The potential for N losses via denitrification beneath a green cane trash blanket

Green cane harvesting of the sugarcane crop is now practised in about 50% of the industry with 10-15 t/ha of organic matter commonly returned to the soil as a trash blanket. This study was conducted to determine the influence of the trash blanket on fertiliser N losses via denitrification and the proportion of gascs emitted as Nz and N20. Microplots were placed in sugarcane rows between stools and KN03 (98.5 atom % 15N excess) applied to the soil surface at 160 kg N/ha. Trash (10 t/ha) was then removed from half the microplots and the experimental area waterlogged using sprinkler irrigation. Covers were placed on all microplots for 1 hour and gas samples taken from the headspace after 0 and 60 minutes for flux measurements. This procedure was repeated every 8 hours for 9 days. Total denitrification and N20-N losses of 7.6 and 1.9 kg/ha, respectively, occurred from the minus trash plots over 9 days. The presence of trash on the soil surface resulted in increased gaseous N losses over the same period with total denitrification and N20-N losses of 9.2 and 2.8 kg/ha respectively, which is an increase in total denitrification losses of 17% and N20 losses of 32%. Under these conditions, the majority of gaseous N loss occurred as N2. Microbial respiration was higher from the trash treatments suggesting an initial effect of soluble C on CO2 production. However, no significant relationship was found between denitrification and CO2 production. Soil moisture content was higher and soil temperature was lower beneath the trash blanket than in the minus trash treatment throughout the study period. Trash blanketing of sugarcane soils resulted in increased microbial activity and greater emission of both Nz and NzO through longer retention of moisture in the soil surface. Some management practices are suggested that may lead to a reduction in denitrification losses from trash blanketed sugarcane fields.