The Mechanisms By Soil Microbes Responding To Plants And Large Herbivore Grazing On Songnen Grasslands

Soil microbes are one of the major components of terrestrial ecosystems, which have the richest and largest number species in soil and exert the primary decomposition. Hence, changes in soil microbial communities would influence the relative ecosystem processes and functions, such as nutrients cycling. Conversely,alterations in plant quantity and quality or the soil properties also have strong direct or indirect impacts on soil microbial communities. Studies had proved that the ecological interactions between plants and soil organisms and their ecosystem functions were often strongly influenced by other trophic levels, especially the impacts of above-ground herbivores. In grassland ecosystems, grazing by large herbivores not only could affect the plant community structure but also alter the efficiency of nutrients transformation; meanwhile, the distribution of plant nutrients also could impact herbivores foraging behavior. In natural grassland, plant diversity and functional groups were judged to be the key factors affecting herbivore foraging among the relative elements. That is to say, integrating the close connections between large herbivores and plants, and further analyzing their effects on below-ground microbial communities could be important for understanding the linkages between above- and below-ground communities.This study was conducted in a meadow steppe in Songnen grassland. Different large herbivore combinations(ungrazed, cattle grazing, and cattle and sheep assemblage) were executed on the sites with lower and higher plant diversity. 454 pyrosequencing technology was used to identify soil bacterial and fungal communities both at plant communities and functional groups levels. Through comparing the differences in soil microbial community structure and amount under different treatments, we try to find the main pathways of large herbivore grazing influencing below-ground microbial communities based on plants and soils, and find the main factors influencing soil microbes and the mechanisms of soil microbial communities responding to above-ground plants and herbivores. This study not only could improve our knowledge and understanding for the changes of soil microbial communities diversity; also, it has important theoretic and practical meanings for further studying the regulation of above- and below-ground communities to ecosystem functions and developing an effective grazing management in grassland ecosystems.Based on a mass of experiment design and methods, we obtained important results and conclusions as follows:(1) The effects of large herbivores and plants on soil physical and chemical indexes are different, and their interactive effects on soil properties depended on specific grazing assemblage and plant diversity context. At the plant community level,especially under the combination of cattle and sheep grazing, soil moisture, organic carbon and total nitrogen increased with the raising of plant diversity. The main reasons included: at lower plant diversity level, the additive effects of cattle and sheep foraging induced the overutilization of dominant grasses, weakening the ability of molding moisture of grasses and reducing the amount of the resources imputing into soil from the above-ground plants; however, at higher plant diversity level, the complementary effects of cattle and sheep foraging leading to a more even usage across all plant species, which increased the plant diversity and biomass facilitating molding moisture and resources in soil. Soil pH and electric conductivity all reached highest value at higher plant diversity level under cattle grazing, which demonstrated that a mass of feces of cattle inputting into soil could improve the accumulation of soil salt ions, the cycling of soil ions and the increase of soil H+concentration. Yet, at the plant functional group level, based on the context of higher plant diversity and grazing by cattle and sheep, higher plant species richness and lower cellulose content in forbs than perennial rhizome grasses facilitated the cycling and accumulation of soil resources and moisture; meanwhile, the higher nitrogen content in the soil of forbs could reduce the power of soil hydrogen. Our data demonstrated that plants and grazing all had significant effects on specific soil properties, of which the increasing in forbs at higher plant diversity context, moderate and the cattle and sheep assemblage were best for improving soil moisture, resources and environments.(2) Pland diversity and large herbivores grazing all had significant effects on soil bacterial community diversity, and the improvement of soil bacterial community diversity with increasing plant diversity was maintained by large herbivore grazing.The improvements in soil and plant resources induced by large herbivores at the site with higher plant diversity facilitating soil bacterial community diversity; also, the structural equation modeling proved that plant biomass and species richness were the main direct factors of grazing influencing bacterial community diversity. Soil bacterial community composition was just affected by grazing, and only specific bacterial phyla responded to the indirect effects of large herbivores, including Planctomycetes, Gemmatimonadetes, Nitrospirae and Verrucomicrobia. Yet, the major phyla Proteobacteria and Actinobacteria were insensitive to grazing as the numerous amount and stabilization mechanisms. Therefore, alters in the quantity and quality ofresources were the main factors of grazing influencing soil bacterial community diversity and specific phyla at different plant diversity context.Plant functional groups and large herbivore grazing also had strong effects on soil bacterial community diversity and composition, and the effects of grazing on soil bacterial community diversity under different plant functional groups just existed at higher plant diversity. Forbs increased bacterial community diversity mainly through the reduction of soil electric conductivity and pH, and soil electric conductivity was also the main factors influencing bacterial community composition. Actinobacteria,Acidobacteria, Gemmatimonadetes, Chloroflexi and Verrucomicrobia were the main phyla having responses to plant functional groups. These results showed that the soil salt concentration was the main factors influencing soil bacterial community structure by different plant functional groups; higher plant diversity context and large herbivores’ assemblage were best for the soil bacterial community diversity.(3) Different from the results of soil bacteria, the fungal community diversity had no significant responses to plants and grazing treatments whether at the plant community or at functional group level. The main reason for the stabilization of soil fungal communities might be caused by the complex network of their hypha and most plants intensively connecting many soil fungal species. At the level of whole plant communities, the fungal community composition was influenced by plant diversity context. The phylum Ascomycota increased following the raising of plant diversity was the main reason for soil fungal composition responding to plant diversity context,yet, the Basidiomycota, Glomeromycota, Zygomycota and Chytridiomycota had no responses. Soil moisture, nutrients and pH were the most important factors impacting soil fungal community composition. Our results demonstrated that the changes of plant diversity just had strong effects on specific fungal phyla; soil fungal community structure might more stable than bacteria.Soil fungal community composition had obvious responses to plant functional groups and large herbivore grazing treatments. The relative abundance of Ascomycota of forbs was higher than that of perennial rhizome grasses at lower plant diversity; yet,Basidiomycota, Glomeromycota and Zygomycota were sensitive to the grazing treatment, which all were lowest under the cattle grazing. Soil electric conductivity and aboveground biomass were the main factors influencing soil fungal community composition. Based on the above results, we could get that at the plant functional groups level, the changes of plant functional groups and large herbivore grazing all had strong effects on specific fungal phyla; and, different levels of plants could alterboth the rules and the mains factors for the soil fungal community structure.(4) The number of gene copies of soil bacteria were about 200-1200 times than soil fungi, so the soil bacteria was the dominant microbes on Songnen grassland. The amounts of soil fungi and bacteria all had no significant responses to the treatments of plant diversity context and large herbivore grazing, which might be caused by the moderate grazing intensity and the similar plant species richness at the sites with pre-razing lower and higher plant diversity after four years grazing. It is interesting that large herbivore grazing could affected the fungal : bacterial ratio through soil moisture and electric conductivity. Soil electric conductivity could inhibit the ratio of fungi and bacteria, and fungal : bacterial ratio lowest under cattle grazing might be the reduction of soil moisture and increase of soil electric conductivity with cattle feces inputting into soil. Based on the above results, in the meadow steppe characterised by high salt and alkali, soil bacteria was the dominant microbes; the amount of soil fungi was more easily affected by soil conductivity and moisture compared to bacteria.In conclusion, large herbivores and plant communities had strong effects on specific soil physic-chemical properties and microbial communities, and their effects were often interactive; cattle and sheep assemblage with the increasing in forbs at higher plant diversity context were best for improving soil resources, bacterial community diversity and soil environments. Through the systematical analysis of the rules of soil microbial communities, we found that soil bacterial community structure was more sensitive to the treatments of plants and large herbivores, soil fungal community structure was more stable and had stronger disturbance resistance; yet, the resposes of the fungal : bacterial ratio to large herbivores reflected that the sensitivity of fungal amounts to soil resources and environments. As stated above, soil microbes could regulate their community structure and amounts responding to the effects of plants and large herbivores. The comprehensive study about the relationships between herbivores, plants and soil microbes obtained: 1) The distinct responses of soil bacterial and fungal community structure to the interaction between plants and large herbivores are useful for understanding relationships between above- and below-ground biodiversity and the relative ecosystem functions; 2) Highest microbial diversity under cattle and sheep grazing is valuable for mixed grazing livestock grazing management building in grassland ecosystems.