Study On Variation Characteristics Of Grassland Under Different Conversion Periods On Soil Properties In Loess Hilly-gully Region

Since the implementation of the project of conversion cropland to forest and grassland on the Loess Plateau region in China,grassland has obvious improvement effects on the basic properties and structural characteristics of soil.In order to clarify the mechanism of grassland impacts on soil with different returning years,expand the grassland restoration area and rationally plan the planting structure.In this paper,three vegetation lands?the restoration period is about 5 years,15 years,and 25 years of grassland?and the additional cropland which located in Changwu Loess tableland slope and Zhifanggou Small Watershed in Ansai County,Yan'an City,Shaanxi Province,Central region of the Loess Plateau were selected as research objects.The basic properties,aggregates characteristics,pore characteristics and infiltration capacity of the soil among 0³0cm in these plots were studied and compared.The results are as follow:?1?The basic properties of the topsoil?0³0 cm?of vegetation in different conversion years are different.The sand content showed G_5a>G_15a>Cropland>G_25a;the silt content showed G_25a>G_15a>G_5a>Cropland;the clay content showed Cropland>G_25a>G_15a>G_5a.Soil bulk density showed Cropland>G_5a>G_15a>G_25a.Soil saturated moisture content and >0.25 mm water-stable aggregates showed the same trend G_25a>G_15a>G_5a>Cropland.Soil organic matter content and Total nitrogen content showed G_25a>G_15a>Cropland>G_5a.Root biomass and Root area index showed G_25a>G_15a>Cropland>G_5a;Root length density showed G_25a>G_15a>G_5a>Cropland;Root average diameter showed G_25a>G_15a>Cropland>G_5a.Overall,the soil basic properties of Grassland with a conversion period of 15 or 25 years were better than that of Grassland with 5 years of conversion and cropland.?2?The MWD?GMD and fractal dimension?D?of the topsoil?0³0 cm?aggregates of vegetation in different conversion years are different.The MWD and MWD showed G_25a>G_15a>G_5a>Cropland;the fractal dimension?D?showed Cropland>G_5a>G_15a>G_25a.Grassland with 5 years of conversion owned higher >5 mm aggregates content.Therefore,MWD and GMD value are higher.The MWD and GMD value of cropland are at a low level.Therefore,the stability and the erosion resistance of soil is poor.Overall,the soil aggregates content of Grassland with a conversion period of 15 or 25 years were better than that of Grassland with 5 years of conversion and cropland.?3?The infiltration capacity of the topsoil?0³0 cm?of vegetation in different conversion years are different.Initial infiltration rate showed G_25a>G_15a>Cropland>G_5a;stable infiltration rate showed G_25a>G_15a>G_5a>Cropland.Grassland with 5 years of conversion and cropland showed lower infiltration capacity.Therefore,the longer the period of vegetation conversion,the stronger the improvement of soil infiltration capacity.The infiltration capacity of grassland soil is stronger than that of Cropland.?4?The pore characteristics of the topsoil?0³0 cm?of vegetation in different conversion years are different.The average porosity and pore number showed G_25a>G_15a>Cropland>G_5a;the pore rounding rate showed Cropland>G_5a>G_15a>G_25a;the pore fractal dimension showed G_25a>G_15a>G_5a>Cropland.In the depth of 0¹5 cm,macropores in four types of plots developed well.With the deepening of soil layer,the porosity and pore number are significantly decreased faster,the pore fractal dimension decreases slowly and the change of pore roundness is not obvious.There was a significant?P<0.001?or?P<0.05?correlation between soil root biomass,soil organic matter content,soil clay content and soil pore parameters.Their changes had an important influence on soil pore characteristics variation.The results of our study showed that grassland can significantly improved the basic properties,aggregate characteristics,pore characteristics and infiltration capacity of soil.It is beneficial to enhance the structure and stability of soil,promote rainfall infiltration,and improve soil erosion resistance.It is beneficial to enhance the structure and stability of soil,promote rainfall infiltration and improve soil erosion resistance.Therefore,the Loess Hilly-Gully region should implement the project of conversion cropland to forest and grassland,expanding the area for grassland restoration,rationally planning the planting area,and coordinating the functions of water and soil conservation and environmental greening of vegetation.