Responses Of Soil Carbon,Nitrogen And Microorganisms To Different Land Management Practices And Freeze–thaw Cycling In The Ecology Fragile Area Of Semi-arid Loess Plateau,China

The semi-arid region of Loess Plateau is a typical rain-fed agricultural area,as well as one of the most severely degraded ecosystems in the world.Increased soil disturbance caused by high intensity of agricultural production resulted in the destruction of soil structure,which enhanced soil degradation.Grassland recovery is a low cost,effective and environment friendly way to improve soil quality.However,the effects of grassland recovery on soil quality are various in different environment conditions.Therefore,assessing the change of soil properties under different land management practices were crucial to recovery soil quality in ecology fragile area of semi-arid Loess Plateau.This study is aimed to explore the impacts of different land management practices on soil carbon and nitrogen stocks,and soil microorganisms.To discuss the responses of soil carbon,nitrogen and microorganisms in the soils of different land management practices which was subject to freeze–thaw cycles was to judge if grassland recovery resulted in different soil carbon and nitrogen stocks,and soil microbial community functional diversity which differed from cropland.The objective of present study is to provide a guideline for improving and optimizing soil quality in the ecology fragile area of semi-arid Loess Plateau.We designed and selected five kinds of land using different land management practices at the Semi-arid Ecosystem Research Station of Loess Plateau run by Lanzhou University in 2014 and 2015:(1)enclosed and uncultivated grassland(13 years old),(2)middle-age artificial(Medicago sativa L.)alfalfa land(15 years old),(3)young artificial alfalfa land(2 years old),(4)fallow land,(5)cropland,to discuss the responses of aboveground biomass,soil moisture,soil bulk density,soil organic carbon,total nitrogen,total phosphorus,available phosphorus,soil microbial biomass,and soil enzyme activity to different land management practices.Moreover,we selected three typical land management practices:(1)enclosed and uncultivated grassland(13 years old),(2)middle-age artificial alfalfa land(15 years old)and(3)cropland(wheat land),to discuss the responses of soil carbon,nitrogen and microbial community functional diversity to freeze–thaw cycles.The orders of aboveground biomass in different land management practices were shown as: cropland > young artificial alfalfa land > middle-age artificial alfalfa land > enclosed and uncultivated grassland > fallow land(no aboveground biomass).Until October 2015,soil water storage in 0-3 m of soil profile was lowest in enclosed and uncultivated grassland and middle-age artificial alfalfa land,followed by young artificial alfalfa land and cropland,and was highest in fallow land.Soil water consumption in the growing season was different in different land management practices and standing ages.Enclosed and uncultivated grassland,cropland and young artificial alfalfa land mainly consumed soil water in the 0–1 m and 1–3 m of soil profile,respectively.However,middle-age artificial alfalfa land and fallow land consumed little soil water in the 0–3 m of soil profile.Soil bulk density in the topsoil(0–20 cm)was found in the order: enclosed and uncultivated grassland < cropland < artificial alfalfa land.Soil organic carbon,total nitrogen,total phosphorus content and carbon and nitrogen stocks in the topsoil(0–20 cm)were shown as: enclosed and uncultivated grassland > middle-age artificial alfalfa land > young artificial alfalfa land ? cropland ? fallow land.Soil organic carbon and total nitrogen stocks were significantly negatively correlated with aboveground biomass,indicating aboveground biomass outputs affected soil organic carbon and total nitrogen storage.Soil microbial biomass carbon and nitrogen contents in enclosed and uncultivated grassland and middle-age artificial alfalfa land were significantly higher than in other treatments.Soil microbial biomass carbon and nitrogen were significantly negatively correlated with aboveground biomass,but were significantly positively correlated with soil organic carbon stock,indicating soil microbial biomass was closely related to soil organic carbon storage.Soil dissolved organic C in enclosed and uncultivated grassland and middle-age artificial alfalfa land was higher than in young artificial alfalfa land,fallow land and cropland,but soil dissolved organic N was lowest in enclosed and uncultivated grassland and middle-age artificial alfalfa land.Soil acid and alkaline phosphatase activities were highest in enclosed and uncultivated grassland,followed by middle-age artificial alfalfa land,and were lowest and showed no significant difference in young artificial alfalfa land,cropland and fallow land.There were significantly positive correlation relationships between soil acid and alkaline phosphatase activities and soil organic carbon stock.The results of freeze–thaw cycles in laboratory incubation showed that,soil respiration in three land management practices decreased with successive freeze–thaw cycles,and increased during the +3 °C phase,decreased during the –2 °C phase,indicated the sensitive response of soil microbial activity to soil temperature fluctuating.Within each successive freeze–thaw cycle,soil respiration,microbial biomass C and N were highest in the enclosed and uncultivated grassland with highest soil organic carbon stock,followed by middle-age artificial alfalfa land,and was lowest in cropland.Using Biolog EcoPlate ecology fragile area and principal component analysis(PCA),we found that the AWCD value which indicated soil microbial activity and the McTntosh indicator which illustrated soil microbial species,were both highest in the enclosed and uncultivated grassland,followed by middle-age artificial alfalfa land,and was lowest in cropland,indicating that soil microbial community had formed different functional groups,and the native soil microbial communities might have changed in different land management practices.Therefore,soil microbial community could better adapt to the stress in the land management practice with highest soil organic carbon stock.In conclusion,enclosed and uncultivated grassland and middle-age artificial alfalfa land could efficiently improve soil organic carbon and nitrogen storage,increased soil microbial biomass and microbial community functional diversity in the ecology fragile area of semiarid region Loess Plateau.Moreover,for recovering soil quality,we should reduce the aboveground biomass output during the artificial alfalfa land management in the ecology fragile area of semi-arid Loess Plateau.