Irrigation increases forage production of newly established lucerne but enhances net ecosystem carbon losses

Abstract In New Zealand, dairy farmingfaces increasing scrutiny for its environmental impacts, including those on soil carbon(C) stocks; hence, alternative management practices are required. One such practice is usage of deep-rooting forage, such as lucerne ( Medicagosativa L.). We measured the C and water exchange of two neighbouring lucerne fields on stony, well-drained soil for 3 years, following conversion from grassland. One field received irrigation and effluent; the other received neither. Net CO 2 exchange and evaporation were measured by eddy covariance, drainage and leaching with lysimeters, and water inputs with rain gauges. Biomass removal from harvesting and grazing was recorded by direct sampling. In the conversion year, irrigated lucerne was C-neutral despite two harvests and losses from the conversion process. In the 2nd and 3rd years combined, the biomass-C removal exceeded net CO 2 uptake, causing net losses of 450 g C m −2 and 210 g C m −2 for irrigated and non-irrigated lucerne, respectively. Leaching losses accounted for 1 to 9% of annual net C uptake from the atmosphere. The ratio of ecosystem respirationto gross photosynthetic productivity (GPP) increased from