Understanding & managing N loss pathways : Minimising nitrogen losses to improve use efficiency in summer crops

Abstract

• Over the past 3 years, we have had 6 experiments with isotope-labelled (15N) urea fertiliser in northern NSW and a further 11 in southern Qld, all focussed on measuring the fate of applied N fertilizer in summer sorghum. Normal fertiliser contains 14N so the use of 15N allows us to trace the fate of urea-N applied to the soil from sowing through to harvest. • Between 56 and 93% of the applied N was found in the soil and plant at harvest, with in-season rainfall (both timing and amount) and soil C and N status having a major impact on the seasonal loss potential. • Avoiding unnecessarily high N rates, delaying or splitting N fertiliser so that peak N availability coincides with peak crop N demand and relying on residual N from legume rotations all significantly reduced gaseous N losses from dryland sorghum, although the effectiveness of any management strategy varied with seasonal conditions. • Nitrification inhibitor-coated urea significantly reduced nitrous oxide emissions in all studies, but did not improve grain yields enough to justify the additional cost on an agronomic basis. • Depending on the season, delaying/splitting N applications gave either no yield benefit (dry season) or a significantly greater yield (good in-crop rainfall). Much of the unused N after a dry season remained in the soil and, provided loss events were not experienced during the fallow, significantly benefited the following crop.
As reliance on N fertilizer increases, getting good crop recovery of applied N is essential Crop N recovery is most efficient when N is distributed through the root volume Gaseous losses of fertilizer N can be substantial, with denitrification of greatest concern for summer sorghum. Ensuring fertilizer N is in the crop or deeper in the soil profile is the best defence against loss Controlled release fertilizers can reduce denitrification but are rarely economic; legume N is effective