Using fertiliser to maintain soil inorganic nitrogen can increase dryland wheat yield with little environmental cost

Abstract Nitrogen (N) deficiency is responsible for a large proportion of the exploitable yield gap that exists in Australian wheat production. In order to maximise N use efficiency (NUE), growers attempt to match N fertiliser to seasonal water limited yield potential, which in the southern grain growing region is largely determined by in-crop rainfall. Australia’s variable rainfall makes water limited yield potential very difficult to predict, and growers tend to under fertilise, fearing economic and environmental losses of N if crops are over-fertilised. However, environmental losses of excess N are low in the semi-arid, winter dominant rainfall environments with high water holding capacity soils that comprise much of the wheat producing regions of south eastern Australia. This is particularly the case when high carbon (C) to N ratio crop residues are retained which facilitate rapid immobilisation of any mineral N surplus to crop requirements. We therefore propose that the exploitable yield gap in wheat could be closed if a longer-term strategy of N fertiliser application was adopted. In this strategy, growers would simply use N fertiliser to maintain an environmentally appropriate base level of inorganic N (N bank) rather than attempting to match fertiliser inputs to seasonal conditions. We used field studies with 15N labelled fertiliser and cropping systems simulations to demonstrate that fertiliser N applied at rates calculated to maintain fixed thresholds of inorganic N (N bank criteria) and top-dressed immediately prior to the period of rapid crop uptake can substantially increase yields and profit in comparison to current practice, even when applications exceed crop demand in some years. A further benefit of this strategy is an increase in soil organic matter. Both field and simulation studies indicate that the environmental cost of such a strategy would be minimal, with only a small increase in leaching and denitrification despite higher overall rates of N application. This strategy needs to be compared with current practices in longer term field experiments to confirm production increases and lack of environmental impacts.