Long-term impact of rotation, tillage and stubble management on the loss of soil organic carbon and nitrogen from a Chromic Luvisol

Abstract

Farm management practices have led to low soil organic carbon (SOC) and total nitrogen (TN) in Australia. Change in SOC and TN during 21 years of different rotation, tillage and stubble management systems were studied on a red earth, a Chromic Luvisol, at Wagga Wagga, NSW, Australia. The rotations included lupin/wheat (L/W), subterranean clover/wheat (S/W) and wheat/wheat (W/W) which had 0 or 100 kg N ha −1 per year as urea fertilizer. Soil was either direct drilled or cultivated three times prior to sowing in L/W and S/W and cultivated three times prior to sowing in W/W. Stubble was either burnt or retained in L/W, retained in S/W and burnt in W/W. The SOC and TN levels were high at the start of the experiment following subterranean clover based pasture for most of the previous 19 years. At the end of 21 years, the change in SOC in the surface 0.1 m ranged from a loss of 8.2 t ha −1 for W/W when stubble was burnt and soil tilled, to a gain of 3.8 t ha −1 for S/W when stubble was retained and soil direct drilled. The addition of N fertilizer to W/W, when stubble was burnt and soil tilled, had no significant effect on SOC but L/W had higher levels of SOC than W/W without N fertilizer. The rate of loss of SOC in L/W and S/W rotations was considerably less with direct drilling than with three tillage passes. Similarly, stubble retention in L/W maintained higher levels of SOC than stubble burning. The proportion of stubble carbon incorporated into SOC was estimated to be 5.6%. The effects of management treatment on soil TN were similar to effects on SOC. Where change in SOC and TN occurred, there was no evidence that equilibrium had been reached, although a change in slope had occurred in many treatments. The C:N ratio generally increased but was only significantly different than the original when stubble was burnt and no N fertilizer was applied.