Nitrogen losses spotlighted in research
Their quest is being assisted by comprehensive research aimed at furthering the understanding of how N is lost from cropping soils and investigating management solutions.
The research is being led by Queensland Alliance for Agriculture and Food Innovation (QAAFI) Principal Research Fellow Professor Mike Bell and Dr Graeme Schwenke from the New South Wales Department of Primary Industries (NSW DPI) and supported by the Grains Research and Development Corporation (GRDC).
Northern growers typically apply nitrogen fertilisers such as urea directly into the soil (banding) or broadcast it on the surface then incorporate.
This is done to reduce the potential for ammonium-containing (eg. sulfate of ammonia) or ammonium-producing (eg. urea) fertilisers volatilising into the atmosphere as the gas ammonia.
However, N can also be lost through leaching with the drainage of water through the profile, or through nitrate denitrification - a biological process that occurs within the soil profile wherever there is sufficient available nitrate, labile carbon substrate, and low oxygen conditions such as in slowly draining soils.
Professor Mike Bell said understanding N loss pathways and how they are influenced by seasonal conditions and management strategies was a critical first step in optimising the efficiency and profitability of applied N use.
“Given growers’ substantial investment in N fertilisers, there needs to be considerable attention given to factors that affect the efficiency of use of applied N (NUE), with indices such as crop recovery of applied N (kg fertiliser N accumulated in the crop or in the grain/kg N applied) and the agronomic efficiency of N use (kg additional grain produced/kg N applied) used to benchmark NUE,” Professor Bell said.
“Any loss of applied N will affect NUE by reducing the pool of N that a crop can use to produce biomass and grain yield.”
A recent survey of more than 150 advisors throughout NSW and Queensland showed the overwhelming majority recognise that N losses exist and can be significant, and highlighted a perception of an increased risk of losses in summer compared to winter cropping.
There was also a perception among survey respondents of greater potential N losses - as much as 20-40% of applied N - in the northern part of the region, but given the unpredictability of environmental conditions that favour losses few advisors were found to actually factor those losses into fertiliser recommendations.
Over the past three years, the research team has conducted six 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 fertiliser in summer sorghum.
According to Professor Bell, the results found between 56% and 100% of the applied N was retained in the soil and plant at harvest, with in-season rainfall, both timing and amount, and soil carbon (C) and N status having a major impact on the seasonal loss potential.
“The research also found that 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,” Professor Bell said.
“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.”
The trial work also investigated the impact of using nitrification inhibitor-coated urea, which significantly reduced nitrous oxide emissions in all studies but did not improve grain yields enough to justify the additional cost on an agronomic basis.
Professor Mike Bell, QAAFI, UQ
07 4160 0730
Sarah Jeffrey, Senior Consultant Cox Inall Communications
0418 152 859
GRDC Project Code UQ00063