The effect of stubble management on the availability of ¹⁵N-labelled residual fertilizer nitrogen and crop stubble nitrogen in an irrigated black earth

Abstract

Field experiments on an irrigated alkaline black earth soil of the Darling Downs, south-east Queensland, examined transformations of nitrogen (N) and its subsequent availability for the growth of wheat after stubble had been removed, mulched or incorporated. Two crop sequences were used: sorghum-3- month fallow-wheat (S-W); and wheat-7-month fallow-wheat (W-W). The crops were grown in microplots enclosed by steel cylinders (75 cm diam. and 35 cm deep) to a depth of 30 cm. For the initial crop, some plots were fertilized with l5N-labelled ammonium sulfate and others with unlabelled ammonium sulfate (50 kg N/ha). After harvest of the initial crop, labelled stubble was added to unlabelled soil, either as a mulch or incorporated, and unlabelled stubble was similarly added to soil labelled with residual 15N from the fertilizer application. Uptake of 15N by a test wheat crop and distribution of 15N in the soil-plant system were then determined. In the test crop fertilized with unlabelled urea (50 kg N/ha), incorporation of stubble depressed plant growth and N uptake by 35% in the S-W sequence but had no effect in the W-W sequence. Residual fertilizer 15N in the soil was more available to the test crop than was 15N in retained stubble (6 v. 2% and 12 v. 6% for the S-W and W-W sequences respectively). However, the test crop obtained only 0.9-1.2% of its total N uptake from residual fertilizer N and 0.4-2.9% from the stubble of the initial crop. The effects of stubble management on the availability of N from these two sources were small. If suitable rates of N fertilizer are applied, it is unlikely that crop yields will be adversely affected by stubble retention in this subtropical environment.