Evolvement Characteristics And Quantification Of Soil Aggregate Microstructure In The Process Of Vegetation Restoration In Loess Hilly-gully Region

Soil structure is central to fulfil the functions of soil.The recovery of good soil structure by restoring vegetation is key to improve soil quality.Soil aggregate is one of important soil structural units,and their internal structure play a key role in determining the transport of water,gases,and nutrients in soils and biological activity.Until now,the effect of vegetative restoration on the pore network of aggregates,however,is not well understood.In this study,soil samples were collected from the rehabilitated lands of different rehabilitation ways and different ages on the Loess Plateau,China.The response of different soil physical and chemical properties to the revegetation was studied,and the microstructures of soil aggregates were quantified using X-ray computed tomography and image analysis.The relationships between the aggregate pore parameters and aggregate stability were obtained by quantitative analysis.Based on the soil structural index,the evolvement of soil structure in the process of vegetation restoration was also evaluated.The main results are as follows:1.The evolution characteristics of soil physical and chemical properties in the process of vegetation restoration in Loess hilly-gully region were clarified,and the main breakdown mechanism of soil aggregates was revealed.Soil organic carbon,total nitrogen,soil water holding capacity,saturated hydraulic conductivity and soil available water content increased with the increase of revegetation age.Although soil total nitrogen content,total soil porosity and capillary porosity were higher after revegetation,there was no significant difference between the abandoned sites and cropland site.The content of large aggregates increased significantly and the content of small aggregates decreased gradually with abandonment age.During the process of natural succession,aggregate stability had reached a steady state after six years of abandonment.Different revegetation types had varying effects on soil physical and chemical properties.Soil organic carbon,total nitrogen,soil water holding capacity,saturated hydraulic conductivity,soil available water content,root mass density,>2 mm aggregate content and aggregate stability were higher and soil bulk density was lower in shrubland,natural grassland and woodland than in artificial grassland and cropland.There was no significant difference in soil total porosity and capillary porosity between different vegetation types.The structures of the aggregates were subjected to varying degrees of breakdown by three wetting treatments of Le Bissonnais method.The mean weight diameter(MWD)was shown as fast-wetting treatment < wet-stirring treatment < slow-wetting treatment,indicating that slaking by fast wetting was most likely the main mechanisms of aggregate breakdown in the study area.2.The evolution characteristics of soil aggregate pore in the process of vegetation restoration in Loess hilly-gully region were revealed according to quantitative characterization of the 2D and 3D internal structures of soil aggregates.Age and revegetation type both significantly affected the development of soil structure.The 2D and 3D microstructure of soil aggregates indicated that the microstructure of soil aggregates developed from a very dense massive microstructure to a more complex porous microstructure after vegetation restoration.Total porosities,>75 ?m porosities,fractions of elongated pores,3D mass fractal dimensions,and connectivity markedly increased and the numbers of pores,<75 ?m porosities,the mean pore-shape factors,and the fractions of regular and irregular pores markedly decreased after revegetation.The improvement of aggregate microstructure increased with abandonment age.Different revegetation types had varying effects on the microstructure of soil aggregates,and differences in those pore properties among the different revegetation sites may be attributed to different plant roots and organic carbon content.The fractal dimension was more sensitive for monitoring the quality of the soil structure in this area.We suggest that the fraction of elongated pores can be used as an important indicator for monitoring the recovery of soil structure,due to their important role in the storage and transmission of soil water and gases.3.The quantitative relationship between the pore structure of aggregates and the stability of aggregates was revealed,based on which the main pore factors controlling the wet aggregate stability were confirmed.Total porosity,total number of pores,>100 ?m porosity,fraction of elongated pores and 3D mass fractal dimensions were identified as the primary factors controlling the wet aggregate stability of three wetting treatments according to partial least squares regression.Total porosity,>75 ?m porosity,fraction of elongated pores and 3D mass fractal dimensions played a positive role in the stability of soil aggregates,while total numbers of pores played a negative role in aggregate stability.Revegetation decreased total number of pores and increased total porosities,>100 ?m porosity,fraction of elongated pores and 3D mass fractal dimensions,which could improve the pore hydrophobicity,and reduce the wetting speed and slaking stress,thereby enhancing the stability of aggregates.4.Response of soil structural quality to vegetation natural restoration process was assessed.The relationship between soil structural quality and some important soil physical and chemical factors was analyzed.Among the pore properties,fraction of elongated pores,the mean pore-shape factors and MWD were found to be most important for assessing soil structural quality.Significant linear relationships between soil structural index and soil organic carbon,saturated hydraulic conductivity and soil available water content were obtained.Soil structural index(SSI)increased with the increase of restoration years,and the SSI of four abandoned sites were significantly higher than that of the cropland site.The development of SSI did not continue at the same pace with that of soil organic carbon,saturated hydraulic conductivity and soil available water content during the natural vegetative succession.The SSI values were higher under these four types of revegetation than the cropland and grouped into three categories: high SSI for shrubland and natural grassland,intermediate SSI for woodland and artificial grassland,and low SSI for cropland.In order to promote the improvement of soil quality of eroded land more effectively,especially the reconstruction of poor soil structure,we recommend shrub plantation and natural grassland for the revegetation of degraded land in semi-arid areas.