Longterm mineral fertilizer input or crop monoculture management can result in soil quality degralotion and soil carbon and nitrogen depletion.This study focused on soil physical properties during the conversion from conventional farming to organic farming of soybean field.Soil aggregate composition and carbon and nitrogen storage were analyzed by using soil physical aggregate isolation method. Results showed that<0.053 mm aggregate fraction and soil organic carbon content of<0.053 mm aggregate in microaggregates occluded water-stable macroaggregates decreased

while 0.053-0.25 mm aggregate fraction and soil organic carbon content of 0.053-2 mm in microaggregates-occluded water-stable macroaggregates increased significandy after the conversion from conventional soybean field to organic soybean field.Higher aggregate stability and carbon and nitrogen storage were detected in organic farming system.Organic farming is better than conventional farming ecosystems in conserving soil fertility and system stability.