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

Abstract

Background and aims: 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 (eCO₂) and warming on soil N availability and transformations in a low-fertility native grassland in Tasmania, Australia. Methods: 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. Results: Soil N availability varied strongly over time but was more than doubled by experimental warming of 2°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 eCO₂ and increased by warming. Conclusions: These results suggest that warming should increase soil N availability more strongly than it is suppressed by eCO₂ in low fertility grasslands such as this, stimulating terrestrial carbon sinks by preventing eCO₂-induced nitrogen limitation of primary productivity.