| In the paper, water-stable aggregates of different diameter (＞4mm, 2-4mm, 1-2mm, 0.5-1mm and 0.25-0.5mm) were gained and determined by drying-wetting sieving method in three kinds of eroded ( slight, medium and severe) red soils. The distributions, water-stability and the percentage of disrupting aggregates were studied in the paper. The composition of organic matter, combined humus, the nutrition distributions, and oxidation-stabilities of eroded Red soils were analyzed. At last, the contents of water-stable macroaggregates after inputting humic acid were studied in this paper. The aim was to prove the characteristis of water stable aggregate and organic matter in different eroded Red soils, reveal the influence of organic cementing material to Stability of aggregates, and show the reason of structure degradation and fertilizer decreasing because of erosion in Ultisol. These results indicated that:1. Regardless of the degree of erosion, the percentage of＞4 mm aggregates (～40%) was the highest after dry sieving. And as aggregate size decreased, so did the percentage of respective aggregate. The outcome indicated that before wet sieving, big aggregates occupied more. After wet sieving, the percentage of micro-aggregates (＜0.25mm) was the highest, occupying about 40%, regardless of erosion degree. There was little difference among the other four size fractions in terms of percentages, the values ranging from 2.67% to 26.80%. With the enhancing of erosion degree, the content of＞0.25mm water-stable aggregate (WSA) took on a falling trend, but the percentage of aggregate disruption (PAD) went up.2. The higher erosion degree was, the lower the carbon content of humus acid (total humus acid, humic acid and fulvic acid) was. So do those of aggregates. In slight and medium erosion Red soils, the carbon content of total humus acid in soil was smaller than those of aggregates, but the situation was contrary to that of severe erosion Ultisol. The carbon contents of humic acid in big aggregates (＞4mm and 2-4mm) were higher than those of soils, but these were opposite to small aggregates (1-2mm, 0.5-1mm and 0.25-0.5mm). In slight and medium eroded red soils, the carbon contents of fulvic acid in aggregates were higher than those of corresponding soils, but that was contrary to that of severe eroded red soil. All the composition of soil humus acid had a positive correlation with the content of the＞0.25mm water-stable aggregates and soil nutrients (total N, total P, and OM), and among them, humic acid had the largest influence on those.3. After inputting different concentrations (0.06g/Land 0.rg/L) humic acid, the content of micro-aggregates (＜0.25mm) reduced, with the range of 2.50%-13.91%, but macro-aggregates (＞0.25mm) increased in three kinds of eroded red soils. Compared with humic acid of low concentration, the inputting of humic acid of high concentration made more for the forming of water-stable aggregates. The contents of small aggregates (0.25-0.5mm) had the biggest increase ranging from 1.44% to 7.72% because of the inputting of humic acid.4. In both eroded red soil and aggregates, tightly-combined and loosly-combined humus substance played important roles in the makeup of humus substance, steadily-combined was the least. With the increase of erosion degree, the organic carbon contents of three kinds of combined humus substance all reduced. Aggregates (＞4mm and 2-4mm) had the largest influence on the carbon content of tightly-combined and loosely-combined humus substance in soil, but there were significant differences in carbon contents between different aggregates and soil.5. With the increase of the eroded degree, the contents of main nutrients in soil (OM, total N and total P) reduced gradually, and there was no obvious change in the content of total K. In slight and medium eroded red soils, with the increase of the aggregate size, the contents of chief nutrients in soil (OM, total N and total P) all increased, but the status of severe erosion was contrary to it. As for water stable macro-aggregates, in slight eroded red soil, the contribution rates of aggregates to soil nutrients(OM, total N and total P) consisted basically with the order: [＞4mm]＞[0.5-1mm]＞[2-4mm]＞[1-2mm]＞[0.25-0.5mm], and that of total K followed the below order: [＞4mm]＞[0.5-1mm]＞[1-2mm]＞[2-4mm]＞[0.25-0.5mm]. In the medium eroded red soil, the contribution rates of aggregates to soil nutrients (OM, total N, total P and total K) were in the sequence of [＞4mm]＞[0.5-1mm]＞[1-2mm] ＞[2-4mm]＞[0.25-0.5mm], but there was no obvious regularity in severe eroded red soil. There was a positive correlation between soil nutrients (OM, total N and total P) and water stable macro-aggregates (＞0.25mm), but total K was negative related to it.6. With the increase of the erosion degree, the contents of total organic carbon, readily-oxidation carbon and hard-oxidation reduced gradually. As to all the examined soils, the oxidation stability coefficients were in the order: slight erosion(0.38)＜medium erosion(0.52)＜severe erosion(1.48). The contents of total organic carbon, readily-oxidation carbon, hard-oxidation carbon were positive correlated with the content of water-stable maroaggregates (＞0.25mm), and among them, the readily-oxidation carbon had the closest relationship with it.
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