EFFECT OF MANAGEMENT OF PREVIOUS CEREAL STUBBLE ON NITROGEN-FERTILIZER REQUIREMENT OF WHEAT

The effect of either burning stubble, or incorporating it in the soil, on the nitrogen (N) fertiliser requirement of the following wheat crop was examined over 10 years (1978-87) in a continuous wheat system at 2 sites (Wongan Hills and Nabawa), and in both continuous wheat and wheat-fallow systems at one site (Merredin). There were significant grain yield increases in response to N fertiliser in all years at Nabawa. At Wongan Hills there was no response in 1978 and 1985, a yield reduction in 1979, and a yield increase in all other years. At Merredin, there was no response in 1980, a yield decrease in 1984 and 1985, and an increase in all other years. In some years grain yield responses were small at Wongan Hills and Merredin. The only significant overall effects of stubble treatment were at Nabawa in 1978 (P<0.01) and 1985 (P<0.05). The interaction between stubble treatment and N rate was significant at Wongan Hills in 1980 and 1981 (P<0.05), and at Merredin in 1981 (P<0.001), 1983, and 1985 (both P<0.05). Response to N fertiliser was higher where the stubble was incorporated than where it was burnt. There was also a tendency for higher optimum economic rates of N fertiliser with stubble incorporated rather than burnt, but differences were not large. At Merredin, the overall yield increase with fallow was significant (P<0.001) in 1979 and 1983. The fallow x N fertiliser rate interaction was significant in all comparison years except 1987. Responses to N were greater in the non-fallow treatments. Soil organic carbon (C) levels were higher with stubble incorporation than where the stubble was burnt, and fallowing resulted in lower organic C. There was a downward trend with time, especially when fallowing was carried out. Effects on total N levels in the soil were similar to those for organic C but were less marked. The study indicates that at a level of stubble residues of 1-3 t/ha with continuous wheat in this winter rainfall environment in Western Australia, stubble treatment is unlikely to be a major factor in determining the rate of N fertiliser required for a wheat crop.