Long term trends in fertility of soils under continuous cultivation and cereal cropping in southern Queensland. IV. Loss of organic carbon from different density functions

Abstract

Six southern Queensland soils used for cereal cropping for cultivation periods ranging from 20 to 70 years were subjected to density fractionation. The soils were separated into fractions with densities of <2, 2.0-2.2, 2.2-2.4 and >2.4 Mg m-3 using bromoform-ethanol mixture. The < 2 Mg m-3 fraction (light fraction) contained only 1.8-3.2% of the total soil weight, but accounted for 15-32% of total soil organic C. In five clay soils the rate of loss of organic C from the light fraction was 2-11 times greater than that from the heavy fraction (>2 Mg m-3). The higher the clay content the larger was the difference between these two fractions in rate of loss of organic C. It is inferred that the heavy fraction was closely associated with clay in these soils. In a sandy soil, rate of loss of organic C from the heavy fraction was similar to that from the whole soil.