Stability And Nutrition Of Soil Aggregates In Degraded Alpine Grassland

This study is mainly focused on the lightly,moderately and heavily degraded grasslands from Alpine steppe and Alpine meadows respectively,which were dominant ecosystems on the Qinghai-Tibet Plateau.The particle size distribution and stability of the soil aggregates within different degrees of degradation in alpine grassland were analyzed and compared,and the response regularity of soil aggregates organic carbon(SAOC),total nitrogen(SATN)and total phosphorus(SATP)nutrients were studied,and to explore the intrinsic relationships between the physical compositions and the characteristics of soil nutrients.The main results were displayed as following:(1)Grassland degradation impacted the distribution characteristics and structural stability of soil aggregates in both Alpine steppe and Alpine meadows.With the aggravation of grassland degradation,the soil aggregates in these two grassland types tended to change from large size to medium and small size,and the stability of soil aggregates tended to decrease.(2)With the aggravation of degradation degree,except for the same grain size SATP content in Alpine steppe showed a trend of light degradation > heavy degradation > moderate degradation,the same grain size SAOC and SATN content in both Alpine steppe and Alpine meadows and the same grain size SATP in Alpine meadows showed a decreasing trend.With the decrease of soil aggregate size,the content of SAOC,SATN and SATP in different grades of Alpine grassland decreased first and then increased.While with the aggravation of grassland degradation in Alpine meadows,the content of SAOC,SATN and SATP within soil aggregates tended to increase first,and then decrease,and finally increase again.Overall,the contents of SAOC,SATN and SATP were higher within the soil aggregates with particle size more than 0.25 mm.(3)The ratio of SAOC/SATN of the same grain size in two different types showed heavy degradation > moderate degradation > light degradation,and the ratio of SAOC/SATP and SATN/SATP in Alpine steppe were moderate degradation > light degradation > heavy degradation,while the ones in Alpine meadow were light degradation > moderate degradation > heavy degradation.Under the same degradation level,the maximum values for SAOC/SATN,SAOC/SATP and SATN/SATP in the two types of grassland were mostly distributed in 2-0.25 mm particle size.(4)The content of soil total nutrients closely related with both the particle size compositon of soil aggregate and the content of nutrients and distribution characteristics in aggregates.In Alpine steppe,the content of soil organic carbon(SOC),total nitrogen(STN),total phosphorus(STP)and SOC/STN,SOC/STP,STN/STP were positively correlated with 2-0.25 mm soil aggregate content and negatively correlated with 0.25-0.053 mm soil aggregate content.The content of SOC,STN,STP and SOC/STN,SOC/STP,STN/STP were positively correlated with 2-0.25 mm soil aggregate content and negatively correlated with <0.25 mm soil aggregate content in Alpine meadow.The results of stepwise regression analysis indicated that the contents of SOC,STN and STP in Alpine grassland were mainly related to the content of SAOC,SATN and SATP in large grain size,and the stoichiometric ratio of soil total nutrient was mainly related to the nutrient content of small-grain soil aggregates.(5)Under the same degree of degradation,the contribution rate of SAOC,SATN and SATP to SOC,STN and STP were consistent in Alpine steppe and Alpine meadow,Particularly,the contribution rate of <0.053 mm was the lowest.