Effects of Straw Returning on Soil Organic Matter and Characteristics of Soil Aggregates in Tobacco Planting Field

A field experiment from 2009 to 2012 were conducted to study the effects of different amount of wheat and maize straw returning on soil organic matter and characteristic of soil aggregates in tobacco planting field by dry and wet sieving. The result showed that the soil organic matter, mean weight diameter(MWD) and geometric mean diameter(GMD) all increased with the increasing straw rate. But the effect of different straw returning on soil organic matter and characteristic of soil aggregates were different. The dry sieving results indicated that the fractal dimension decreased with the increasing straw rate. The wet sieving results indicated that the fractal dimension of soil aggregates was significantly lower in the high amount of maize straw returning treatment (7500 kg/ha) than that in the low amount of maize straw returning treatment (1500 kg/ha). The fractal dimension decreased with the increasing amount of wheat straw returning. Maize straw returning had a better effect on improving the soil organic matter and characteristic of soil aggregate than the wheat straw returning under the condition of the same straw rate. Significantly positive correlations were obtained between soils organic matter and MWD and GMD by dry sieving, and soil organic matter was positively correlated with GMD and the >0.25 mm aggregates by wet sieving. In conclusion, straw returning could influence the distribution of aggregation and improve the stability of soil aggregates by increasing the soil organic matter.