Variability of ecosystem scale water-use efficiency in a nutrient manipulation experiment

Nitrogen (N) and phosphorus (P) are the two most important limiting soil nutrients reducing carbon sequestration globally. Through anthropogenic N-deposition, stoichiometric imbalances in plant-available N and P are expected in terrestrial ecosystems. This will result in increased P-limitation to plants and associated, but yet understudied, implications for ecosystem carbon sequestration, water-use efficiency (WUE), and biophysical properties. Here, we show results of a large-scale fertilization experiment designed to quantify effects of stoichiometric N:P ratio imbalances on WUE in a semi-arid tree-grass ecosystem. At the ecosystem-scale, the addition of N increased leaf area index, canopy chlorophyll content, and WUE. The addition of P, which relived the N:P imbalance, resulted in a further increase of WUE, more fixed carbon per transpired water. We conclude that increased N and combined N+P addition leads to shifts in many aspects of ecosystem functioning and biophysics, in particular related to water use strategies.