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The influence of rotation, tillage, stubble management, and nitrogen (N) fertiliser on soil organic carbon (C) and total nitrogen (N) was studied between 1979 and 1993 in a field experiment at Wagga Wagga, New South Wales, on a red earth. The rotations included lupin-wheat (LW), subterranean clover-wheat (SW), and continuous wheat (WW) with and without N fertiliser (100 kg N/ha). At the start of the experiment the soil organic C and N in the surface 10 cm were high following many years of subterranean clover based pasture. The trends in soil organic C varied considerably between treatments from near equilibrium levels for SW direct-drilled and stubble-retained to annual losses of 400 kg/ha for WW conventionally cultivated and stubble burnt. Similarly, total soil N content over time varied from equilibrium levels to highly significant declines of 53 kg/ha. year for WW conventionally cultivated and stubble burnt. Both direct drilling and stubble retention reduced the losses of organic C and N compared with conventional cultivation and burning, with greatest loss occurring when cultivation and stubble burning were combined. SW and LW produced a similar contribution of fixed N to total N product removal, but greater benefits to following wheat crops were provided by SW rotations. Where losses of organic C and N were recorded there was no evidence of equilibrium levels being reached after 14 years.
Heenan et al. (Sun,) studied this question.