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Title: Contrasting Effects of Elevated CO2 and Warming on Nitrogen Cycling in a Semiarid Grassland

Author
item Dijkstra, Feike
item Blumenthal, Dana
item Morgan, Jack
item PENDALL, ELISE - University Of Wyoming
item CARRILLO, YOLIMA - University Of Wyoming
item Follett, Ronald

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2010
Publication Date: 6/25/2010
Citation: Dijkstra, F.A., Blumenthal, D.M., Morgan, J.A., Pendall, E., Carrillo, Y., Follett, R.F. 2010. Contrasting Effects of Elevated CO2 and Warming on Nitrogen Cycling in a Semiarid Grassland. New Phytologist. 187:426-437.

Interpretive Summary: Nitrogen (N) is an important nutrient affecting plant productivity, species composition, and C sequestration in rangelands. As such, it is essential to better understand how global change factors such as atmospheric CO2 enrichment and warming affect N cycling in rangelands. In a field experiment in Cheyenne, WY, we observed that by increasing the atmospheric CO2 concentration from 390 to 600 'l L-1 reduced inorganic N availability in the soil, most likely because of increased amounts of N being taken up by soil microorganisms. On the other hand, warming the plant canopy by 1.5 °C during the day and 3 °C during the night, increased soil inorganic N availability, most likely because of increased decomposition and mineralization of soil organic matter. Therefore, in a world where atmospheric CO2 and temperature are both increasing, warming effects on soil inorganic N availability could offset negative effects of elevated CO2 on soil inorganic N availability. Our results provide important information for computer simulation models in predicting the long-term effects of climate change on plant productivity and C sequestration in rangelands.

Technical Abstract: Simulation models indicate that the nitrogen (N) cycle plays a key role in how other ecosystem processes such as plant productivity and carbon (C) sequestration respond to elevated CO2 and warming. However, combined effects of elevated CO2 and warming on N cycling have rarely been tested in the field. We studied N cycling under ambient and elevated CO2 (600 ppm), and ambient and elevated temperature (1.5/3.0 ºC warmer day/night) in a full factorial semi-arid grassland field experiment in Wyoming, USA (Prairie Heating And CO2 Enrichment, PHACE). We measured soil inorganic N availability (using resin probes), N pools and NO3- uptake (using a 15N tracer) in plants and microbes, and activity of six extracellular enzymes. Soil inorganic N availability significantly decreased under elevated CO2, most likely because of increased microbial N immobilization, while soil inorganic N availability significantly increased with warming, most likely because of increased gross and net N mineralization. We observed no CO2*warming interactions on any of the parameters we measured, except for the activity of the enzyme leucine amino peptidase. This CO2*warming interaction was most likely caused by changes in soil moisture altering the stability and production of this enzyme. Our results indicate that elevated CO2 and warming cause imbalances in the supply of and demand for inorganic N, resulting in decreased availability of soil inorganic N under elevated CO2 and increased availability with warming. Sustained increases in soil inorganic N availability with warming could ultimately alleviate the potential for progressive N limitation under elevated CO2.