Characteristics of Organo－Mineral Complexing of Microaggregates in Paddy Soils Developed from Purple Soils

This paper deals with characteristics of organo-mineral complexing of microaggregates in the paddy soils developed from purple soils in Sichuan, China. Results show that the contents of organic matter in microaggregates are in the order of 1--0.25 mm > smaller than 0.05 mm > 0.05-0.25 mm. But the organic matter in 1-0.01 mm microaggregates accounts for 68.1%-78.7% of that in soil. The organic matter in<0.05 mm microaggregates is complexed humus on the whole, of which the degree of organo-mineral complexing varies between 96.1% and 99.5%, which is higher than that of the soil or>0.05 mm microaggregates. The contents of loosely combined humus and the ratios of loosely and tightly combined humus markedly decline with the size of microaggregates. Fresh soil humus formed from semi-decomposed organic material or organic manure added is combined first with<0.001 mm clay, and then aggregated with other organic and mineral particles to form larger microaggregates, in which the aging of humus happens at the same time; whereas organic matter of the light fraction is mainly involved in the formation of>0.05 mm microaggregates.

Composition and Characteristics of Organo-Mineral Complexes of Red Soils in South China

The objective of the present study is to reveal the composition and characteristics of organo-mineral complexes in red soils (red soil, lateritic red soil and latosol) of south China in terms of chemical dissolution and fractional peptization methods. In the combined humus, most of the extractable humus could dissolve in 0.1 M NaOH extractant and belonged to active humus (H1), and there was only a small amount of humus which could be further dissolved in 0.1 M Na4P2O7 extractant at pH 13 and was stably combined humus (H2). The H1/ H2 ratio ranged from 3.3 to 33.8 in red soils, and the proportions of both H, and total extractable organic carbon (H1+H2) in total soil organic carbon and the ratios of H1 to H2 and H1 to (H1 +H2) were all higher in lateritic red soil and latosol than in red soil. The. differences of combined humus composition in various red soils were directly related to the content of Fe and Al oxides. In organo-mineral complexes, the ratio of Na-dispersed fraction (G1) to Na-ground-dispersed fraction (G2) was generally smaller than 1 for red soils, but there was a higher G1 / G2 ratio in red soil than in lateritic red soil and latosol. G1 fraction had a higher content of fulvic acid (FA), but G2 fraction had a higher content of humic acid (HA). The ratios of H1 to H2 and HA to FA were higher in G2 than in G1. The differences in the composition and activity of humus between G1 and G2 fractions were related to the content of free Fe and Al oxides. The quantities of complex Fe and Al, the Fe/ C and Al/ C atomic ratios were higher in G2 than in G1, and the ratio of Al/ C was much higher than that of Fe/ C. It may be deduced that aluminum plays a more important role than iron in the formation process of organo-mineral complexes in red soils.