Fjölrit RALA - 05.12.1999, Page 142

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BIOMASS AND SOIL NUTRIENT POOLS IN NEWZEALAND
ductions in biomass compartments would be reflected in some soil enrichment, which
might have persisted for some time, just as we have surmised from the burning of for-
est in the history of North Island pasture development.
In our current results we can discem no clear evidence of reduction in nutrients in
soil pools, associated with the reduction in biomass and nutrient pools above-ground.
We do not conclude that nutrients have not been lost from the soil, but we have found
no evidence here to support any claim that they have been lost. We accept that there is
some evidence of change in soil nutrient regimes following transformation of tall tus-
sock grasslands to short grasslands. Mclntosh et al. (1981) in an intensive topsoil sur-
vey at two Upper Waitaki localities, Glencaim and Longslip, recorded clear differ-
ences in topsoil (0-75mm) C, N, total P, Olsen P and exchangeable bases in relation to
topographic position and precipitation regimes. Many differences were attributed to
“soil rejuvenation”, especially on sunny slopes, terrains from which, even at Glencaim
with only 500 mm rainfall, tall tussock had probably been destroyed, in our view. by
early fire and grazing.
We endorse the suggestion of those authors that the depletion of vegetation cover
and consequent higher inorganic P and Olsen P values which they recorded in soils
under short tussock grassland are likely outcomes of pastoral culture. We suspect that
such effects of fíre and grazing in transforming tall tussock grassland to short grass-
land have persisted since the nineteenth century. In the case of Longslip, the more
humid of the two localities, further buming and grazing in this century may have con-
tinued or renewed the process. We interpret the demonstrated difference in Olsen P in
topsoil between sites under Chionochloa rigida and sites in short grassland where
soils are otherwise similar, as evidence of increased mineralisation of organic P. We
ascribe it to ecological degradation during pastoral occupation.
Likewise, we accept the evidence reviewed by Mclntosh (1997), from Glencairn
dry sites (500 mm rainfall) revisited after 15 years, of signifícant changes in nutrient
status of 0-75 mm soil layer in unimproved grasslands under grazing. (Mclntosh et al.
1996). We agree that such changes in soil N as were measured there are greater than
would be explained by grazing transfers alone, and that buming losses were not in-
volved. Mclntosh’s suggestions of soil erosion or accelerated mineralisation of soil
organic matter and leaching “in areas depleted of tussock cover” point to the likeli-
hood of incipient desertification as a precursor to nutrient loss, rather than as its con-
sequence. Mclntosh et al. (1997) have recently demonstrated in the same locality, that
desisting from grazing for 15 years and countering the depleting effects of grazing by
oversowing weedy short grasslands resulted in increased biomass above and below
ground and higher total biomass pools of Ca, Mg, P, S, K and N. This supports their
earlier inference that current grazing use was associated with continuing nutrient loss.
It is noteworthy that, like our revegetation with legumes at Mt Possession over a
similar time span. their oversowing with luceme without fertiliser did not result in
significant differences in total nutrients in the soil compartment, 0-250 mm, above the
levels of the grazed or ungrazed-without-oversowing treatments, except in the case of
exchangeable K.
How then do we reconcile our present failure to detect differences in soil pools at-
tributable to past ecological changes with this evidence from unimproved grasslands