AEROBIC EMISSIONS OF N2O AND N-2 FROM SOIL CORES - FACTORS INFLUENCING PRODUCTION FROM N-13 LABELED NO3- AND NH4+

Gas-stripping procedures, with air as the stripping and carrying gas, were used to investigate the effects of amendments on production in so il cores of N2O and N-2 arising from N-13-labelled NO3- (denitrificati on) or NH4+ (nitrification). These cores had been used previously to i nvestigate ambient gas emission rates; the rates after amendment were compared with these ''natural'' emission results. Using (NO3-)-N-13, t he results of amendment with H2O, glucose, NO3- or glucose + NO3-, sev eral hours before the experiment, were usually explicable in terms of effects on microbial numbers and soil O-2 status. However, results obt ained immediately after amendments were made, before de novo enzyme sy nthesis or change of O-2 status would have occurred, were generally di fficult to interpret. Amendments which included NO3- gave spuriously l ow N2O and N-2 emission rates, probably due to isotopic dilution. The immediate effect of amendments may vary from time to time and depend u pon soil conditions at the time of sampling. Only glucose consistently stimulated gas emissions, presumably because it enhanced activity of the existing denitrifying population. Using (NH4+)-N-13, amendment wit h H2O increased production of N-2, but not N2O, and amendment with glu cose reduced emissions of both gases. The advantages and disadvantages of this N-13 method for investigating the influence on N2O and N-2 pr oduction of various soil amendments, and the relationships of these re sults with those of more conventional studies, are discussed.