Warming has a larger and more persistent effect than elevated CO2 on growing season soil nitrogen availability in a species-rich grassland

The terrestrial biosphere's ability to capture carbon is dependent upon soil nitrogen (N) availability, which might reduce as CO2 increases, but global warming has the potential to offset CO2 effects. Here we examine the interactive impact of elevated CO2 (eCO(2)) and warming on soil N availability and transformations in a low-fertility native grassland in Tasmania, Australia. Using ion exchange membranes, we examined soil nitrogen availability during the growing season from 2004 to 2010 in the TasFACE experiment. We also estimated soil N transformation rates using laboratory incubations. Soil N availability varied strongly over time but was more than doubled by experimental warming of 2A degrees C, an impact that was consistent from the fifth year of the experiment to its conclusion. Elevated CO2 reduced soil N availability by 28%, although this varied strongly over time. Treatment effects on potential N mineralisation also varied strongly from year to year but tended to be reduced by eCO(2) and increased by warming. These results suggest that warming should increase soil N availability more strongly than it is suppressed by eCO(2) in low fertility grasslands such as this, stimulating terrestrial carbon sinks by preventing eCO(2)-induced nitrogen limitation of primary productivity.