Effect Of Shrub On Soil Organic Carbon Stability Of Grassland In Arid Area

The stability of soil organic carbon（SOC）refers to the resistance of SOC to external interference in a certain environment,which determines the carbon cycle and turnover.Shrubbery refers to the significant increase of shrub coverage,density and biomass in grassland,which is particularly common in arid areas.The study of the impact of shrub on the stability of grassland SOC in arid areas is of great significance to understand the global carbon cycle,climate change and the relationship between them.In this study,four types of grassland distributed along the altitude were selected in the Middle East section of the northern slope of Tianshan Mountain in typical arid area of Xinjiang.Samples were taken under and between shrub patches respectively.Thermal analysis technology,soil aggregate separation technology and nuclear magnetic resonance spectroscopy technology were used to comprehensively reveal the impact of shrub on the SOC stability of grassland in dry and arid area.The main results are as follows:（1）From temperate desert to mountain meadow,the contents of SOC,soil total nitrogen and total phosphorus increased,while p H and C/N ratio decreased with the increase of altitude.In temperate desert,the total phosphorus of soil under shrub was significantly lower than that between shrubs,but there was no significant difference in SOC,p H,soil total nitrogen and C/N ratio;In temperate grassland desertification,the soil total nitrogen under shrub was significantly lower than that between shrubs,while the C/N ratio was significantly higher than that between shrubs.There was no significant difference in SOC,p H and soil total phosphorus;In temperate desert grassland,SOC,C/N ratio,soil total nitrogen and total phosphorus under shrub were significantly higher than those between shrubs,but there was no significant difference in p H;In mountain meadow,SOC and soil total nitrogen under shrub were significantly higher than those between shrubs,but there was no significant difference in p H,soil total phosphorus and C/N ratio.（2）The proportion of aromatic carbon（C）among shrubs decreased gradually from temperate desert to mountain meadow along the altitude.In temperate desert,temperate grassland desert,temperate desert grassland and mountain meadow,the ratio of alkyl C to alkoxy C increased by 0.07,0.12,0.03 and 0.20respectively.The phenomenon of shrub increased the chemical structure stability of SOC.In temperate steppe deserts,temperate desert grasslands and mountain meadows,the ratio of the mass of easily decomposable SOC（SOC decomposed at lower temperature）to the total mass of SOC under shrubs（%Exo₁）and the corresponding temperature of SOC at half of energy release(DSC-T₅₀)are higher than those between shrubs,while the temperature at half of SOC decomposition(TG-T₅₀)is lower than that between shrubs,indicating that in these grasslands,the phenomenon of shrubbery increases the thermal stability of SOC.In temperate desert,although the indexes%Exo1,TG-T₅₀ and DSC-T₅₀ are not different between shrubs,the proportion of SOC combustion energy released in total combustion energy（Q）decreases from shrub to shrub at low temperature,while the energy released by SOC combustion increases at high temperature,which also indicates that shrubbery increases the thermal stability of SOC.（3）Shrubbery affected the composition of water stable aggregates in arid grassland.Compared with that between shrubs,in temperate desert,the soil under shrubs is<53μm.The proportion of aggregates increased;In temperate grassland desertification,the proportion of aggregates>2 mm in soil increases;In temperate desert grassland,the proportion of aggregates>2 mm in soil increased and 53μm～0.25 mm aggregate and<53μm.The proportion of aggregate decreased;In mountain meadow,The proportion of 53μm～0.25 mm aggregates decreased.Shrubbery increased the stability of warm grassland desertification,warm desert grassland and mountain meadow aggregates.The average weight diameter（MWD）and geometric average diameter（GMD）of soil water stable aggregates under shrubbery were higher than those between shrubberies.Shrubbery increased the SOC content of>2μm aggregates in the soil under the shrub of temperate desert grassland and>2 mm aggregates and<53μm aggregates in the soil under the shrub of mountain meadow.The content of SOC in aggregates had no significant change in other grasslands and their particle size aggregates.