inorganic N (Yamaya and Oaks, 2004). Whilst most plants can use either nitrate or ammonium (or amino acids) as an N source, the added bioenergetic cost of reducing nitrate back to ammonium means that – in theory – ammonium would be a preferred source (Touraine, 2004; Yamaya and Oaks, 2004). More detailed field data in cropping environments under e[CO2] will be required to develop realistic strategies. It has also been suggested that e[CO2] will increase the importance
of N-fixing legumes in ecosystems (Rogers et al., 2009). Translated to agricultural systems, this would, for example, advocate crop rotations with legumes and cereals, which are considered beneficial also for other purposes. However, for legumes P may become particularly limiting under e[CO2] and will need to be carefully managed (Sa and Israel, 1998). There is already a strong emphasis on breeding and biotechnological efforts for more nutrient efficient crops, and a range of target traits were identified, for example for wheat2009). A number of the target traits selected in that review will be
affected by e[CO2] or become even more important under e[CO2]. For example, leaf N allocation and remobilisation will have to be reevaluated for the changes in C allocation. Maintaining or improving N-uptake and N assimilation, such as by selection or manipulation of enzymes involved in N-uptake, assimilation, and transport will only gain importance under e[CO2]. In particular, N translocation dynamics into the developing grains appear to be affected by e[CO2] and will become more important to maintain grain quality traits.