Aerobic emissions of N2O and N2 from soil cores: measurement procedures using 13N-labelled NO3− and NH4+

Abstract The formation of N 2 O and N 2 in soil cores treated with 13 N-labelled NO 3 − and NH 4 + and maintained under aerobic conditions was investigated using a gas-stripping procedure with air as the stripping and carrier gas. The extreme sensitivity of detection of this short-lived ( t 1 2 = 10 min ) radioisotope allowed experiments of short duration (∼60 min) and, with minuscule (∼10 −13 g) amounts of 13 N-label applied to the cores, natural equilibria and process reaction rates prevailing in the soil would have been unaffected. Ambient or background gas emission rates from 13 NO 3 − and 13 NH 4 − were extremely variable in cores taken from ∼4 m 2 of apparently uniform grazed pasture. Denitrification rates (ng N g −1 soil h −1 ), estimated by multiplying the soil NO 3 − concentration by the proportion of label emitted as gaseous products, ranged from 0.1–183 for N 2 O and 0.2–125 for N 2 . Gas emission rates were always greater from NO 3 − than from NH 4 + . With both substrates, N 2 O-to-N 2 ratios were initially very high and then generally declined. The patterns of gas production from denitrification and nitrification were broadly similar and strongly support the contention that gas emissions during nitrification are due to coupled denitrification of the NO 2 − intermediate. The advantages and disadvantages of this 13 N method for investigating N 2 O and N 2 production, including its potential value in mechanistic studies, are discussed.