Spatial variability of organic carbon in a tussock grassland, Manorburn, Central Otago, New Zealand

The variability of topsoil organic carbon (OC) to a depth of 7.5 cm was mea sured in two adjacent tussock grassland plots to determine the effect of th e proximity to tussock plants on OC levels and the effect of tussock abunda nce on OC variance and spatial variation. The plots were on Tawhiti soils n ear the Upper Manorburn Reservoir in Central Otago. A substantial cover of red tussock (Chionochloa rubra) was present on one plot but absent from the other. In both plots OC showed weak spatial dependency with nugget more th an 50% of the sill. The range of spatial dependency of about 2 m was consis tent with the natural spacing of tussock plants which was less than 1.5 m, with a mode of 0.5 m. However, no direct relationship was observed between OC levels of individual samples and tussock proximity. Weak spatial anisotr opy in the variability was indicated with higher variance in the across-slo pe direction than the down-slope direction. Mean values of OC were signific antly lower in the plot with low tussock cover than the plot with substanti al cover of red tussock, but variance and spatial variability of carbon wer e not significantly influenced by the type of vegetative cover. Sampling st rategies on similar soils should not have to take into account either speci es abundance or the proximity of samples to tussock plants. Sampling must b e randomised to avoid a natural periodicity of about 2 m wavelength. At lea st 26 samples are required for an estimate of a mean with a 95% confidence interval of about +/-0.25% OC concentration. OC values showed periodicity w ith wavelengths of about 2, 4, and 8 m when plotted along the sampled trans ects. This was confirmed by semivariograms. The maxima and minima were appr oximately circular in shape. The periodicity may be related to the clusteri ng of tussock plants but there is no positive correlation between cluster d ensity and OC.