Trade-off between blue and grey water footprint of crop production at different nitrogen application rates under various field management practices

In irrigated crop production, nitrogen (N) is often applied at high rates in order tomaximize crop yield. With such high rates, the bluewater footprint (WF) per unit of crop is low, but the N-related greyWF per unit of crop yield is relatively high. This study explores the trade-off between blue and greyWF at different N-application rates (from 25 to 300 kg N ha(-1) y(-1)) under various field management practices. We first analyse this trade-off under a reference management package (applying inorganic-N, conventional tillage, full irrigation). Next, we estimate the economically optimal N-application rate when putting a price to pollution. Finally, we consider the blue-grey WF trade-off for othermanagement packages, a combination of inorganic-N or organic-N with conventional tillage or no-tillage, and full or deficit irrigation. We use the APEX model to simulate soil water and N balances and crop growth. As a case study, we consider irrigated maize on loam soil for the period 1998-2012 in a semi-arid environment in Spain. The results for the reference package show that increasing N application from 50 to 200 kg N ha(-1), with crop yield growing by a factor 3, involves a trade-off, whereby the blue WF per tonne declines by 60% but the N-related grey WF increases by 210%. Increasing N application from 25 to 50 kg N ha(-1), with yield increasing by a factor 2, is a no-regret move, because blue and grey WFs per tonne are reduced by 40% and 8%, respectively. Decreasing N application from 300 to 200 kg N ha(-1) is a no-regret move as well. The minimum blue WF per tonne is found at N application of 200 kg N ha(-1), with a price of 8 $ kg-1 of N load to water pollution the economically optimal N-application rate is 150 kg N ha(-1). (C) 2018 Elsevier B.V. All rights reserved.