An analysis of variability in the activity of nitrifiers in a soil under pasture .I. Spatially dependent variability and optimum sampling strategy

Abstract

Variability in the activity of nitrifying organisms in a Tokomaru silt loam (Typic Fragiaqualf) under a ryegrass-white clover pasture was measured using a short-term nitrification assay (SNA). Spatial dependence of the variability in SNA value and associated soil properties was examined for an area of 625 m2 using geostatistical analysis. The values of SNA, soil nitrate (NO3-N), and exchangeable ammonium (Ex-NH4+) were all highly variable and conformed to lognormal distributions. The range of spatial dependence in the variability of SNA, NO3-N, and the pH of the SNA incubation media, which was assumed to approximate the field soil pH, was 3.3, 12.0 and 13.8 m respectively. EX-NH4+ showed no spatial dependence. SNA, NO3-N and incubation pH varied isotropically within the sampling area, but soil moisture content was strongly anisotropic. These results are discussed in relation to the most efficient sampling strategy for estimation of the mean field NO3- concentration. It was concluded that sufficient small localized clusters of samples should be taken to give a low standard error of the mean, with each cluster separated by at least 12 m. In the case of the Tokomaru silt loam, 12 clusters, each comprising bulked subsamples, would be required for estimation of the mean field nitrate concentration with 95% probability of �, the sample mean, being within � 5% of �, the true mean. This represents a large sampling effort.