Effects of nutrient supply on carbon and water economies of C4 grasses

C-3 plants can increase nutrient uptake by increasing transpiration, which promotes the flow of water with dissolved nutrients towards the roots. However, it is not clear if this mechanism of nutrient acquisition, termed mass flow', also operates in C-4 plants. This is an important question, as differences in mass flow capacity may affect competitive interactions between C-3 and C-4 species. To test if mass flow can be induced in C-4 species, we conducted an experiment in a semiarid seasonal savanna in South Africa. We grew six C-4 grasses in nutrient-poor sand and supplied no nutrients, nutrients to the roots or nutrients spatially separated from the roots. We measured the rates of photosynthesis and transpiration, water-use efficiency (WUE), nitrogen gain and biomass. For all species biomass, N gain, photosynthesis and transpiration were lowest in the treatment without any nutrient additions. Responses to different nutrient positioning varied among species from no effect on N gain to a 50% reduction when nutrients were spatially separated. The ability to access spatially separated nutrients showed a nonsignificant positive relationship with both the response of transpiration and the response of WUE to spatial nutrient separation. This indicates that nutrient acquisition is not regulated by decreasing WUE in C-4 grasses. Overall, our study suggests that under elevated CO2, when evaporative demand is lower, C-4 species may be at a competitive disadvantage to C-3 species when it comes to nutrient acquisition.