Denitrification in grass swards is increased under elevated atmospheric CO 2

Abstract

Emissions of N 2O and N 2 were measured from Lolium perenne L. swards under ambient (36 Pa) and elevated (60 Pa) atmospheric CO 2 at the Swiss free air carbon dioxide enrichment experiment following application of 11.2 g N m −2 as 15NH 4 15NO 3 or 14NH 4 15NO 3 (1 at.% excess 15N). Total denitrification (N 2O+N 2) was increased under elevated pCO 2 with emissions of 6.2 and 19.5 mg 15N 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 N 2O producing process under ambient pCO 2 whereas denitrification was predominant under elevated pCO 2. The N 2-to-N 2O ratio was often higher under elevated pCO 2 suggesting that previous estimates of gaseous N losses based only on N 2O emissions have greatly underestimated the loss of N by denitrification.