The gaseous losses of N from conventional-till (CT) and zero-till (ZT) crop fields were 3 to 7 and 12 to 16 kg N ha-1 y-1, respectively. In contrast, losses from CT and ZT fallow were severalfold higher, namely, 12 to 14 and 34 kg N ha-1, respectively. The more dense surface soil and consistently higher moisture content (lower air-filled porosity) were identified as major factors affecting increased denitrification under ZT. The potential denitrification rates were markedly higher under ZT, and the population of denitrifiers was up to six times higher than in CT soil samples. The contribution of lower soil horizons towards gaseous N losses was found to be low on both CT and ZT fields, and this finding was confirmed from a survey carried out on three other widely differing soils. Volumetric soil moisture and air temperature were the only two of several factors that accounted for a significant portion of the variations in gaseous N fluxes under field conditions. The average mole fraction of N2O ranged from almost 100% to as low as 28% of the total gaseous products and showed a negative relationship with soil moisture.
