Response And Mechanism Of Soil Nematodes In Alpine Meadow To Grazing Intensity

As one of the most important grasslands,the alpine meadow faces the chanllege of soil degradation and ecological function loss due to increasing human activities in recent years.Soil nematodes are sensitive to environment changes and play a pivotal role in soil food web and ecosystem funtionality.Here,the study explores the effect of different grazing intensities(fence ungrazed,moderate grazing and intense grazing)on soil nematods based on long-term grazing experiment since 2009.The results are as follows:(1)Root mass as reprensented by dry weight concentrated in 0-10 cm soil depth,and root mass and nitrogen(N)content was highest while carbon(C)/N ratio was the lowest subjective to the moderate grazing.Grazing decreased soil water content and increased pH at 0-10 cm depth.Particulate organic C mainly distributed at 0-10 cm depth and decreased with increasing grazing intensity.Moderate grazing led to the lowest contents of nitrate,dissolved organic C and dissolved organic N in soils.Grazing increased soil microbial biomass,especially the moderate grazing increased fungal biomass and fungi/bacteria ratio significantly at 0-10 cm depth(P<0.05).Grazing did not affect soil bacteria biomass.(2)At 0-10 cm depth,grazing significantly affected most parameters of nematode community.Compared to the ungrazed level,total abundance of nematode was higher at moderate whiler lower at the intsense grazing level at 0-10 cm depth.Grazing reduced the taxa number of nematodes.The moderate grazing significantly stimulated the abundance of plant-feeding and fungal-feeding nematodes,while the intense grazing significantly decreased the abundance of plant-feeding nematodes and omnivores-predators.Under the moderate and intense grazing,the maturity index of free-living nematodes Nematode Channel Ratio decreased,indicating a bacteria-dominated decomposition pathway.(3)Multiple regression equation showed that grazing affected soil nematode communities mainly through altering root-related resource availability and quality as well as microbial community.Further,structural equation model(SEM)confirmed that roots rather than soil abiotic properties determined the effects of grazing on soil nematodes.The relationship between soil nematodes and ecosystem function(i.e.enzyme activities)under different grazing intensity was further explored.The results are as follows:(1)Grazing increased enzyme activities involved in C cycling such as a-glucosidase,cellulase and xylanase,but the reverse was true for ?-glucosidase.Grazing also promoted enzyme activities involved in N cycling such as N-acetyl-P-D-glucosaminidase but not leucine aminopeptidase.The lowest and the highest normalized enzyme activity for C and N cyling were found in soils under moderate grazing at 0-10 cm depth.Grazing decreased the specific enzyme activities per microbial bioamss at 0-10 cm and 10-20 cm depth,indicating microbial physiology was altered by grazing.(2)Nematode communities were positively correlated with the C-acquiring enzymes but not the N-acquiring enzymes,suggesting nematodes mighgt be more important in C cycling than that in N cycling.In summary,grazing changed soil nematode communities mainly through direct or indirect effects of plant roots and mainly occurred at 0-10 cm depth.Moderate grazing could improve root quality and quantity,but accompanied by a decrease in soil water content and organic matter content.Besides,the relationships between nematode community and enzyme activities highlighted the high potential of nematodes indicating soil ecosystem functional state.