Water, nitrogen and phosphorus use efficiencies of four tree species in response to variable water and nutrient supply

The commonly observed trade-off between plant water use efficiency (WUE) and nitrogen use efficiency (NUE) has been attributed to physiological constraints in the leaf. We examined if a similar trade-off can occur between WUE and phosphorus use efficiency (PUE) and if changes in NUE and PUE in response to water and nutrient supply can be related to microbial N and P immobilisation. We studied water and nutrient use efficiencies in leaves of four tree species (Eucalyptus sideroxylon, Eucalyptus tereticornis, Casuarina cunninghamiana, and Pinus radiata) that were grown under rainout shelters for 16 months at low and high levels of water and nutrient supply. Across all four species, WUE increased when water supply was low and nutrient supply was high, while NUE increased when water supply was high and nutrient supply was low. As a consequence, a trade-off was found between WUE and NUE for all four species. In contrast, no trade-off was found between WUE and PUE, likely because PUE and microbial P immobilisation in the soil unexpectedly increased with high nutrient supply. With variable water and nutrient supply, physiological constraints generate a trade-off between WUE and NUE, but not between WUE and PUE; the latter may have been obscured by microbial control over P supply to plants.