NITROUS-OXIDE AND DINITROGEN EMISSIONS FROM URINE-AFFECTED SOIL UNDERCONTROLLED CONDITIONS

A N-15 labelling technique was used to measure N2O and N2 emissions fr om an undisturbed grassland soil treated with cow urine and held at 30 cm water tension and 20-degrees-C in a laboratory. Large emissions of dinitrogen were detected immediately following urine application to p asture. These coincided with a rapid and large increase in soil water- soluble carbon levels, some of this increase being attributed to solub ilization of soil organic matter by high pH and ammonia concentrations . Emissions of nitrous oxide generally increased with time in contrast to dinitrogen fluxes which decreased as time progressed. Estimated lo sses of N2O and N2 over a 30 day period were between 1 to 5% and 30 to 65% of the urine N applied plus N mineralized from soil organic matte r, respectively. Most of the N2 and N2O originated from denitrificatio n with nitrification - denitrification being of minor significance as a source of N2O. Comparisons of the N-15 enrichments in the soil miner al N pools and the evolved N2O suggested that much of the N2O was prod uced in the 5-8 cm zone of the soil. It is concluded that established grassland soils contain large amounts of readily-oxidizable organic ca rbon which may be used by soil denitrifying organisms when nitrate is non-limiting and soil redox potential is lowered due to high rates of biological activity and high soil moisture contents.