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 CO2 (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 CO2 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 NO3 (-) but had neutral to positive effects on NH4 (+) and dissolved organic N (DON), particularly in a wet year. Warming increased NO3 (-) 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 NO3 (-) availability for plants under eCO(2), the NO3 (-) pool in soil is mainly driven by the negative effects of eCO(2).