Warming prevents the elevated CO2-induced reduction in available soil nitrogen in a temperate, perennial grassland

Rising atmospheric carbon dioxide concentration ([CO2]) has the potential to stimulate ecosystem productivity and sink strength, reducing the effects of carbon emissions on climate. In terrestrial ecosystems, increasing [CO2] can reduce soil nitrogen (N) availability to plants, preventing the stimulation of ecosystem carbon assimilation; a process known as progressive nitrogen limitation. Using ion exchange membranes to assess the availability of dissolved organic N, ammonium and nitrate, we found that CO2 enrichment in an Australian, temperate, perennial grassland did reduce soil N availability, mostly by reducing nitrate availability. Importantly, the addition of 2ºC warming prevented this effect while warming without CO2 enrichment did not significantly affect N availability. These findings indicate that global warming plays an important role in the impact of [CO2] on ecosystem N cycling, potentially overturning CO2-induced effects, with consequences for ecosystem productivity and carbon sequestration. Keywords: elevated CO2, FACE, progressive nitrogen limitation, warming, biogeochemistry