Effects Of Ecological Construction On Soil Aggregates And Their Carbon And Nitrogen In Loess Hilly Area

The loess hilly area is the most serious soil erosion area in the Loess Plateau,also is the key area of soil and water conservation and ecological construction in China.Since the 1980 s,lots of ecological construction projects were carried out in the loess hilly area,which had played a positive role in soil erosion management and the vegetation restoration,and the soil aggregates and their nutrient also had been affected.This paper took wangmaogou,a typical watershed in loess hilly area,as the research object.By means of field sampling and laboratory tests,four kinds of ecological construction sites,wood land,grass land,shrub land and terrace land were selected,and slope land was taken as the comparison,different soil samples were collected for screening and testing.In this study,not only the particles distribution and stability of soil aggregates under different ecological constructions were analyzed,but also the distribution of soil aggregates organic carbon and total nitrogen contents under different ecological constructions were analyzed.And the relationship between soil aggregates composition and soil aggregates organic carbon,total nitrogen contents under different ecological constructions was explored,and so as to reveal the influence mechanism of different ecological constructions on soil aggregates and their carbon and nitrogen contents.The main research results are as follows:(1)Ecological construction has significantly increased the percentage and stability of soil large aggregate(5-2mm).Wet sieve method could more accurately reflect the influence of ecological construction on the composition and stability of soil aggregate particles distribution and soil organic carbon and total nitrogen than dry sieve method.After sloping farmland transformed into wood land,grass land,shrub land and terrace land by ecological construction,soil aggregate stability of soil layer 0-60cm especially 0-20cm enhanced,which showed that ecological construction was beneficial to the improvement of soil aggregate structure.That embodied in the soil>0.25mm aggregates percentage increased significantly(p<0.05),MWD(mean weight diameter)and GMD(geometric mean diameter)value increased significantly(p<0.05),APD(particle size fractal dimension of aggregates)value significantly reduce significantly(p<0.05).Broadly,woodland had the greatest effect on the stability of soil aggregates,followed by grassland and shrub land,and the least was terrace land.The organic carbon and total nitrogen contents in 0-60cm soil layer of all size soil aggregates showed a trend of increasing first and then decreasing with the decrease of soil aggregates(p<0.05).And they were the highest in small aggregates and the lowest in micro aggregates.(2)Ecological construction effectively promoted and protected the accumulation of organic carbon and nitrogen in soil aggregates.In 0-60cm soil layer under different ecological construction in the study area,the deeper the soil layer,the lower its contents.Soil organic carbon contents of 0-40cm soil layer were significantly increased in ecological construction(p<0.05),while soil total nitrogen contents were significantly increased in 0-60cm soil layer(p<0.05).Soil aggregates organic carbon and nitrogen contents of 0-60cm soil layer were highest in small aggregates(2-0.25mm),while least in micro aggregates(<0.25mm).The conversion of sloping farmland into woodland,grassland and shrub land could significantly promote the accumulation and protection of organic carbon and total nitrogen of aggregates of 0-20cm soil layer.(3)The aggregates vector form of soil organic carbon and total nitrogen under ecological construction were defined.>0.25 mm aggregates contributed the most to soil organic carbon and total nitrogen,while micro-aggregate contributed the least.The greater the depth of soil layer,the weaker the physical protection of large aggregates on soil organic carbon and total nitrogen,and the stronger the physical protection of small aggregates and micro aggregates.The contribution rate of large aggregates to organic carbon and total nitrogen in 0-20cm soil layer increased significantly after slope land converted into woodland,grassland,shrub land and terrace land(p<0.05).Ecological construction could promote the stability of soil carbon and total nitrogen interaction in soil aggregates of all particle sizes and in 0-60cm soil layers.