It has long been argued that there are links between cycles of carbon and nitrogen, and between these and other elements and water. Despite the seeming truth of such assertions, evidence in support - at the ecosystem scale especially - remains scarce. At molecular/cellular/leaf scales, links between nitrogen and carbon and water are perhaps best exemplified for C3 plants by relationships between cellular concentrations of the key carboxylating enzyme, ribulose-1,5, bisphosphate carboxylase (Rubisco) and rates of carbon fixation. Rubisco makes up a very large proportion of all leaf N and enhanced nitrogen availability could lead to enhanced Rubisco concentrations and instantaneous rates of carbon fixation. That could in turn reduce the need for stomatal opening (amount, duration), and thus reduce water loss.
Here we draw upon collated leaf and tree-ring data across biomes, and experimental data from a dry tropical region of Australia, to explore how nitrogen availability might affect cycles of carbon and water, and water use efficiency (WUE). Our results demonstrate that increased availability of nitrogen to woody plants increases their WUE. Woody legumes have a distinct advantage when they are truly facultative N-fixers as this provides a means of adaptation to differing light environments. Long-term nitrogen enrichment (such that soil C:N ratios are reduced) may remove that advantage. Woody legumes are also capable under arid conditions of more efficient storage of energy (energy stored per unit carbon) , again related to the increased availability of nitrogen. Increases in WUE across many, if not most, of the world’s major forests – tropical (both wet and dry), temperate, boreal - are arguably one of the most clear and consistent of all major ecological changes, with considerable repercussions for water yield and carbon storage. Even so, we can ascribe far less certainty to the responsible underlying cellular mechanisms (as described above), and their relationship to nitrogen availability. We conclude that quickly growing knowledge of the significance of nitrogen fixation to global cycles of carbon and water is both an essential and exciting prospect, as we seek to understand and then mitigate many global changes.