IMPACT OF MANAGEMENT OF CANE TRASH AND LEGUME RESIDUES ON N MINERALISATION AND CROP N UPTAKE

INAPPROPRIATE management of the fallow legume residues has been shown to result in rapid N mineralisation and potentially large N losses due to leaching. This study examined the impact of tillage system and management of cane trash and legume residues on subsequent N mineralisation and crop N accumulation in a sugarcane plant crop at Bundaberg. Soybean fallow crops increased soil mineral N at planting of the subsequent cane crop by 160–210 kg N/ha, although even bare fallows had mineralised 110–150 kg N/ha at the time of cane planting. Incorporation of the soybean residues using conventional tillage resulted in increased mineralisation of soybean N. Tillage had a negligible effect on N mineralisation from a residual trash blanket, while presence of a decomposing trash blanket had only a minimal impact on mineralisation of soybean N– regardless of tillage system or time of incorporation. Conventionally tilled soybean crops resulted in leaching of NO3-N below the crop root zone (110 cm) at the time of a mid-season sampling 7 months after planting, with further evidence of N leaching during the remainder of the crop growing season. Treatments combining fallow soybeans with trash retention and zero tillage showed no evidence of NO3-N leaching at 7 months. However, minimal N accumulation by the cane crop in the latter part of the growing season, combined with high rates of late-season N mineralisation, resulted in leaching losses also being recorded in the zero tilled soybean treatments after harvest. Significant residual mineral N (100–200 kg N/ha) remained in soil profiles after harvest of the plant crop in the soybean fallow treatments. However, the distribution of this N in the soil profile, and measures of Potentially Mineralisable N, suggested N fertiliser applications would be necessary in the first ratoon to provide sufficient mineral N for establishment and early growth. Cane yields were greatest in the bare fallow treatments with conventional tillage, although yield differences among treatments were small. Stalk densities were not affected by treatment, but there was a negative association between high soil mineral N concentrations mid-season and individual stalk weight and also ccs. Final sugar yields varied more than cane yields, with the highest yield in the trash retained, bare fallow, conventional tillage treatment. The negative impact of high in-crop mineral N on CCS highlights the need to appropriately manage in-crop N mineralisation for commercial as well as environmental reasons.