Response Of Soil Microbial Community And Its Nutrient Limitation To Livestock Grazing In Songnen Degraded Grassland

Grassland,as one of the major ecosystems in China,is the green production base of animal husbandry and an ecological barrier.Grassland ecosystems play an important ecological role in global nutrient cycling,water conservation and biodiversity maintenance.However,due to the intensification of human activities and climate change,large areas of grasslands have been degraded to varying degrees.Grassland degradation leads to the reduction of biodiversity and the deterioration of ecological environment,and also hinders the sustainable development of animal husbandry.Soil microorganisms are an important part of grassland ecosystems and have an essential role in plant diversity maintenance,organic matter decomposition and nutrient cycling.Soil microorganisms drive nutrient cycling mainly by secreting various soil enzymes.Soil enzymes participate in various biochemical reaction processes and play a vital role in the decomposition of organic matter,material recycling,energy conversion and environmental quality assessment.At present,studies on grassland degradation mainly focus on plant productivity,plant diversity and soil nutrients,but the characteristics of soil microbial community,microbial metabolic nutrient limitations and their influencing factors are relatively lacking.Livestocks is the most extensive land management and practice in grassland worldwide.Grazing may directly and indirectly affect soil microbial community by altering aboveground plant communities and belowground ecological processes via animal trampling,foraging and excretion.Previous studies on the effects of grazing on soil microbial communities have focused on grazing intensity and regimes,but the research on the effects of livestock species and assemblage on soil microbial community in degraded grassland was very weak.Furthermore,the regulation mechanisms of grazing on soil microbial community and their nutrient limitations in degraded grasslands are unclear.In this study,grasslands with different degrees of degradation(non-degraded grassland,moderately degraded grassland,and severely degraded grassland)were selected as the research objects in the Songnen Plain.High-throughput sequencing technology and fluorescence enzyme labeled method were used to systematically analyze the variation trends of soil microbial composition,diversity,co-occurrence network structure,soil enzyme activites and stoichiometric characteristics along the grassland degradation gradient,and to identify the driving factors of soil microbial community structure and their nutrient limitations.Therefor,this study carried out grazing control experiments(no grazing,sheep grazing,cattle grazing and mixed grazing of sheep and cattle)in non-degraded grassland and moderately degraded grassland,respectively,to examine the effects of grazing livestock assemblages on soil microbial community composition,diversity,network structure and metabolic nutrient limitation,and to explore the response mechanism of the diversity and nutrient limitation of soil microbial community to grazing.We report the main results and conclusions as follows:(1)There were significant differences in soil microbial community diversity and structure among grasslands in different degradation stages,and the diversity and structure were mainly dominated by soil salinity and p H.The dominant phyla in soil bacterial community were actinobacteria,proteobacteria,chloroflexi and acidobacteria,and their relative abundance was 31.70%-41.74%,19.96%-22.14%,9.00%-18.55% and 5.13%-20.59%,respectively.Soil fungal community were dominated by ascomycota,unclassified＿k＿Fungi,basidiomycota and zygomycota,with relative abundances of 56.85%-75.89%,13.95%-33.11%,2.21%-12.62% and 1.03%-5.52%.The α-diversity of soil bacterial and fungal communities was inconsistent along the grassland degradation gradient.Theα-diversity of bacterial community showed a hump-shaped change with the increase of grassland degradation degree,while the α-diversity of fungal community showed a decreasing trend.Grassland degradation had caused significant alteration in soil bacterial and fungal communities structure,and soil salinity and p H were the most important environmental factors controlling microbial community structure.In addition,vegetation characteristics(plant community diversity and nitrogen content in aboveground plant tissues)were also major drivers of soil fungal community.Grassland degradation not only changed the microbial community diversity and structure,but also altered the microbial co-occurrence network structure.Grassland degradation enhanced the interactions between microbial species,and the positive interactions of microbial networks gradually increased with the degree of grassland degradation.This result supported the stress gradients hypothesis,that is,as the stress gradient increased,the interspecific interactions gradually became dominated by positive interactions,and the stronger the stress,the stronger the positive interactions.Soil fungal community is more sensitive to grassland degradation than bacterial community,so soil fungal community composition,diversity and structure can better indicate the degree of grassland degradation.(2)Soil enzyme activities(except L-leucine aminopeptidase,LAP)showed a consistent variation pattern with the increase of grassland degradation degree,and all showed as moderately degraded grassland > non-degraded grassland > severely degraded grassland,indicating that moderate degradation promoted soil microbial activity,while severe degradation inhibited soil microbial activity.The stoichiometric characteristics of soil enzymes were changed during the process of grassland degradation.Severe degradation significantly decreased the enzyme C: N ratio,and moderate degradation significantly increased the enzyme C: P ratio,while the N: P of soil enzyme showed a gradual increase trend with the increase of the grassland degradation degree.Soil microbial community metabolism was limited by both carbon and phosphorus.Soil microbial carbon limitation showed a hump-shaped change along the grassland degradation gradient,while phosphorus limitation showed a decreasing trend.Microbial diversity was a key regulator of soil enzyme activity and microbial carbon limitation,and soil properties were the main factor affecting microbial phosphorus limitation.(3)This study conducted a 3-years(2017-2019)short-term grazing experiment to explore the effects of livestock grazing species and assemblages on soil microbial community in degraded grasslands.Grazing significantly affected soil microbial community and the effect was strongly dependent on the health status of grassland,herbivore species and assemblages,and their interactive effects.In the non-degraded grassland,mixed grazing increased the abundance of bacteria and fungi,while in the moderately degraded grassland,sheep grazing and mixed grazing significantly reduced the abundance of bacteria,but had no significant effect on the abundance of fungi.Grazing significantly increased microbial richness in non-degraded grassland,but had no significant effect on soil microbial richness in moderately degraded grassland.Soil microbial diversity maintained at a high level under cattle grazing.Grazing significantly altered the soil microbial community structure.Due to the difference in feeding behavior of cattle and sheep,cattle mainly feed on dominant plant species,which will have a strong impact on the grassland ecosystem.Therefore,cattle grazing has a greater impact on soil microbial community structure.The main driving factors of soil microbial richness and structure were different under different grazing treatments in non-degraded grassland and moderately degraded grassland.Soil microbial richness and structure were dominated by soil properties under different grazing treatments in undegraded grasslands,while soil microbial communities in moderately degraded grasslands were co-regulated by soil properties and vegetation characteristics.Grazing reduced soil microbial network stability,and bacterial network structure was more sensitive to grazing than fungal community.(4)Grazing changed some soil enzyme activities,and the response of soil enzyme activities to grazing was inconsistent between non-degraded and moderately degraded grasslands.The stoichiometric characteristics of soil enzymes and nutrient limitation of microorganisms were affected by grassland degradation but not by grazing treatments.Among all soil enzymes,only alkaline phosphatase activity(AP)was significantly correlated with soil properties,vegetation characteristics and microbial community characteristics,while the rest of the soil enzyme activities had no significant correlation with environmental factors.The stoichiometric characteristics of soil enzymes and microbial phosphorus limitation are jointly regulated by soil properties,vegetation characteristics and microbial community characteristics,whereas microbial carbon limitation is only closely related to microbial abundance.Based on the above experimental results,this study obtained an in-depth understanding of the variation of soil microbial community diversity,structure and nutrient limitation along the grassland degradation gradient and its response to livestock grazing.Grassland degradation not only altered soil microbial community diversity,structure and nutrient limitation,but also changed co-occurrence network structure,and the variation of network structure was also a strategy for microbial community to adapt to the harsh environment.Soil salinity was a critical factor driving the diversity and structure of soil microbial community in degraded grasslands.Soil fungal community was more sensitive than bacterial communities to the response of grassland degradation gradients,and thus soil fungal community was better indicators of grassland health or the degree of recovery.In this study,the effect of grazing on soil microbial community was further investigated in non-degraded and moderately degraded grasslands,and it was found that the effect of livestock animals on soil microbial community was complicated,and the effect was both dependent on herbivore species and assemblage,and influenced by the health status of grasslands.The effect of livestock grazing on grassland microbial community as a combined effect of foraging,trampling and excretion on soil properties and vegetation characteristics.Selective grazing by livestock species in specific stages of grassland contributed to the maintenance of soil microbial community diversity and function.From the perspective of soil microorganisms,mixed grazing was suitable in non-degraded grasslands,while cattle grazing might be the best grazing management mode for moderately degraded grasslands.This study has complemented the inadequate insights into soil microbial community and their nutrient limitation in degraded grasslands,clarified the main regulatory factors of changes in microbial community and their nutrient limitation during grassland degradation,and revealed the degradation mechanism of Songnen grassland from the perspective of microbial co-occurrence networks,which can provide theoretical reference for predicting the response of soil microbial community to environmental changes,and also provide theoretical basis for restoration and reconstruction of degraded grasslands as well as guiding soil nutrient management in grassland ecosystems.This study also showed that in addition to grazing intensity and regimes,it is also important to differentiate animal species and asemblage of grazing modes according to the health status of the grassland for the management of grassland.This research has provided important insights into degraded grasslands to effectively improve grassland ecosystem function and to provide a direct and solid scientific basis for practical grassland adaptive management.