Controls over soil nitrogen pools in a semiarid grassland under elevated CO2 and warming

Long-term responses of terrestrial ecosystems to the combined effects of warming and elevated CO 2 (eCO 2) will likely be regulated by N availability. The stock of soil N determines availability for organisms, but also influences loss to the atmosphere or groundwater. eCO 2 and warming can elicit changes in soil N via direct effects on microbial and plant activity, or indirectly, via soil moisture. Detangling the interplay of direct- and moisture-mediated impacts on soil N and the role of organisms in controlling soil N will improve predictions of ecosystem-level responses. We followed individual soil N pools over two growing seasons in a semiarid temperate grassland, at the Prairie Heating and CO 2 Enrichment experiment. We evaluated relationships of N pools with environmental factors and explored the role of plants by assessing plant biomass, plant N, and plant inputs to soil. We also assessed N forms in plots with and without vegetation to remove plant-mediated effects. Our study demonstrated that the effects of warming and eCO 2 are highly dependent on individual N form and on year. In this water-constrained grassland, eCO 2, warming and their combination appear to impact soil N pools through a complex combination of direct- and moisture-mediated effects. eCO 2 decreased NO 3 - but had neutral to positive effects on NH 4 + and dissolved organic N (DON), particularly in a wet year. Warming increased NO 3 - availability due to a combination of indirect drying and direct temperature-driven effects. Warming also increased DON only in vegetated plots, suggesting plant mediation. Our results suggest that impacts of combined eCO 2 and warming are not always equivalent for plant and soil pools; although warming can help offset the decrease in NO 3 - availability for plants under eCO 2, the NO 3 - pool in soil is mainly driven by the negative effects of eCO 2.