The cost of uncertainty for nitrogen fertilizer management: A sensitivity analysis

Uncertainties in soil nitrogen (N) supply and crop N demand present a challenge to farmers deciding on N fertilizer rates. While field studies have documented the improvements in N use efficiency possible with site-specific N management approaches that address these uncertainties, a general understanding of the importance of uncertainty across a wide range of cropping systems is yet to emerge. Here a general model of N rate decision-making is presented which computes the optimal N rate that maximizes expected profit given uncertainties in N supply and demand. The cost of uncertainty is measured as the difference in N rate when soil N supply and crop N demand are unknown versus known perfectly. Eliminating uncertainty in soil N supply (but not crop demand) would reduce average N rates by ∼5–15% in typical irrigated rice systems, 10–30% in wheat, and 20–40% in maize. Perfect knowledge of potential crop N demand (but not soil supply) would reduce rates by ∼3–10% in all systems. Simultaneous knowledge of both factors reduced N rates by significantly more than the sum of their individual effects, reflecting important interactions between supply and demand uncertainties. This indicates that the value of information for one factor is inversely related to the level of uncertainty in the other. Studies that separately consider information on soil N supply or crop N demand may therefore underestimate the total benefit of management approaches that address both these uncertainties. Site-specific N management could lead to substantial reductions of N rates without yield loss in a wide range of cropping systems, thereby improving profitability and environmental quality.