Variation Characteristics Of Soil Extracellular Enzyme Activity And Microbial Residue Carbon In Grassland On The Loess Plateau

In recent years,a series of ecological projects such as returning cropland to forest（grass）have been carried out on the Loess Plateau,and significant results have been achieved.With the restoration of vegetation,the secondary succession of vegetation increases the number and species diversity of ground plants,resulting in changes in soil nutrient cycle and ecosystem.At present,a large number of studies have been carried out on the source,input and output,carbon pool storage and carbon sink process of soil organic carbon in the Loess Plateau and its influencing factors（such as plant components,restoration years,nutrients,etc.）.However,there is a lack of in-depth research to investigate the impact of vegetation succession on soil nutrient acquisition,extracellular enzymes and microbial residues as a part of soil C pool,and there are fewer studies at a depth of less than 30 cm.Taking the soil of Yunwu Mountain grassland ecosystem as the research object,this study researched the microbial necromass carbon and soil extracellular enzyme activity of grassland soil with different enclosure years.To understand the source and accumulation of soil organic carbon in grassland ecosystem of the Loess Plateau,analyze the relative contribution of microbial source carbon in grassland ecosystem to soil nutrient cycle mechanism,and clarify the different regulatory factors and nutrient constraints of soil organic matter pool in grassland ecosystem,so as to provide data support and theoretical basis for the theory of soil organic carbon forming"carbon pump".The main conclusions are as follows:（1）With the increase of enclosure years,the content of soil basic properties and microbial biomass carbon,nitrogen and phosphorus increased significantly,and decreased with the deepening of soil layer.The contents of SOC,TN and TP showed a non-linear growth process with grassland enclosure.The SOC content was 30 years>40 years>10years>20 years>15 years>1 year>cropland.In the vertical distribution of different enclosure years,the contents of SOC,TN,TP,MBC,MBN and MBP in grassland show the law of gradual decrease from the surface layer to the deep layer of the soil,and the content is the highest in the 0-10cm soil layer.The range of SOC content in 1-40 years of enclosure is 19.74～30.08 g·kg^-1,TN is 1.88～3.14 g·kg^-1,TP is 6.83～8.62 g·kg^-1,MBC is389.77～624.70 mg·kg^-1,MBN is 28.14～43.87 mg·kg^-1 and MBP is 11.47～22.66 mg·kg^-1.（2）Compared with cropland,the extracellular enzymes and their stoichiometry of grassland from 1 year to 40 years increased significantly.The activity of soil extracellular enzymes showed a nonlinear growth process of increasing.In the vertical distribution of different enclosure years,the activities ofαG、βG、βX、NAG、LAP、AP、BG/（NAG+LAP）and BG/AP decreased gradually from the surface layer to the deep layer,and the（NAG+LAP）/AP showed the opposite change.The extracellular enzyme activity in 0-40cm soil layer was significantly higher than that in the lower layer,and the activity was the highest in 0-10 cm soil layer.The range ofαG activity is 16.12～37.00 nmol·h^-1·g^-1,βG is50.83～80.29 nmol·h^-1·g^-1,βX is 12.48～40.39 nmol·h^-1·g^-1,NAG is 8.24～18.82nmol·h^-1·g^-1,LAP is 182.64～270.88 nmol·h^-1·g^-1,AP is 58.09～110.22 nmol·h^-1·g^-1.（3）In the whole process of enclosure succession,there is a strong N limit in grassland soil microbial community.With the increase of soil depth,the microbial C limit decreases.In the 0-10 cm soil layer,the vector length and angle（from 1.23 to 1.50 and 36.2to 40.6,respectively）changed significantly during the enclosure succession.The years of enclosure,soil depth and their interaction had a significant effect on soil extracellular enzyme activity and stoichiometry.There was a strong positive correlation between soil extracellular enzyme activity and soil microbial biomass,and a significant negative correlation with soil p H.（4）In cropland,the content of bacterial residues is about 3-10 times that of fungal residues,while in grassland,the content of fungal residues is about 2-5 times that of bacterial residues.Glu N is abundant in fungal cell wall.Compared with cropland,the contents of Glu N,Gal N and Man N in enclosed grassland are significantly increased,and the content of fungal residues is about 7-10 times that of cropland(0.27～0.85 g·kg^-1).Mur N only comes from bacterial cell wall,and its content in cropland is significantly higher than that in enclosed grassland.The content of bacterial residues in cropland(1.84～5.73 g·kg^-1)is 3-5 times higher than that in enclosed grassland from 1 year to 40years.It shows that the contribution of bacterial residues in cropland is higher than that in grassland,on the contrary,the contribution of fungal residues in grassland is higher than that in cropland.（5）The contribution of microbial necromass carbon（MNC）to soil SOC in cropland is higher than that in enclosed grassland.On the vertical profile with a depth of 1m,the MNC/SOC of cropland is about 59%～92%,while the MNC/SOC ratio of enclosed grassland（1-40 years）is about 14%～52%.It shows that most of the C in cropland comes from the accumulation of microbial necromass carbon,while a small proportion in grassland may indicate that a large amount of SOC comes from the accumulation of plant residues.The years of enclosure,soil depth and their interaction had significant effects on Glu N,Man N,Gal N,Mur A,fungal residues,bacterial residues and MNC/SOC.Glu N,Man N,Gal N and fungal residues had a strong positive correlation with SOC,TN,extracellular enzyme activity and soil microbial biomass,and a negative correlation with soil p H.