A comparison of nitrogen fertiliser decision making systems to profitably close grain yield gaps in semi-arid environments

Context: Nitrogen (N) deficiency is the single biggest cause of the yield gap in Australian wheat production. Nitrogen fertiliser is a costly input and prediction of crop seasonal demand for N in Australia's variable climate is difficult, so farmers are conservative with investment in N fertiliser, leading to under-fertilisation and over reliance on soil organic N. Objective: We evaluated the ability of different N decision-making systems to close yield gaps, reduce mining of soil organic N and minimise accumulation of soil nitrate. Methods: A 5-year (2018–2022) field experiment was conducted in a rainfed Mediterranean environment at Curyo, Victoria in Australia with different N decision-making systems, namely N bank (NB) targets (100, 125 and 150 kg N ha−1), Yield Prophet® (YP) at different yield probabilities (25, 50, 75 and 100 %), annual Australian national average N rate (NA45, 45 kg N ha−1), replacement of N in exported grain (R) and a nil control, as treatments in a randomised complete block design with four replicates. Results: After five years, YP25, YP50, YP75 and NB125 applied on average 49, 30, 4 and 18 kg ha−1 more N per year than NA45, respectively, and achieved or exceeded economic yield (EY), i.e. 80 % of water-limited potential yield (PYw), as opposed to 72 % of PYw achieved in NA45. These systems also had a higher 5-year mean gross margin (AUD 469–550 ha−1) compared to the NA45 (AUD 401 ha−1). Positive 5-year partial N balance (total N input minus total N exported in grain over 5 years) was observed only in the YP25, YP50, NB150 and NB125 treatments (4–93 kg N ha−1). However, apart from NB125 these treatments had consistently higher soil mineral N levels to 1-m depth compared to NA45 and <2 marginal return:cost ratio. Also nitrate content at 0.7–1.0 m depth in the YP25 and NB150 treatments were consistently higher (p <0.05) than that in NA45. Conclusions: Low soil nitrate level, achievement of EY and higher gross margin in the NB125 compared to NA45 makes it the N management system best suited for this environment. Additionally, the positive partial N balance (4 kg N ha−1) observed in the system suggests that it is less likely to mine soil organic N compared to NA45 (-39 kg ha−1). Significance: Adoption by growers of the best performing systems should reduce grain yield gaps and reduce mining of soil organic N with no increased risk of environmental N loss.