Effects Of Litter Types On Soil Micro-food Web Structure And Function In Degraded Grassland

Grassland,which contents multiple ecological functions,is under threat from continuing degradation,and its ability of supporting biodiversity and maintaining ecosystem functions has been severely weakened.The grassland degradation retrogrades plant community succession,and then changes the dominant species,resulting in varying in the quality of litter.Litter is an important component of grassland ecosystems and most net aboveground productivity is returned to the soil by litter.Litter is the main biomass and energy source for soil organisms.The nutrients and mineral elements released by litter decomposition entering into the soil,then changing the soil micro environment,thus affecting the community composition of soil organisms.Among soil organisms,soil micro-food web composed of soil microorganisms and nematodes is an important part of soil detrital food web.It is mainly involved in soil organic matter decomposition and nutrient turnover.Soil micro-food web can regulate nutrient flow in soil and control plant nutrient supply.Through predating,the function of food web is regulated,and further affects the ecological function of the terrestrial ecosystem.As the main components the of soil micro-food web,soil nematodes and soil microorganisms play important role in nutrient cycling and energy flow through their predation relationships.However,the structure and function of soil micro-food web in degraded grassland are still unclear.In this paper,the effects of litter types on the structure and function of soil micro-food web in degraded grassland were studied.Through the litter decomposition experiments,the effects of Chloris virgata and Leymus chinensis litters on the structure and function of soil micro-food web were compared at different degradation levels.The research results will reveal the response of different trophic levels of soil micro-food web to different litter types in degraded grassland.The main findings were as follows:（1）The effect of litter types on soil physicochemical factors varied depending on litter decomposition time and soil degradation levels.The effect of C.virgata litter on the content of fast-acting nutrients such as DOC,NH₄⁺,NO₃^-were significantly higher than L.chinensis litter,while the opposite effects were observed on the content of total soil nutrients such as total carbon,nitrogen and phosphorus.With the litter decomposition,the physicochemical factors of L.chinensis litter were significantly lower than C.virgata litter.The nutrient elements in litters showed the dynamic release patterns of"leaching-release"and"leaching-enrichment"with the litter decomposition.The carbon element in C.virgata litter of highly degraded level was a"leaching-enrichment"pattern,while the other treatment was a"leaching-release"pattern.The nitrogen content in L.chinensis litter of highly degraded level showed a"leaching-release"pattern,while the other treatments showed a"leaching-enrichment"pattern.The phosphorus element in all litters showed a"leaching-enrichment"pattern in all degraded levels.（2）The effects of litter types on soil microorganisms in different degraded grasslands showed that C-and N-acquiring enzyme activity decreased gradually with the litter decomposition time increased,but oxidase activities showed an increasing trend.Litter types had different effects on soil extracellular enzyme activity and stoichiometry with different degraded levels.Except for fungi and ratio of fungi to bacteria,soil microorganisms content decreased gradually with the litter decomposition time increased.The effects of litter types on soil microorganisms community had the same trend:on the 23-day,the contents of all phospholipid fatty acids（PLFA）on L.chinensis litter were significantly lower than those on C.virgata litter in lightly degraded level,but the effects in moderately and highly degraded levels were opposite.On the 60-day,in lightly degraded level,the effect of L.chinensis litter on all PLFA contents was significantly higher than the C.virgata litter treatment.Soil extracellular enzyme activity and stoichiometry,as well as PLFA content,were mainly affected by soil p H,electrical conductivity（EC）,soil nutrient and litter nutrient.（3）The effects of litter types on soil nematodes showed that the absolute abundance of nematode trophic groups increased gradually with the litter decomposition time increased.The effects of litter types were mainly found on fungivores,bacterivores and omnivore-predator nematodes which was absolute abundance of those trophic group were significantly higher in L.chinensis litter than those in C.virgata litter.The Evenness index,Generic richness and Shannon diversity decreased gradually with litter decomposition time increased,while Simpson index increased.There was no significant difference between two litter types on soil nematode diversity.Except Simpson index,soil nematode diversity index decreased gradually with increasing degraded levels.Soil nematode abundance and diversity were mainly affected by soil p H,EC,NH₄⁺,soil total carbon,nitrogen and phosphorus contents,and litter total nitrogen and phosphorus contents.（4）The effects of litter types on the soil micro-food web functions in different degraded grasslands.It showed that the soil micro-food web changed gradually from a non-disturb state to a disturb state.There was no significant difference in the nematode metabolic footprint（NMF）among different litter treatments.On the 23-day of litter decomposition,the energy channels of soil micro-food web were mainly herbivore channel and bacterivore channel.On the 240-day,the energy channels gradually changed to bacterivore channel and fungivore channel.The C-,N-and P-acquiring enzyme activity,the contents of bacteria,fungi,total PLFA and absolute abundance and diversity of fungivores and bacterivores play an important role in soil carbon,nitrogen and phosphorus cycles operation.