Fjölrit RALA - 05.12.1999, Page 144

BIOMASS AND SOIL NUTRIENT POOLS IN NEWZEALAND
We infer from the earlier model of nutrient balances in tussock grasslands
(O’Connor and Harris 1992) and especially from its recent revision by Harris and
O’Connor (1998), that animal transfers under extensive grazing practice and periodic
rabbit plagues, and not fires, have been the most significant agents of continuing nu-
trient wastage from unimproved rangeland systems in the past. We recall the espe-
cially severe and prolonged declines in livestock numbers in drier regions of the high
country' (O’Connor 1986). We infer that depletion compounded the problem of dete-
rioration in semi-arid and dry sub-humid regions.
We surmise that sustained depletion of grass cover in semi-arid and dry sub-humid
districts that led to incipient desertification, may have thereby led to further loss of
nutrients by surface soil erosion or accelerated mineralisation of organic matter and
seasonal leaching, in line with recent research by Mclntosh et al. (1996). We believe
from the evidence of the first Longslip study referred to (Mclntosh et al. 1981), that
this kind of effect of severe depletion of vegetative cover could be significant, even in
humid districts when biomass destruction on steep slopes led to exposure of bare soil
or incomplete occupation of soil systems by vegetation.
In review, high country pastoralism accomplished its principal transformations of
vegetation very early in pastoral occupation. In the process of that biomass reduction,
considerable amounts of nutrients were lost, especially those volatilised in fire. For
another 100 years, many tall tussock grasslands persisted despite periodic buming in-
volving íurther nutrient loss, because grazing pressure after fíre was seldom sufficient
to prevent vegetation recovery. During all that time, in tall grasslands and in short,
extensive pastoralism continued to wear away at nutrient capital, principally through
animal grazing and nutrient transfer.
For all this attrition of system nutrients, soil pools may show little net change, ex-
cept where surface erosion and mineralisation of organic matter has been promoted by
depletion of cover. Cultural revegetation with exotic herbaceous or forest cover may
have as dramatic effects in rebuilding biomass nutrient pools without having much
obvious effect on soil nutrient pools. The apparent ability of less arid high country
soils and their organic regimes to buffer ecosystems against dramatic change in nutri-
ents warrants further research attention. This should accompany research into the sig-
nificance of organic matter maintenance in drier soils. High risks of Ioss of nutrients
from biomass pools may affect forestry and animal husbandry systems in mountain
environments, if systems are not well buffered or well managed. Both land uses share
with natural vegetation systems the prospect and the need of rebuilding systems with
nutrient frugality.
References
Buchanan, J. 1880. Manual of the Indigenous Grasses of New Zealand. Colonial Museum and Geo-
Iogical Survey Department, Wellington.
Buchanan, J. 1867. Botanical notes on the Kaikoura Mountains and Mount Egmont. New Zealand
Geological Survey, Reports of Geological Explorations 1, 1-16.
Buchanan, J. 1868. Sketch of the botany of Otago. Transactions N.Z. Institute 1 (Part III), 22-53.
Butler, S. 1862. A First Year in Canterbury Settlement. Longman, Roberts and Green, London.