial community diversity under the condition of dry farming, which aims to provide theoretical basis for the utilization and popularization of conservation tillage technology in Heilongjiang province.

2 Changes in soil microbial communities under conservation tillage systems

2.1 Effects of tillage measures on soil microbial communitiesSoil tillage can affect the change of soil microenvironment by changing soil physical, chemical and biological properties, in a short time, change the habitat space of soil microorganisms, affect the relationship between soil biological populations within soil ecosystems, change the dominant distribution of species, and then change the relative size of microbial populations and the diversity of communities[6]. It is concluded that under no-tillage conditions, the activity and quantity of soil microorganisms are significantly improved compared with tillage, and the microbial soil quality parameters and tillage frequency of surface soil are inversely related[7]. Many studies have proved that the main reason for conservation tillage to improve soil microbial diversity is to increase soil organic matter content and improve soil structure, while soil microbial diversity is closely related to the content of organic matter and affects the functional diversity of microorganisms[8]. However, although other factors of farmland management (e.g., organic fertilizer application, crop species and pesticides) can also have important effects on soil microbial community structure, soil microbial communities are more strongly influenced by tillage measures. Overall, as tillage intensity decreased, soil community richness and diversity increased[9]. However, the response to tillage measures varies considerably regardless of the type of microbial population considered. When tillage measures change from tillage to no-tillage, the vertical distribution of soil microbial community in soil layer can be improved.

2.2 Effects of tillage measures on soil nutrient response to soil microbial communitiesDifferent tillage measures have different rates of release and decomposition of soil organic nutrients, but traditional tillage increases soil porosity through tillage, which in turn increases soil air flow rate and enhances mineralization rate of organic nutrients. Our research group found that less tillage and no-tillage was beneficial to the formation of soil aggregates >0.25 mm, while traditional tillage measures reduced the amount of soil macroaggregates, thus accelerating the rate of microbial decomposition of soil organic nutrients, resulting in a decrease in soil microbial activity. However, the relationship between soil aggregates and microbial diversity has not been found through extensive literature review[10], and similar conclusions are obtained by studying microbial biomass differences in fallow and cultivated land. Therefore, it is necessary to study and explore this aspect in depth in the future.

2.3 Response of changes in soil microbial community structure to soil tillageMost organisms are sensitive to tillage measures and different organisms can show different responses[3]. In the rice rapeseed rotation no-tillage test study[11], no-tillage has a higher bacterial diversity than traditional tillage, in which conservation tillage soils contain bacterial populations capable of nitrogen fixation and degradation of toluene and bromide, suggesting that no-tillage measures improve the ecological function of soil microbial communities. Fungi are more sensitive than bacteria to different soil management measures. Studies have shown that the effects of tillage measures on microbial ecology can be reflected by mycorrhizal fungi (arbuscular mycorrhizal). Fungal hyphae form extended networks in dryland soils and are activated by contact with seedlings, which are divided by tillage and may result in loss of intracellular lysates[12]. In addition, tillage affects fungal spore formation, resulting in the deposition of the propagule on the soil surface, exposing it to higher soil temperature and strong antagonistic pressure, while the mycorrhizal system is more stable under no-tillage measures.

3 A Survey of soil microbiological studies under two rotation systems

3.1 Changes in soil microbial diversity in rotation systemsSoil microbial community diversity is significantly associated with the heterogeneity of different crops. Some studies have shown that under a single wheat planting system, different rotation patterns can change the diversity of soil microorganisms. Therefore, different planting crops and crop residues have significant effects on soil microbial biomass and activity. During the rotation process, the more the substances remained on the soil by different crops, the more the microorganisms, so rotation may be more conducive to maintaining the diversity and activity of soil microorganisms than single cultivation tillage[13]. The fungi in the rhizosphere soil of continuous cropping soybean were enriched, and the dominant fungi were purple blue mold, which showed strong inhibition on soybean growth and development[14]. However, research on soil microbial communities under specific crop production systems, such as dryland farmland ecosystems, with different tillage and rotation management measures has been less reported.

3.2 Response of rhizosphere soil microbial communities to crop rotation measuresRhizosphere soil refers to the soil around plant roots affected by root activities, and plant root activities make the rhizosphere form a unique soil microenvironment. Compared to the non-rhizosphere soil, the physical properties and biochemical processes of the soil around the root system are directly affected by root activity, and the rhizosphere deposition released by the root causes changes in microbial communities[15]. It has been found that the rotation system with the involvement of legume crops is beneficial to the propagation of rhizosphere plant growth-promoting flora. The differences in root morphology and biomass and the distribution of organic carbon components secreted by roots all influence the microbial population density and activity in rhizosphere soil. Bean crop rotation is an effective farming measure to increase the yield of gramineous crops. Legume stubble changes the soil biochemical process, increases nitrogen mineralization rate and phosphorus utilization rate, increases soil pH value and mycorrhizal infection rate of arbuscular bacteria, reduces plant parasitic pathogen nematodes, and increases the yield of later gramineous crops[16].

4 Outlook

(i) Anthropogenic interference, exogenous carbon addition, crop type and crop growth period can significantly affect soil microbial activity, community function and structural composition. Although a large number of studies have been carried out in the present stage to find out the change trend of soil microorganisms in dry farming areas of Heilongjiang Province, the internal effects of different influencing factors under the rotation system are unknown, so it may be necessary to further investigate the types and contents of root exudates and the chemical composition of different crop stalks under rotation system. Gene technology such as high-throughput sequencing, which has developed rapidly in recent years, will provide more comprehensive information on microbial diversity. (ii) The mechanism between crop-soil nutrient cycling and nutrient efficient utilization mediated by microorganisms has not been thoroughly studied, and some studies of crop soil microbes under crop rotation systems remain in the description of phenomena and need to be further explored. (iii) Many studies on soil microbes in soybean tend to focus on the soil in a certain growing period, and there are few studies on the comparative comprehensive analysis of soil microbial community structure throughout the growing period of soybean. Soybean growth and development is carried out under an environmental system that interacts with soil and microorganisms. Soybean development is bound to affect soil microbial community structure, and vice versa. Therefore, it is necessary to study the dynamic characteristics of soil microbial community structure and function during soybean growth, which lays a theoretical foundation for the study of some microbial genes and functions in soil. (iv) Screening soil dominant bacteria and developing biological bacterial fertilizer. When some factors are determined, the development and utilization of dominant bacteria can not only improve the yield and quality of crops, but also reduce the occurrence of pests and diseases. (v) Genetic variation in microorganisms will lead to changes in the microbiota and activity, and we should therefore intensify research on changes in the microbiota at the molecular level. With the decrease of sequencing cost and the increase of sequencing depth in recent years, the parallel of macro genome, transcriptome, proteomics and metabolomics can study the community structure of microorganisms at different levels. The development of these technologies has a broad application prospect in microbial research. Therefore, the influence of various factors on soil microorganism should be studied from the aspect of action mechanism, not only the increase or decrease of quantity or the change of species.