Trade-Offs and Synergies Between the Efficiency in the Use of Water and Nitrogen.

Understanding the interactions between water and nitrogen is important for rainfed crops. Here I focus on tradeoffs and synergies to emphasise the risk of ignoring interactions and the benefit of considering both resources simultaneously. The nitrogen-driven trade-off between yield per unit evapotranspiration and yield per unit nitrogen uptake is illustrated with experimental and modelling data for wheat, maize and rice and explained with physiological and agronomic principles. This trade-off is hardwired in the crop; it cannot be broken. Hence, growers have to find their own solutions regarding the water and nitrogen use efficiency at which they operate.

With few exceptions, breeding for drought adaptation has ignored the role of nitrogen and reciprocally, attempts to genetically improve the capture and efficiency in the use of nitrogen have paid little attention to the interaction with water. Direct selection for yield of Australian wheat between 1958 and 2007 increased yield linearly with no change in evapotranspiration, hence the improvement in yield per unit ET at a rate of 0.06 kg ha⁻¹ mm⁻¹ y⁻¹. Improvement in both harvest index and biomass explained yield gains. Enhanced biomass at anthesis resulted from increased radiation use efficiency (0.012 g MJ^-1 y^-1), which in turn resulted from increased nitrogen uptake (0.4 kg N ha^-1 y^-1) and changes in the profiles of foliar nitrogen and radiation. The improved water use efficiency of wheat in Australia thus partially resulted from improved capture and allocation of nitrogen.