Effects of Long-term Application of Organic Manure and Chemical Fertilizers on Organic Carbon in Aggregates of a Sandy Loam

To evaluate the effects of long-term application of organic manure and chemical fertilizers on the concentration of soil organic matter,fluvo-aquic soils collected in the North China plain were separated into different size aggregates by wet sieving and the particulate organic matter of them were further segregated through combining density fractionation,dispersion and wet-sieving.The water-stable aggregates were dominated by the size fraction of 53 – 250 μm in the tested soil.Organic manure addition significantly increased the size fraction of 250 – 2 000 μm,but lowered the proportion of the fraction of 53 – 250 μm.The application of chemical fertilizers increased soil organic C concentrations in aggregates but did not change ratio of aggregates in soils.Soil organic carbon in aggregates mainly existed as fine particulate organic matter.Application of organic manure sharply increased concentrations of mineral-combined organic C and the proportion of fine particulate organic matter and mineral-combined organic matter.In contrast,Chemical fertilizer amendment nearly evenly increased organic carbon concentrations in different fractions of aggregates.