SPATIAL ANALYSIS OF THE RELATIONSHIP BETWEEN NITROGEN SURPLUS AND YIELD IN SUGARCANE

DATA ON NITROGEN losses from sugarcane to ground and surface water are sparse, mainly due to the high cost of in-paddock measurement. To overcome this lack of data the nitrogen surplus is often used as a surrogate for nitrogen losses. The nitrogen surplus is simply the difference between applied nitrogen and nitrogen exported in harvested material. As the nitrogen surplus increases, the potential for nitrogen losses also increases. Yield of sugarcane can vary over short distances, and can often exhibit temporally stable ‘production zones’ at a sub-block scale. Hence, we hypothesised the nitrogen surplus also varied at a sub-block scale and was correlated to yield. In a six-hectare block of third and fourth ratoon Tellus growing in the Burdekin region, we measured fresh and dry weight yield, and nitrogen concentration of above ground biomass at harvest at 22 and 25 random sites in 2012 and 2013. We determined nitrogen surplus at these sites. Fresh weight sugarcane yield averaged 117 t/ha (range 67 to 146) in 2012 and 81 t/ha (range 41 to 112) in 2013. Nitrogen surplus exhibited a wider range, averaging 114 kg N/ha in 2012 (range 54 to 171) and 68 kg N/ha in 2013 (range –7 to 136). There was a strong relationship between yield and nitrogen surplus; the higher nitrogen surplus sites were also the lower yielding sites. These results suggest lower yielding areas of the block are most likely to be where there is higher potential for nitrogen losses under uniform management. Management actions targeted at lowering nitrogen application rates to lower yielding areas of blocks, such as variable rate nitrogen application, could be implemented to mitigate nitrogen losses from sugarcane.