Resumen
Nutrient management is central in water footprint analyses as it exerts strong control over crop yield and potentially contributes to pollution of freshwater, the so-called grey water footprint. In the frame of grey water footprint accounting, two methods are suggested, the constant leaching fraction approach (10% of applied fertilizer N) and the N surplus approach. We compared both approaches and expected that the N surplus approach gives lower estimates of N leaching (and fertilizer-induced freshwater pollution) when the N surplus is small and higher N leaching estimates when the N surplus is high. We compared N fertilizer application at which the N balance = 0 with the N application at which profit is highest. We further expect pronounced differences in N surplus between farm sites and years, due to yield and soil fertility differences. N response trials were conducted at several locations over three years in Germany. Fertilizer-induced N surplus was calculated from the difference between applied N fertilizer and grain N removal. N fertilizer application at which N balance = 0 (NBal = 0) was lower than economic optimum N application rates (NEcon). N surplus at NEcon was linearly correlated with the additional N applied. Pooled over years and sites the median N surplus was 39 kg N ha-1. Differences between sites rather than between years dominated variation in fertilizer-induced N surplus. Estimated N leaching at NEcon was on average 9% of applied fertilizer N. The product water footprint was on average 180 m3 per ton of grain, but differences between sites were substantial with values varying between 0 and >400 m3 per ton. Yield and protein contents were lower at NBal = 0 compared to NEcon indicating a trade-off between freshwater protection, yield, wheat grain quality and economic optimum N application. Site-specific fertilizer strategies which consider soil type, crop development, annual field water balance, in-season nutrient dynamics and crop rotational effects are key to minimize fertilizer-induced leaching of N into groundwater.