Nitrogen loss through denitrification in a soil under pasture in New Zealand

Denitrification on several contrasting topographical sites in a New Zealand dairy-farm pasture was measured periodically over a year, using the acetylene inhibition technique, by incubating undisturbed soil cores in a closed system. The measured denitrification rates varied considerably both spatially and temporally. High coefficients of variation (CV) and log-normal distributions of denitrification rate were often observed. The spatial variance in denitrification rate changed temporally and was apparently related to soil moisture content and the grazing pattern. Denitrification rates followed a marked seasonal pattern, with highest rates being measured during the wet winter and lowest rates during the dry summer and early autumn. Differences in denitrification rates among sites were not consistent. However, slightly higher denitrification rates were usually detected in the floor of a gully and in a gateway area than on other sites. Mean denitrification rates from individual dates were positively correlated to soil moisture content. However, there was a negative correlation between denitrification rate and soil nitrate concentration, respiration rate and temperature. An annual nitrogen loss of 4.5 kg N ha−1 through denitrification was estimated in this legume-based dairy-farm pasture. Low soil moisture content was the primary factor limiting denitrification during the dry summer and early autumn. Low denitrification rates were also caused by lack of available soil NO3−-N.