Wednesday, August 8, 2007

PS 41-22: Elevated CO2 increases long-term N mineralization and plant N uptake in a semiarid grassland: results from a five-year 15N tracer study

Feike A. Dijkstra1, Elise Pendall2, Arvin R. Mosier3, Jennifer Y. King4, Daniel G. Milchunas5, and Jack A. Morgan1. (1) USDA-ARS, (2) University of Wyoming, (3) USDA-ARS (retired), (4) University of Minnesota, (5) Colorado State University

While rising atmospheric CO2 has the potential to enhance plant growth and biomass accumulation, rates of these processes will be constrained by soil nitrogen (N) availability.  Despite much effort, it is still uncertain how elevated CO2 affects long-term soil N dynamics.  To examine the effect of doubling atmospheric CO2 concentration on N dynamics in a semiarid grassland ecosystem we used open-top chambers, three at 720 ppm CO2 and three at ambient CO2, located at the USDA-ARS Central Plains Experimental Range in northeastern Colorado, USA.  The dominant species were the C4 grass Bouteloua gracilis, and the C3 grasses Pascopyrum smithii and Stipa comata.  In the first year 0.5 g m-2 of ammonium nitrate-N, 99.9 atom% 15N, was added to each plot.  We estimated N mineralization and plant N uptake by tracking 15N and total N in plant and soil the following five years.  We found that averaged over all five years, elevated CO2 significantly increased total N content in aboveground biomass (up to 68%).  Stipa comata took up relatively more and B. gracilis relatively less N with elevated CO2.  The fraction of labeled N in aboveground biomass declined over time and this decline was greater under elevated CO2.  The amount of labeled N in the soil did not change with time and was unaffected by elevated CO2.  These results suggest that with time, (unlabeled) N released from mineralization in the soil diluted the labeled N in aboveground biomass and that this dilution effect caused by N mineralization was greater under elevated CO2.  We conclude that elevated CO2 enhanced soil N mineralization and plant N uptake, particularly in S. comata.  Because N concentrations were on average significantly lower with elevated CO2, it remains unclear whether enhanced N mineralization could be sustained for longer than five years in this system under elevated CO2.

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