Characteristics Of Soil Organic Carbon Of Paddy Soil Under Different Land Uses In Jianghan Plain

Distribution, composition and stability of soil organic carbon (SOC) in soil profiles, aggregates, light and heavy fractions in soils from Jianghan Plain were measured. The characterstics of SOC under different land uses were discussed. The main results were as following:1 The SOC contents in soil profiles were different. It was high in top layer soils, which decreased with the depth increasing. The contents of SOC and active SOC were different under different land uses. They were in the sequence of paddy soil > paddy soil changed to dryland soil > rotation soil > dryland soil > orchard soil > dryland soil changed to paddy soil. Among, the SOC content of rice culture soil was the highest and its SOC stability was the lowest.2 . The degree of humification in the tested top soil was high. The carbon content of humus was in the order of humic acid > fulvic acid > humin, and the content of humus was in the sequence of paddy soil > paddy soil changed to dryland soil > rotation soil > dryland soil > orchard soil > dryland soil changed to paddy soil. The percentage of heavy fraction in the top soils was higher than that of the light fraction. However, the SOC content in light fraction was higher than the heavy fraction's. About 65.83%~76.06% in SOC distributed in the heavy fraction, and the sequestration of heavy fraction was bigger than that of light fraction.3> Soil was mainly composed by the aggregates in size of 2~20 μ m, <2 μ m and 20~50 μ m. The proportion of <50 μ m aggregate was the main aggregate which accounted for 80.83%~94.13% in all aggregates. The content of SOC in aggregate was related to the aggregate size. The content of SOC in 200~2000 μ m aggregate was the highest, and the content of SOC in >50 μ m aggregate was higher than in <50 μ m aggregate. The stability of SOC in aggregates was in the order of 2~20 u m, <2 μ m and 20~50 μ m. And the stability of SOC in aggregate in dry farming soil was higher than in rice culture soil. There was 66.30%~86.56% SOC distributing in the <50μm aggregate, and the sequestration of <50 μm aggregate was higher than >50 μ m aggregate.4 The content of SOC has significantly positive correlation with contents of amorphous iron and 1:1 phyllosilicate and has significantly negative correlation with pH value and 2:1 phyllosilicate content. It may be related to microbial activity, adsorption of opposite charge and iron oxides's bond bridge in SOC accumulated in phyllosilicate.5 Rice culture was benefited to soil fertility and greenhouse effect, specially in <20μ m aggregate play an important role. Therefore, it is helpful to make soil play an active part in agriculture production and climate change by using the reasonable agriculture ways.