Denitrification in grass swards is increased under elevated atmospheric CO₂

Emissions of N2O and N-2 were measured from Lolium perenne L. swards under ambient (36 Pa) and elevated (60 Pa) atmospheric CO2 at the Swiss free air carbon dioxide enrichment experiment following application of 11.2 g N m(-2) as (NH4)-N-15 (NO3)-N-15 or (NH4)-N-14 (NO3)-N-15 (1 at.% excess N-15). Total denitrification (N2O + N-2) was increased under elevated pCO(2) with emissions of 6.2 and 19.5 mg N-15 m(-2) measured over 22 d from ambient and elevated pCO(2) swards, respectively, supporting the hypothesis that increased belowground C allocation under elevated pCO(2) provides the energy for denitrification. Nitrification was the predominant N2O producing process under ambient pCO(2) whereas denitrification was predominant under elevated pCO(2). The N-2-to-N2O ratio was often higher under elevated pCO(2) suggesting that previous estimates of gaseous N losses based only on N2O emissions have greatly underestimated the loss of N by denitrification. (C) 2003 Elsevier Science Ltd. All rights reserved.