Fjölrit RALA - 05.12.1999, Blaðsíða 143

Kevin F. O’Connoretal.
141
in North Otago of system losses of nutrients, even affecting topsoil pools? We believe
some factors differentiate our situations from those of the North Otago studies. First,
in the more moist regimes and less open vegetation which we have examined, there
are indications of nutrient conservation in the soil system. This could be, in part at
least, an outcome of the phenomenon described and quantified by Robinson and Mac-
donald (1964) for a weedy short grassland ecosystem of the Craigiebum terraces,
where mineralisation of soil organic matter occurred at a very low rate, reflecting the
scarcity of available energy substrate for the dominant bacterial flora. Williams et al.
(1977a) have already suggested a phenomenon of “pseudo-ageing” as central to nutri-
ent conservation of such short grassland systems, here characterised as suffering from
“deterioration”, in Cockayne’s parlance. In North Otago, in contrast, where grassland
“depletion” is more characteristic on sunny slopes of dry sub-humid short tussock
grasslands (Glencairn), or even among open tall tussock grasslands, especially on
steep slopes (Longslip), soil erosion and mineralisation of residual soil organic matter
are likely mechanisms of nutrient loss from soil itself.
While further research along such lines in different climatic regimes may reveal the
mechanisms of accelerated nutrient loss as well as those of nutrient conservation in
degraded systems, we can indicate the empirical answer for South Island, New Zea-
land to our earlier question of the relationship of nutrient loss to desertification: Is loss
of nutrients from ecosystems a contributor to range deterioration and thereby to deser-
tification, or is it a consequence of desertifícation?
We conclude that initial fire and grazing achieved most biomass destruction
through transformation of tall tussock grasslands and scrub to short grasslands. We
conclude that range deterioration and nutrient loss from biomass destruction have
been concomitant effects of early pastoral burning and grazing. We infer that contin-
ued grazing, sustained for long periods in seasonally repetitive fashion, has allowed
animal preferences to reduce the already limited quality of forage available. We inter-
pret this as the primary feature in range deterioration, a deficiency of suitable forage
which might have been assuaged for the moment, but could not be permanently coun-
teracted, by periodically resorting to fire.
So long as tall tussock grasslands continued to be bumed during the last 100 years,
they would lose more nutrients from biomass, especially N and probably S. So long as
they recovered from fire, tussock grasses would have to draw on their own reserves,
perhaps for some years, drawing also on mineralised nutrients ffom soil organic mat-
ter for themselves and for the adventive plants that were now able to flourish amongst
them (O’Connor and Lambrechtsen 1964, Payton et al. 1986, Yeates and Lee 1997,
Ross et al. 1997). The fact that induced or modified vegetation recovers from such pe-
riodic fire, does not gainsay the evidence that nutrients are lost from the system and
the inference that levels of nutrients in soil organic matter pools are lowered. More
thorough understanding of this resource-exploiting situation has given rise to more
vigorous challenge to its sustainability (Mark 1994). We do not see the evidence of
such rates of nutrient loss from biomass destmction leading to desertification, even
though we believe that the practice of repeated buming without replacement of nutri-
ents and vegetation cover of soil is unsustainable.