AEROBIC EMISSIONS OF N2O AND N-2 FROM SOIL CORES - MEASUREMENT PROCEDURES USING N-13 LABELED NO3- AND NH4+

The formation of N2O and N-2 in soil cores treated with N-13-labelled NO3- and NH4+ 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 (similar to 60 min) and, with minuscule (similar to 10(-13) g) amounts of N-13- label applied to the cores, natural equilibria and process reaction ra tes prevailing in the soil would have been unaffected. Ambient or back ground gas emission rates from (NO3-)-N-13 and (NH4+)-N-13 were extrem ely variable in cores taken from similar to 4 m(2) of apparently unifo rm grazed pasture. Denitrification rates (ng N g(-1) soil h(-1)), esti mated by multiplying the soil NO3- concentration by the proportion of label emitted as gaseous products, ranged from 0.1-183 for N2O and 0.2 -125 for N-2. Gas emission rates were always greater from NO3- than fr om NH4+. With both substrates, N2O-to-N-2 ratios were initially very h igh and then generally declined. The patterns of gas production from d enitrification and nitrification were broadly similar and strongly sup port the contention that gas emissions during nitrification are due to coupled denitrification of the NO2- intermediate. The advantages and disadvantages of this N-13 method for investigating N2O and N-2 produc tion, including its potential value in mechanistic studies, are discus sed.