Effects of irrigation and addition of nitrogen fertiliser on net ecosystem carbon balance for a grassland.

The ability to quantify the impacts of changing management practices on the components of net ecosystem carbon balance (NB) is required to forecast future changes in soil carbon stocks and potential feedbacks on atmospheric CO2 concentrations. In this study we investigated seasonal changes on the components of net ecosystem carbon balance resulting from the application of irrigation and nitrogen fertiliser to a temperate grassland in New Zealand where we simulated grazing events. We made seasonal measurements of the components of NB using chamber measurements in field plots with and without irrigation and addition of nitrogen fertiliser. We developed models to determine the physiological responses of gross canopy photosynthesis (A), leaf respiration (RL) and soil respiration (RS) to soil and air temperature, soil water content and irradiance and we estimated annual NB for the first year after treatments were applied. Overall, irrigation and nitrogen addition had a synergistic effect to increase annual estimates of above-ground components of carbon balance (A, RL and carbon exported through simulated grazing, Fexport), but there was no effect from adding nitrogen alone. Annual RS remained unchanged between treatments. The treatments resulted in increases in above-ground biomass production, but, with the high intensity of simulated grazing, these were not sufficient to offset ecosystem carbon losses, so all treatments remained a net source of carbon. There were no significant differences between treatments and annual NB ranged from -540gCm-2y-1 for the treatment with no irrigation and no nitrogen addition and -284gCm-2y-1 for the treatment with irrigation and nitrogen addition. Our findings from the first year of the treatments quantify the net benefits of addition of irrigation and nitrogen on increasing above-ground production for animal feed but show that this did not lead to a net increase carbon input to the soil.