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nitrogen, phosphorus, potassium, calcium and magnesium from atmospheric depos-
itions - dust and rain - but these are also rapidly leached away so the net accumula-
tion rates are very low, and perhaps even negative in the case of potassium. Coloniza-
tion seems to start 25-55 years after tipping has ceased, and consists at first of heath-
er, gorse and broom with a few other plants. This community is later invaded by goat
willow Salix caprea , followed by woodland vegetation with rhododendron, birch and
oak when the tips are 75-120 years old.
Once the vegetation is established, nitrogen actually accumulates faster than
would be expected from atmospheric inputs alone. This is due to nitrogen fixation by
the root nodules of gorse and broom (see chapter 3 ); gorse can fix 26 kg nitrogen per
hectare per year, compared with a measured atmospheric deposition of only 9 kg/ha/
year. At a few sites, tree lupin Lupinus arboreus is the first colonist when the tips are
only 10-20 years old. This is not a native British species but it has become naturalized
in some areas, and is an important plant on these sand wastes because it is able to fix
relatively large amounts of nitrogen - about 72 kg/ha/year. Individual plants die after
about six years, but the boost they give to the nitrogen economy of the developing soil
is sufficient for other species, that do not fix nitrogen, to become established around
them. The critical factor, then, for building up organic matter in these wastes, and
stimulating plant succession, is the capture and cycling of nitrogen for which woody
legumes are the prime agents. It seems that non-leguminous shrubs and trees do not
begin to colonize china clay sand wastes successfully until a nitrogen capital of some
1000 kg/ha has been built up, of which about 300 would be in the living plants at any
one time, and 700 in the soil. This story seems simple in retrospect, but it involved a
team of scientists under the leadership of A. D. Bradshaw to discover the principles
and to work out the details. Figure 66 shows some of the experimental plots that were
set up.
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