[Illustration: _Plant ecologists “tagging” experimental forest plots
with radioactive cesium for long-term studies._]
Natural radionuclides find their way into plants’ metabolic processes.
Man-made radionuclides also are so incorporated—even some, such as
uranium or radium, that have no known metabolic role. The man-made
nuclides, whether they reach the earth in fallout or by other means, mix
with the stable nuclides to which they are chemically related,
increasing by small fractions the total amount of each element available
to participate in plant growth cycles. Because artificial radionuclides
behave so typically, they present, on the one hand, a possible long-term
hazard and, on the other, the expectation that their detectability will
reveal much about the biological courses of minerals and nutrients.
The disposition of man-made radioactivity on land is determined in part
by such factors as topography and the presence or absence of water.
Topography may influence the distribution by setting patterns of
drainage and exposure of surface soils to wind and rain. Water may
affect dilution, or it may leach radionuclides out of surface soils and
thus remove them from the level in which plants are rooted. The leaching
may carry radionuclides elsewhere, however, possibly causing mild
contamination of the water table.
[Illustration: _Trench dug on Rongelap Island to expose soil strata and
root systems to determine penetration of radionuclides in coral-sand
“topsoil”._]
Plants take up radionuclides through their roots or through their
foliage. But the role of soils is significant. Some radionuclides are
bound as ions to clays and thus are withheld in large measure from entry
into the plant system. Cesium-137, for example, is held so tightly by
soils that uptake through plant roots is slight, and thus a more
significant mode of entry of cesium-137 into food chains is by direct
deposit on plant leaves. Variables are introduced by the physical
configuration of the plant itself, by seasonal differences in plant
metabolism, and by the effects of rain and snow. In the case of
iodine-131, a short half-life—8 days—virtually precludes the possibility
of extensive uptake through plant roots. But the half-life is not too
short to prevent grazing cattle from ingesting radioiodine deposited in
fallout and thus allow the appearance of radioiodine in milk.
[Illustration: _Survey of pasture grasses to determine whether
radioactive materials are present. If they are, they could be passed
from the grasses to cows and then from the cows’ milk to humans._]
Much attention has been devoted to strontium-90 and to its availability
to man by deposit on plants and soils. Because strontium bears a close
chemical relation to calcium, a unit expressing this relation, the
_strontium unit_ (one picocurie[15] [1 × 10^-12 curie of strontium-90
per gram of calcium]) is used in following strontium-90 through food
chains. Soils, however, present confusing factors. Experiments and
fallout observations show that strontium-90 does not penetrate soils
deeply. In typical instances it remains in the upper inch or two of the
soil surface, where its availability to root systems is as variable as
the conditions of mixing, leaching, and plant growth. Experiments have
shown that plant uptake of strontium from soils can be reduced by
introduction of calcium in available form into the soil.
Radiobiological developments on land result from combinations of
environmental influences. Studies in the Rocky Mountains show that
ecological conditions above the timberline, particularly in areas where
snowbanks accumulate, are efficient in concentrating fallout
radionuclides. Concentrations thus take place in the snow-packed heights
that are the sources of mountain streams flowing to the plains far
below.