Hewitt et al.--Spatial variability of organic carbon in a tussock grassland

New Zealand Journal of Agricultural Research abstracts

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

A. E. HEWITT

Landcare Research
Private Bag 1930
Dunedin, New Zealand

S. J. BURGHAM

Landcare Research
P.O. Box 69, Lincoln
Canterbury, New Zealand

R. S. GIBSON

Landcare Research
P.O. Box 282
Alexandra
Otago, New Zealand

Abstract The variability of topsoil organic carbon (OC) to a depth of 7.5 cm was measured in two adjacent tussock grassland plots to determine the effect of the proximity to tussock plants on OC levels and the effect of tussock abundance on OC variance and spatial variation. The plots were on Tawhiti soils near 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 than 50% of the sill. The range of spatial dependency of about 2 m was consistent 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 anisotropy in the variability was indicated with higher variance in the across-slope direction than the down-slope direction. Mean values of OC were significantly lower in the plot with low tussock cover than the plot with substantial cover of red tussock, but variance and spatial variability of carbon were not significantly influenced by the type of vegetative cover. Sampling strategies on similar soils should not have to take into account either species abundance or the proximity of samples to tussock plants. Sampling must be randomised to avoid a natural periodicity of about 2 m wavelength. At least 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 with wavelengths of about 2, 4, and 8 m when plotted along the sampled transects. This was confirmed by semivariograms. The maxima and minima were approximately circular in shape. The periodicity may be related to the clustering of tussock plants but there is no positive correlation between cluster density and OC.

Keywords soil organic carbon; soil organic matter; tussock grasslands; spatial variability; geostatistics; sampling; high country; soil

New Zealand Journal of Agricultural Research, 1998, Vol. 41: 613-622

0028-8233/98/4104-0613 $7.00/0 (c) The Royal Society of New Zealand 1998

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