SOIL AGGREGATE AND ITS RESPONSE TO LAND MANAGEMENT PRACTICES

This paper provides a broad review of the existing study on soil aggregate and its responses to land management practices. Soil aggregate is used for structural unit, which is a group of primary soil particles that cohere to each other more strongly than other surrounding particles. The mechanism of soil particle aggregation may be expressed by a hierarchical model, which is based upon the hypothesis that macroaggregates （〉250μm） are collections of smaller microaggregates （〈250μm） held together with organic binding agents. Primary particles form microaggregates and then macroaggregates. Carbon （C）-rich young plant residues form and stabilize macroaggregates, whereas old organic C is occluded in the microaggregates. The interaction of aggregate dynamics with soil organic carbon （SOC） is complex and embraces a range of spatial and temporal processes within macroaggregates and microaggregates. The nature and properties of aggregates are determined by the quantity and quality of coarse residues and humic compounds and by the degree of their interaction with soil particles. The mechanisms resulting in the binding of primary soil particles into stable aggregates vary with soil parent material, climate, vegetation, and land management practices. Land management practices, including tillage methods, residue management, amendments, and soil fertility management, enhance soil aggregation. However, there is still much uncertainty in the dynamics of organic matter in macroaggregation and microaggregation, and research is still needed to understand further the mechanisms of aggregate formation and its responses to human activities.

Effects of land use patterns on soil aggregate stability in Sichuan Basin,China

Soil aggregate stability as a key indicator of soil structure, is a product of interactions between soil environment, management practices, and land use patterns. The objective of this study was to analyze the impact of various land use patterns on soil aggregate stability in Sichuan Basin of southwestern China. The dry- and water-stable aggregate size distributions were determined by manual dry sieving procedure and Yoder＇s wet sieving procedure, respectively, while microaggregates and its mechanical and chemical stabilities by Kachisky＇s method, oscillator method, and citrate-dithionate （C-D） reagent method, separately. The results indicated that fractal dimension and surface fractal dimension were useful indicators to reflect soil aggregate distribution. Land use patterns have an obvious influence on soil aggregate stability. In the study area, water stability, mechanical stability, and chemical stability followed the sequence, Barren land 〉 forestland 〉 orchard 〉 cropland, and the original stability and collapse velocity were sensitive to soil properties and soil structure. The difference of aggregate stability under different land use patterns is mainly due to the intensity of human disturbance and cultivation. Improper land use patterns will lead to breakdown of unstable aggregates, producing finer and more-easily transportable particles and microaggregates. In the future, inappropriate cultivation and land use patterns should be changed to protect soil structure, to improve soil aggregate stability and soil fertility in Sichuan Basin.

The relative contributions of soil hydrophilicity and raindrop impact to soil aggregate breakdown for a series of textured soils

Soil aggregate breakdown is the first key factor that causes soil erosion.At present,research on the mechanisms of soil aggregate breakdown during rainfall is common.However,the research to on quantifying the relative contributions of internal and external forces to aggregate breakdown remains limited.This paper was conducted to analyse the relative contribution of internal and external forces to aggregate disintegration and the factors affecting aggregate stability during rainfall.Soil aggregates with a series of textures were selected as test soil samples;deionized water was employed as the soaking solution and rainfall material in static disintegration experiments and rainfall simulation tests.The effect of internal force(soil hydrophilicity)on aggregate disintegration was analysed by the static disintegration method,and the combined effects of internal force(soil hydrophilicity)and external force(raindrop impact)on soil aggregate breakdown were analysed by rainfall simulation experiments.The results indicated that external force caused more severe soil aggregate breakdown than internal force,and the crushed aggregate was mainly distributed in the range of 2–0.25 mm.With increasing rainfall kinetic energy,the degree of aggregate breakdown increased gradually,and the degree of aggregation of the soil particles decreased gradually.Furthermore,soil aggregates with a high clay content(>30%)were more stable than medium-clay(20–30%)and low-clay(<20%)soil aggregates,and the correlation coefficient provided a good representation of the relationship between the clay content and soil aggregate stability index(ASI).Therefore,external force contributed more to soil aggregate breakdown than internal force during rainfall,and clay plays an important role in aggregate stability.The results of this study are of great significance for elaborating the mechanism and factors affecting aggregate breakdown.