Distribution Of Organic Carbon And Nitrogen In Soil Aggregates Of Revegetation In The Loess Plateau

The Loess Plateau was the most serious water and soil loss region in China.After the 1950 s,under government regulations of reforestation/planting grass,the eco-environment was improved.With the containment of soil and water loss,the soil effect and the carbon sink function of vegetation restoration are becoming the urgent scientific problems.This study aimed at the revegetated soil of Loess Plateau and analyzed the distribution characteristics of soil organic carbon and total nitrogen in soils and aggregates under different revegetation ages.In order to reveal the relationshiop between revegetation,ages and the dynamic changes of soil organic carbon and total nitrogen.This study can provide a theoretical foundation to evaluate accurately the function and sustainability of soil carbon sequestration of revegetation.And it can also contribute to understanding the terrestrial ecosystem carbon cycle and climate system feedback function which dominated by soil organic carbon cycling.The mianly conclusions are as follows:(1)Revegetation can increase the stability of aggregate and improve the soil structure in the Loess Plateau.Revegetation can also increase soil MWD,MGD and macro-aggregate(>0.25 mm),there was positively correlated between the three.The p H in 0-10 cm soil layer was less than 10-20 cm soil layer under different vegetations.The pH in wind-water erosion crisscross region is above 8 and revegetaion has not significant effect on soil pH.Soil pH in Robinia pesudoacacia plantations showed spatial and temporal variability.Soil bulk density in 0-10 cm soil layer was less than 10-20 cm soil layer under different vegetations.In the wind-water erosion crisscross region,soil bulk density was very high(1.28-1.53 g·cm-3)and revegetation has not significant effect on soil bulk density.The soil bulk density in Robinia pesudoacacia plantations was different in different locations.In Yan'an,Fuxian,Yongshou and Fufeng,planning Robinia pesudoacacia on farmland could reduce soil bulk density,while it is increased in Binxian.(2)In the wind-water erosion crisscross region,all the three revegetation practices increased the concentrations of soil organic carbon and total nitrogen in the 0-10 cm soil layer,but their effects differed with revegetation age.Compared with adjacent farmlands,the concentrations of soil organic carbon dan total nitrogen in the 0-10 cm soil layer in grassland did not change within 10 years of succession,but increased after 20 years of succession.In alfalfa land,the concentrations of soil organic carbon and total nitrogen increased by 52%-83% and 43%-67% in the 0-10 cm soil layer,with the increasing rates of stocks of soil organic carbon and total nitrogen being 0.17-0.46 and 0.015-0.043 Mg·ha-1·a-1,respectively.However,soil organic carbon and total nitrogen were not affected by revegetation age in alfalfa land.The increases of concentrations of soil organic carbon and total nitrogen remained high in the first 20 years after conversion of farmland to peashrub land,but decreased after 40 years of conversion.In conclusion,the conversion of farmland to peashrub land or alfalfa land could be better in contributing to high stocks of soil organic carbon and total nitrogen than natural succession of grassland.However,their positive effects on the enrichment of soil organic carbon and total nitrogen may not be sustainable due to the scarcity of soil moisture and high water consumption of these two revegetation plants.(3)Planing Robinia pesudoacacia on farmland increased the concentrations and stocks of soil organic carbon and total nitrogen in the 0-10 cm soil layer in Loess Plateau,and the increasing range was the highest in Yongshou and lowest in Binxian.Compared with the adjacent farmland in the 10-20 cm soil layer,soil organic carbon and total nitrogen increased significantly in Yan'an and Fuxian,and in Fufeng and Yongshou had no significant change,but decreased in Binxian.The concentrations of soil organic carbon and total nitrogen in 0-10 cm soil layer were 1.4-2.8 times of 10-20 cm.The changes of soil organic carbon and total nitrogen in different locations were different with revegetation age.(4)In the Loess Plateau,revegetation can increase the concentrations and stocks of soil organic carbon and total nitrogen in aggregates,and the increasing in 0-10 cm soil layer was larger than that in 10-20 cm.The concentrations and stocks of soil organic carbon and total nitrogen in aggregates depended on revegetation age.In the natural succession grassland,the concentrations and stocks of soil organic carbon and total nitrogen in >0.25 mm aggregate increased in the early succession,then those in 0.25-0.053 mm and <0.053 mm aggregates also increased in the following succession.In peashrub land,the concentrations and stocks of soil organic carbon and total nitrogen in aggregates increased significantly in the first 20 years,but decreased in 40 years.The changes of the concentrations and stocks of soil organic carbon and total nitrogen in aggregates fluctuated with revegetation age.There was no significant change in the stocks of organic carbon and total nitrogen in aggregates in Binxian.In Yan'an,the stocks of organic carbon and total nitrogen in >0.25 mm and 0.25-0.053 mm aggregates increased with the revegetation age,but decreased in <0.053 mm aggregate.In Fuxian,the stocks of organic carbon and total nitrogen in >0.25 mm aggregate did not change,while those in 0.25-0.053 mm and <0.053 mm aggregates increased.In Yongshou,the stocks of organic carbon and total nitrogen in >0.25 mm aggregate increased gradually,but decreased in <0.053 mm aggregate with revegetation age.In Fufeng,the stocks of organic carbon and total nitrogen in >0.25 mm and 0.25-0.053 mm aggregates increased significantly with revegetation age.The soil organic carbon and total nitrogen were mainly accumulated in >0.25 mm aggregate in Fuxian,Binxian,Yongshou,Fufeng Robinia pesudoacacia plantations,while those in the wind-water erosion crisscross region and Yan'an Robinia pesudoacacia plantations were mainly concentrated in >0.25 mm and 0.25-0.053 mm aggregates.(5)The distribution of soil organic carbon and nitrogen in revegetated soils in the Loess Plateau was closely related to the distribution of aggregates and soil organic carbon and nitrogen in aggregates.The organic carbon and total nitrogen in aggregates were mainly concentrated in >0.25 mm and 0.25-0.053 mm aggregates in the wind-water erosion crisscross region and Yan'an Robinia pesudoacacia plantations,due to the dominated the proportion of 0.25-0.053 mm aggregate and the concentrations of organic carbon and total nitrogen in >0.25 mm aggregate.Because of the absolute predomianance of the proportion of >0.25 mm aggregate and the concentrations of organic carbon and total nitrogen in >0.25 mm aggregate,the soil organic carbon and total nitrogen were mainly accumulated in >0.25 mm aggregate in Fuxian,Binxian,Yongshou,Fufeng Robinia pesudoacacia plantations.