Influences Of Simulated Grazing On Soil Nematode Community Characteristics In Typical Grasslands On The Loess Plateau

The typical grassland on the Loess Plateau is located in the northern agricultural pastoral ecotone,which is sensitive to human activities.Soil nematodes occupy multiple trophic levels,participating in the nutrient cycle and energy flow of the ecosystem,and thus playing an important role in maintaining the structure and function of the ecosystem.Grazing is the most important and direct way of utilizing natural grasslands,which influences the structure and function of grassland ecosystems through defoliation,trampling,and excreta return.Although some studies have explored the impact of grazing on soil nematodes,whether and how different pathways of livestock grazing(i.e.,defoliation,trampling,and excreta return)affecting soil nematode communities remain largely unexplored.In this study,by performing a simulated grazing experiment in a semiarid grassland on the Loess Plateaut,including control(CK),mowing(M),trampling(T),addition of dung and urine(DU),and M+T+DU(mixed treatment),we investigated the effects of different pathways of livestock grazing on soil nematode community composition,abundances of total nematodes and different trophic levels,community characteristics.By integrating vegetation community characteristics and soil physicochemical properties,we disentangled the mechanisms of different pathways of grazing affecting nematode community structure.By synthesizing the two consecutive years data in 2020 and2021,we obtained the following results:(1)The extremlely dominant genera(>10%)under different simulated grazing treatments for the two years were different,but the dominant genera(5%-10%)were similar.In 2020,the extremlely dominant genera under all the simulated grazing treatments were Acrobeles and Tylenchorhynchus;In 2021,the extremlely dominate genus under mowing was Criconrmoides,under trampling was Criconrmoides and Mesodothylaimus,and under addition of dung and urine and mixed treatment was Criconrmoides;In the control plot,the extremlely dominant genera for two years were Tylenchorhynchu.The dominant genera were consistent under different simulated grazing treatments for the two years,i.e.,Nygolaimus,Campydora,Axonchium,Cephalobus,Dorylaimoides,Acrobelides,Kochinema,and Dolichorhynchus.(2)Simulated grazing treatments,sampling years,and their interactions significantly affected the abundances of total nematodes and different trophic groups.In 2020,there was no significant difference in the abundance of total nematodes between each simuated treatment and the control.In 2021,the addition of dung and urine and mixed treatment significantly increased the abundance of total nematodes.With regard to different trophic levels,in both 2020 and 2021,the addition of dung and urine and mixed treatment significantly increased the abundance of bacterivores,but had no significant impacts on the abundance of fungivores,herbivores,and omnivores.In 2021,trampling and the addition of dung and urine significantly decreased the abundance of predators.(3)Simulated grazing treatment,sampling year and their interactions significantly influenced shannon diversity,generic richness and dominance index of nematodes.In 2020,all simulated grazing treatment had no significant impacts on shannon diversityand generic richness of nematodes,and the addition of dung and urine significantly reduced its dominance index.In 2021,trampling,dung and urine addition and mixed treatment significantly reduced the shannon diversity and generic richness of nematodes,and mixed treatment significantly increased the dominance index of nematodes.(4)Simulated grazing treatment had significant effect on different ecological indices of nematodes,showing similar trends for the two years.The mixed treatment to some extent reduced the parasitic index and maturity index of nematodes,while trampling,dung and urine addition,and mixed treatment increased the Wasilewska index and channel ratio of nematodes.In addition,the enrichment index of the soil food webs is lower than the structural index,indicating that soil nutrients in our study area are poor and the food web is in a gradual process of stabilization.(5)Our results indicate that the nematode community characteristic indices was mainly driven by soil physicochemical characteristics.The abundance of total nematodes,bacterivores,herbivores,omnivores,nematode channel ratio,and Wasilewska index are significantly positively correlated with soil available phosphorus.The maturity index,structure index,and enrichment index of nematodes are significantly negatively correlated with soil available phosphorus.The dominance index of nematodes was significantly negatively correlated with soil ammonium nitrogen.This study explored the effects of different pathways of grazing,i.e.,defoliation,trampling,and excreta return on the characteristics of soil nematode communities in typical grasslands of the Loess Plateau.By combining vegetation community characteristics and soil physicochemical properties,we analyzed the main factors driving soil nematode community.Our findings facilitate to understand aboveground-belowground multiple trophic interactions,Meanwhile,our study provides scientific support for evaluating the health of grassland ecosystems and the restoration of degraded grasslands in the Loess Plateau.