Impact of Fertilizer N Application on the Grey Water Footprint of Winter Wheat in a NW European Temperate Climate
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.