The Characteristics Of Soil Nematode Communities And Their Response To Livestock Grazing In Major Grasslands In Northern China

Grasslands are a crucial component of terrestrial ecosystems,providing important ecosystem services,such as climate regulation,water conservation,biodiversity maintenance and carbon sequestration.Soil nematodes are widely distributed in terrestrial biosphere and are the most abundant and diverse animals on the Earth.They occupy key trophic levels of the soil food web and play critical roles in regulating organic matter decomposition,maintaining soil biodiversity and nutrient mineralization.The geographical distribution and driving mechanism of organisms have always been one of the core issues in ecology and biogeography.In recent years,ecologists have paid more attention to the geographical distribution of belowground biological communities at the regional scale.However,there is still limited knowledge on the distribution patterns and driving mechanisms of soil nematodes in grasslands at the regional scale.Further,it is not clear about the geographical distribution characteristics and driving mechanisms of nematodes in different grasslands.In addition,livestock grazing is the most extensive land management and practice in grasslands worldwide.Livestock have profound influences on soil ecological process and plant community structure through foraging,trampling and excretion,thus directly and indirectly affecting the nematode community.A large body of studies on the response of nematodes to grazing focused on the effects of short-term grazing and single-species grazing.But there are few reports on the effects of long-term overgrazing and diversification of livestock on the soil nematode community.Furthermore,it is unclear about the differential effects of grazing and mechanisms on soil nematodes in different grasslands.This study was carried out in the main grasslands in northern China,including desert steppe,typical steppe,meadow steppe,temperate meadow and alpine meadow.We conducted long-term overgrazing in situ trials and grazing control experiments in natural grasslands.We examined the variation of soil nematode community composition,abundance,diversity and functional indices among grassland types,and the relationship between nematode communities and environmental factors.Our aim was to reveal the geographical distribution patterns and mechanisms of soil nematode communities in the main grassland ecosystems of northern China.Furthermore,we analyzed the effects of grazing types(no grazing,sheep grazing,cattle grazing and cattle-sheep mixed grazing)and grazing intensity(long-term enclosure and overgrazing)on nematode communities.We studied the regulation of grazing on nematode communities in different grasslands through changes in plant communities and soil characteristics,in order to illustrate the driving mechanism of grazing on nematode communities in different grasslands.(1)There were 82 genera of nematodes identified in major grasslands in northern China,The dominant genera were not consistent among grassland types.Except for the alpine meadow,the other four grasslands were dominated by bacterial-feeders.Based on the study of soil nematode communities in long-term enclosed grasslands in north China,a total of 82 soil nematode genera were found,classified into 43 families.The dominant genera were not consistent among grassland types.In total,30 nematode genera were identified in the desert steppe.Chiloplacus(17.84%),Cephalobus(11.70%),and Acrobeloides(11.16%)were the dominant genera.In the typical steppe,55 nematode genera were observed,and the dominant genera were Chiloplacus(11.24%),Acrobeles(10.08%)and Cervidellus(13.84%).A total of 56 genera of nematodes were found in the meadow steppe and Acrobeles(11.64%)were the dominant genera of nematodes.For temperate meadow,31 nematode genera were found,and the dominant genera were Chiloplacus(11.44%)and Acrobeloides(10.43%).Twenty-four genera of nematode were also identified in the alpine meadow,in which Tylencholaimus(10.08%)and Coslenchus(14.90%)were the dominant nematode genera.The dominant genera of nematodes in alpine meadows were fungal-feeders and plant-feeders,whereas they were bacterial-feeders in the desert steppe,typical steppe,meadow steppe and temperate meadow.(2)Soil nematode communities were significantly different among grassland types with elevation gradients.The spatial distribution pattern was mainly driven by climate and soil factors.Based on the spatial distribution characteristics of soil nematode in grasslands of northern China,it was found that the there was great variation in community composition,abundance and richness of nematode among different grasslands.The abundance and richness of soil nematode were significantly higher in low-elevation grasslands(typical steppe and temperate meadow)than in high-elevation grasslands(desert steppe,meadow steppe and alpine meadow),and were highest in temperate meadow and lowest in alpine meadow.The spatial distribution patterns of nematode abundance and richness were driven primarily by factors of climate and soil factors.The change of nematode abundance was mainly directly affected by climatic factors(mean annual temperature and aridity index),while nematode abundance was indirectly regulated by climatic factors.There were significant differences in nematode community structure among grassland types.Further analysis showed that climatic and soil factors contributed more to the structure of the nematode community than plant communities,and the mean annual temperature was the most important driving factor that affected the structure of the nematode community.Compared with the change of nematode community composition,abundance and diversity,the nematode functional indices were less sensitive to the change of grassland types.The nematode structure index was significantly different among grassland types.It was higher in typical steppe and temperate meadow,indicating that the soil food web was more stable and complex in typical steppe and temperate meadow.These results indicated that climate and soil factors drive the spatial distribution of nematode communities at the regional scale more than the role of plant communities.(3)Grazing had a significant impact on nematode communities and the effect was strongly dependent on grassland types,livestock types and assemblages.Based on the combined effects of grassland types and grazing livestock species on nematode communities,it was found that grassland types,grazing regimes and interaction had significant effects on nematode abundance and diversity.The effects of grazing regimes on nematode communities were not consistent among grassland types.Sheep and cattle grazing both reduced the abundance of nematodes in most grasslands(desert steppe,typical steppe,temperate meadow and alpine meadow),while mixed grazing maintained the abundance except in the desert steppe.Grazing had no significant effect on nematode richness in the grasslands with higher annual average temperature(temperate meadow and desert steppe).However,mixed grazing decreased nematode richness in the typical steppe,meadow steppe and alpine meadow,while cattle or sheep grazing had no effect on nematode richness.Through the linear mixed effect model,further analysis found that in desert steppe and typical steppe with high aridity index,the abundance of nematodes was mainly regulated by aboveground biomass,while in temperate meadow,meadow steppe and alpine meadow with low aridity index,soil physical and chemical properties were the main driving factors.For the change in nematode diversity,grazing mainly affected nematode diversity in the typical steppe and meadow steppe by changing the plant communities,while in the other three grassland types,the change of soil physical and chemical properties induced by grazing contributed significantly to nematode diversity.The results showed that the grassland types,grazing regimes and their interaction had significant effects on the nematode ?-diversity.There were significant differences in the structure of the nematode communities between no-grazing and cattle grazing,as well as between mixed grazing and cattle grazing treatments in temperate meadows,and the structure was similar between different grazing treatments in the other four types of grassland.In conclusion,grazing had a significant effect on nematode communities,which was strongly dependent on grassland types,livestock species and assemblage,and the mechanism of grazing on nematode abundance and diversity differed among grassland types.(4)Overgrazing had adverse effects on soil nematodes in typical steppe by modifying soil nutrients,but had a non-significant effect on nematode communities in the desert and meadow steppe.Overgrazing decreased the relative contribution of plant communities and soil properties to nematode communities.Our study focused on impacts of enclosure and overgrazing on soil nematode communities in different grasslands.We revealed that grassland types and overgrazing had significant interactions on ?-diversity and abundance of all the nematode groups except for fungal-feeders.In the desert and meadow steppe,overgrazing had no significant influence on the abundance and diversity of nematodes.The change in nematode abundance was mainly related to soil conductivity,water content and available nitrogen content.Our study demonstrated that the relative contribution of the plant community and soil properties to nematode abundance was regulated by grazing.In enclosure,both plant community and soil properties were the main modifiers of nematode abundance.In contrast,nematode abundance in the overgrazing treatment was mainly determined by soil properties and its effect was less intense than enclosure treatment.In addition,the interaction between grassland types and grazing treatments had a significant effect on nematode structural index.Overgrazing in desert steppe significantly increased nematode structural index,but grazing in typical steppe and meadow steppe had no effect.Furthermore,we found that nematode structural index was mainly affected by soil water content,and it was positively correlated with soil water content.Based on the above experimental results,this study showed insight into the spatial distribution patterns of soil nematode communities in grassland ecosystems and their response to grazing livestock.Our study found that soil nematode communities were significantly different among grassland types with elevation gradients.Climate and soil factors drove the spatial distribution of nematode communities more than plant communities at the regional scale.This study also showed that climatic factors had direct and indirect effects by altering plant communities and soil characteristics on nematode communities.Our study also revealed that livestock grazing had complex influence on nematode communities,which strongly depended on the grassland types,the types and combination of grazing livestock and the grazing intensity.Furthermore,the regulatory effects of plant communities and soil properties on nematode communities can be altered by grassland types and grazing effects.This study complemented the lack of knowledge on the spatial distribution patterns of soil nematode communities in grassland ecosystems,clarifying the main drivers of nematode community variation among grassland types,and provided important theoretical references for predicting the evolution of ecosystem structure and function under conditions of environmental change.This study also highlighted the differences in the role and mechanisms of grazing on nematode communities in different grasslands,which provided new ideas for the study of the role of grazing on nematode communities.It deepened the understanding of the patterns and mechanisms of changes in ecosystem structure and function under anthropogenic disturbances.This study also showed that in addition to the traditional focus on grazing intensity,grassland management should also distinguish grazing animal species and combination of grazing animal species.This not only had important scientific value for maintaining healthy and sustainable grassland ecosystem development,but also provided theoretical guidance for formulating reasonable and scientific grassland management policies.