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Title: Controls over soil N pools in a semiarid grassland under elevated CO2 and warming

Author
item CARRILLO, YOLIMA - University Of Wyoming
item DIJKSTRA, FEIKE - University Of Sydney
item PENDALL, ELISE - University Of Wyoming
item Morgan, Jack
item Blumenthal, Dana

Submitted to: Ecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/23/2012
Publication Date: 5/11/2012
Citation: Carrillo, Y., Dijkstra, F., Pendall, E., Morgan, J.A., Blumenthal, D.M. 2012. Controls over soil N pools in a semiarid grassland under elevated CO2 and warming. Ecosystems. 15:761-774.

Interpretive Summary: The responses of rangelands to climate change will ultimately be determined by the availability of soil water and nutrients which determine in large part important ecological attributes like plant productivity, species composition and forage quality. Yet there are few experiments that have examined in realistic field settings the combined effects of two or more global climate change factors on soil resources. The Prairie Heating and CO2 Enrichment Experiment is investigating how both simulated warming and a CO2 enriched atmosphere such as will prevail in the second half of this century affect the availability of N for native plants in a Wyoming mixed-grass prairie. While elevated CO2 tended to reduce soil N availability, warming increased it. The results suggest that soil N availability may not be as negatively affected by climate change as previous CO2 enrichment experiments have indicated due to the offsetting, positive effect of warming on soil N.

Technical Abstract: Long-term responses of terrestrial ecosystems to global change will likely be regulated by N availability. Very few studies have addressed the combined effects of warming and elevated CO2 (eCO2) on N availability. The stock of soil N determines availability for organisms, but also for loss to the atmosphere or groundwater. eCO2 and warming can elicit changes in soil N via direct effects on microbial and plant activity, or indirectly, via soil moisture. Understanding the interplay of direct and moisture-mediated impacts and the role of plant and microbial N use on soil N will improve predictions of ecosystem-level responses. We followed soil N pools in a semiarid temperate grassland, at the PHACE (Prairie Heating and CO2 Enrichment) experiment. We evaluated relationships with environmental factors and isolated plant from microbial-mediated effects by assessing N forms under intact vegetation and in the absence of vegetation. Despite being a water-constrained system, in this grassland, eCO2, warming and their combination appear to impact soil N pools through a complex combination of direct and moisture mediated effects. eCO2 decreased NO3- likely due to moisture-mediated enhancement of microbial immobilization. eCO2 had neutral to positive effects on NH4+ and dissolved organic N, the latter associated with increased N mineralization due to a combination of greater moisture and SOM priming. Warming increased NO3 - availability likely due to a combination of indirect drying impacts and direct temperature-driven effects. Notably, warming altered the temporal availability of mineral N by dampening fluctuations via effects on plant demand. Results supported that warming can help offset the eCO2-driven decrease in NO3- availability.