Fjölrit RALA - 05.12.1999, Blaðsíða 141

KEVIN F. 0’CONNOR ETAL.
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displacement and organic matter accumulation at the surface under any revegetation
programme. If we were to make an allowance of only 20 mm soil growth ffom 16
years revegetation with lupins, an estimate which several features indicate is conser-
vative, we would have to add some 470 kg N, 130 kg P, and small amounts of ex-
changeable K and Ca to the values reported for soils at Hakatere. These values are
calculated for an additional 20 mm of the 300-400 soil layer. In the case of pine for-
est, we would also have to make a substantial íurther adjustment due to increased root
volume displacing soil upwards. In cultural revegetation as in degradation, soil com-
partments are slower to change in nutrient contents than are biomass pools.
Discussion and conclusion
Tall tussock grasslands vary considerably in their biomass and nutrient pool charac-
teristics. Some of these variations arise from the physical environment. Some may
arise from differences in past treatment. It is difficult to infer with any confidence ei-
ther the magnitude or the annual or centennial rate of change in carbon or other ele-
ments, by comparison of stages of ecological degradation, except in conditions chosen
with some care to avoid influence by some factor other than the degradation itself.
The outcomes of our attempts to choose comparable sites encourage us to sustain the
work of quantification of nutrient pools in benchmark soil-vegetation systems, chosen
to represent both natural and degraded facets in a continuum of ecological history of
pastoralism in different terrains.
The present study has so far clearly demonstrated only the self-evident, that
weights of above-ground biomass and nutrient pools are substantially lower in trans-
formed and degraded vegetations than they are in the vegetations from which they
were derived. Where transformation of tall tussock grassland is involved, we have
found only limited evidence that the substantial reduction in carbon and nutrients in-
volved in destruction of above-ground biomass has led to a discemible reduction in
carbon or nutrients in the system as a whole. The weedy short grasslands included in
this survey were almost certainly transformed fforn tall grasslands, at least to the stage
of short tussock grasslands, early in pastoral occupation. The net losses of nutrients
which they suffered in early pastoral fires would have been considerable, especially in
volatilised compounds of nitrogen and possibly of sulfur, and in compounds of phos-
phoms and bases that could have been blown away as ash. The examples of N and P
may be used to illustrate.
The mean difference between above-ground biomass of tall tussock grassland and
that for weedy short grassland in our study (152 kg N ha"1) is of the same magnitude
as the loss likely to ensue from one or two fires in the first years of pastoral occupa-
tion, for the above value is approached or exceeded by the above-ground biomass N of
most of our nine tall tussock grasslands. The corresponding margin in above-ground P
between tall tussock grassland and weedy short grassland (22 kg P ha_1) is also of the
same order of magnitude as the above-ground biomass P in our survey. P in ash is less
likely to escape completely from the landscape than are gaseous products of fire, but it
is subject to transfer by redeposition as windblown ash. Transformation of tall tussock
grasslands to short grasslands as a consequence of early fire and grazing induced a
vegetation system no Ionger capable of subtending in its above-ground biomass the
weight of nutrients including bases which had previously been there. P and cation re-