Evaluation of nitrogen bank, a soil nitrogen management strategy for sustainably closing wheat yield gaps

Abstract

Yield gaps of ∼50 % of water limited yield potential occur in wheat globally. Nitrogen (N) deficiency frequently contributes to the gap, but it is difficult to predict an appropriate rate of N to apply because of uncertainty about the forthcoming season and hence water limited potential yield and crop N requirements. We investigated closing the water limited yield gap attributable to N supply by maintaining a “bank” of soil mineral N that is nonlimiting for wheat yields in most seasons. The yield, loss of NO3−-N in leaching and denitrification, and gross margins of wheat crops were simulated at four Australian locations with contrasting soil types and potential yields, in response to N inputs calculated by reference to N banks ranging from 50 to 400 kg N ha-1, the Yield Prophet® decision support tool and the national average N rate. The N bank providing best yields at each location was close in amount to the long-term average rate calculated with Yield Prophet®. Compared to the national average rate of N fertiliser application (45 kg N ha-1), the highest-yielding N bank treatment on average increased the amount of N applied by 3–234 kg N ha-1, the fraction of water limited potential yield by 0.0 to 0.61, and gross margins by AUD 0–1,026 ha-1. Combined losses of N to the environment in NO3−leaching and through denitrification in these strategies also increased but by a disproportionately small amount compared to the increase in the amount applied (-3 to 8.5 kg N ha-1). These losses were relatively insensitive to the amount of N applied at three of the four locations; at the fourth location large episodic losses could occur from sandy soil despite low rainfall when the N bank exceeded water limited potential yield. The N bank approach to determining fertiliser rate has potential to sustainably reduce the yield gap where the target amount is matched to average site-specific yield potential.