Responses Of Soil Nematode To Enclosure And Nitrogen Addition In The Degraded Meadow Steppe Of Stipa Baicalensis

Nematode is the most abundant invertebrate in the soil.It occupies multiple nutrient levels in soil food web and participating in the decomposition of organic matter and nutrient cycling processes,it is of great significance for maintaining the stability of the soil ecosystem.In addition,soil nematodes are an important indicator.In recent decades,with the continuous increase of population and the intensification of global climate change,the grassland around the world has undergone degradation in varying degrees.Enclosure and nitrogen addition are widely used restoration measures of degraded grassland.Both of them can further affect soil nematode communities by changing plant communities,soil physical and chemical properties and soil environment.At present,the effects of enclosure and nitrogen addition on soil nematodes have been studied,but the effects of interaction between the two on soil nematodes and the mechanism of the interaction have not been studied.In this thesis,the degraded meadow steppe of Stipa baicalensis in Hulun Beier was studied.Through the enclosure experiment and nitrogen addition control experiment,studies the restoration process of degraded grassland soil,plant and change the soil nematode community,as well as soil physical and chemical properties and plant community of soil nematodes community,obtain the following main research results and conclusion:(1)In this study,it was found that 88 genera of soil nematodes in degraded meadow steppe of Stipa baicalensis belong to 15 families,27 genera of Plant parasite,11 genera of Fungivores,29 genera of Bacterivores,and 21 genera of Prey-omnivores nematodes.The average abundance of soil nematode individuals was 206(strips /100 g of dry soil).The dominant genus of soil nematodes in degraded grassland in Hulun Beier is Acrobeles.Common genera include 21 genera of Acrobeloides and Plectus;Rare genera include 66 genera including Diphtherphora and Coslenchus.(2)The effects of nitrogen addition and enclosure on the abundance and diversity of soil nematode were affected by the degree of grassland degradation.In moderately degraded plots,enclosure significantly increased nematode abundance and Shannon-wiener diversity.In the severely degraded plots,nitrogen addition,enclosure and their combined action significantly increased the abundance of nematodes and omnivorous nematodes,and the total abundance of nematodes increased from 154(strips /100 g of dry soil)to 216(strips /100 g of dry soil)under enclosure treatment.The nutrient diversity index(TD)of soil nematodes was significantly influenced by both enclosure and nitrogen addition,and was significantly decreased by enclosure & nitrogen addition,indicating that the nutrient structure of soil nematode community tended to be simple.(3)The effects of nitrogen addition and enclosure on the functional indexes of soil nematodes were different in different stages of grassland degradation.Nitrogen addition and enclosure had no significant effect on the functional index of nematodes in mildly degraded plots.In moderately degraded sample plots,enclosure and nitrogen addition had an obvious interaction with the Wasilewska index(WI)of soil nematode,and nitrogen addition & nitrogen application significantly increased it.In the treatment with different degrees of degradation,the WI was greater than 1,indicating that enclosure and nitrogen addition reduced the proportion of phytophagous nematodes,the soil was not easy to be infected by phytophagous nematodes,and the vegetation was in good health.In severely degraded plots,the basic index(BI)of soil nematodes increased under the enclosure treatment,indicating that the degree of soil disturbance decreased and the soil health tended to be stable after grazing pressure was removed.Enclosure and nitrogen addition had no significant effect on soil nematode channel ratio(NCR),structure index(SI),enrichment index(EI),maturity index(MI)and plant parasitic nematode index(PPI).(4)The effects of nitrogen addition and enclosure on soil physical and chemical properties and plant communities were weakest in moderately degraded soils.In mildly degraded grassland,nitrogen addition significantly reduced soil pH,enclosure significantly increased soil total carbon content,and nitrogen addition & enclosure significantly improved soil electrical conductivity,ammonium and nitrate nitrogen contents,and aboveground biomass of plants.In moderately degraded samples,nitrogen addition and nitrogen addition & enclosure significantly increased the content of ammonium nitrogen in soil.In severely degraded samples,nitrogen addition significantly reduced soil pH,nitrogen addition and nitrogen addition & enclosure significantly increased soil nitrate and ammonium nitrogen contents,and nitrogen addition,enclosure and nitrogen addition + enclosure significantly reduced plant species richness and diversity.(5)The effect of soil environment and plant community on nematode changed from favorable to unfavorable with the aggravation of degradation degree.In mildly degraded plots,soil nematode abundance was positively correlated with soil total organic carbon content,and soil nutritive group diversity was positively correlated with plant diversity,while in severely degraded plots,phytophagous nematode abundance was negatively correlated with soil ammonium nitrogen content.Therefore,in mildly degraded grassland,the improvement of soil organic matter and plant diversity contributes to the increase of nematode abundance and diversity;In moderately degraded grassland,soil nematodes were not affected by soil and plant.Ammonium toxicity induced by nitrogen application in severely degraded grassland had adverse effects on soil nematode.To sum up,this study clarified the rules of the effects of encircling and encircling nitrogen on the community characteristics of soil nematodes,and found that the effects of nitrogen encircling and encircling on soil nematodes were different in different degrees of grassland degradation.Nitrogen addition and enclosure had no significant effect on soil nematodes in mildly degraded grassland,but increased the diversity of nematodes in moderately degraded grassland,while increased the abundance of nematodes in severely degraded grassland,but decreased the diversity of nematodes.In addition,the effects of soil properties and plant community on soil nematodes changed from favorable to unfavorable.This study helps to reveal the mechanism of degraded grassland restoration and provides scientific basis for formulating scientific and reasonable policies on degraded grassland restoration.