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

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.