Availability of residual fertilizer nitrogen in a Darling Downs black earth in the presence and absence of wheat straw

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Abstract
Mineralisation and availability of residual fertiliser nitrogen (N) was studied in pots during December-October with and without the addition of straw (0-7.5 t/ha) on a Darling Downs black earth previously cropped with wheat. Soil (0-0.2 m) and straw were collected from field plots in which wheat was grown previously with supplementary irrigation and fertiliser N applied at 0, 100, 200, 300 or 400 kg/ha. At the end of the fallow, in June, there was a net increase in soil mineral N of between 0.7 and 11.1 mg/kg where fertiliser was applied to the preceding crop. The increase represented between 2 and 9% of the original N application and was larger with increasing N application rate and smaller with increasing rate of straw addition. Straw addition caused a substantial decrease in mineral N which was still evident in June and October, 162 and 305 days respectively following straw addition. Soil mineral N decreased linearly at the rate of 5 kg N/t of straw added up to 7.5 t/ha. The net effect of prior N applications on the quantity of N available to wheat plants was equivalent to 10-23% of the quantity of N applied to the preceding crop in the absence of straw and only 4% in the presence of straw. Residual effects of prior N applications on the quantity of N available for wheat plants was generally greater than was evident as soil mineral N in June. During crop growth, additional available N may have been released from the microbial soil N pool, especially where 200 or 400 kg/ha of N had been applied. Straw addition resulted in more microbial biomass throughout the fallow. The larger microbial N pool, however, contained less N than that immobilised due to straw addition. Thus, regardless of prior N application, less N was available to wheat plants in the presence than in the absence of straw of preceding wheat crops.