96 / Basics of Environmental Science
grass and a corresponding increase in the area growing cereals. In 1938, less than 1.2 million ha
was sown to barley and wheat; in 1966 those crops occupied 3.3 million ha. During the same
period, the area devoted to permanent and temporary grassland fell from 8.4 million ha to 6.8
million ha. The 2.1 million ha increase in the cereal area was achieved by reducing the grassland
area. (MAFF, 1968, p. 34)
Thus the change from grassland to cereal cropping led inevitably to an increase in the movement
of nitrate from the soil and into surface and ground water. The widespread introduction of
soluble, nitrogen-based fertilizers exacerbated the problem, especially when heavy applications
were followed by very wet weather, but the fertilizer contribution should not be exaggerated. In
1964, for example, nitrogen runoff was measured following 114 mm of rain in two falls in
Missouri (SMITH, 1967). Bare soil, which had received no fertilizer, lost 0.9 kg N ha-1;
unfertilized maize and oats lost 0.3 kg N ha-1; and continuously grown maize, fertilized with
195 kg N ha-1, lost 0.1 kg N ha-1.
This is not the only source of nitrogen reaching both land and water. Substantial and increasing
amounts also arrive from the air. Elemental nitrogen is oxidized by lightning, in the course of burning
plant materials, and in high-compression internal combustion engines, and biologically by the action
of nitrogen-fixing soil bacteria. Urine from farm livestock releases ammonia, also a soluble compound.
It has been found that in the mid-1970s much of Europe received 2–6 kg N ha-1 yr-1 and that some
areas now receive 60 or more kg N ha-1 yr-1. This level of fertilization may be altering the composition
of certain ecosystems, especially those established on nitrogen-poor soils (MOORE, 1995).
Plants have similar physiological requirements whether they grow on dry land or in water. If
plant nutrients enter water, therefore, they will stimulate the growth of aquatic plants. Nitrate
alone is not enough, of course. The full range of nutrients must be supplied and plant growth is
limited by the availability of the nutrient in shortest supply (in water this is usually phosphorus);
this is the ‘law of the minimum’ first stated in 1840 by the German chemist Justus von Liebig
(1803–73). Other nutrients are less mobile than nitrate, so nitrate leaching has less effect on
plant life than might be supposed.
Agricultural change apart, the movement of nutrients from the land and into water is an entirely
natural process, an inevitable consequence of the drainage of rain water. As water moves
downward through the soil to join the ground water, soluble soil compounds dissolve into and
are carried by it. Were this not so, freshwater aquatic plant life would be severely restricted.
Water draining into surface waters, such as rivers and lakes, also carries fine particulate matter
that is deposited as sediment when the power of the stream falls below a certain threshold. Fast-
flowing streams rapidly remove material that enters them and accumulations occur only in slow-
moving rivers and still water. It is there, and only there, that sedimentation and eutrophication
may cause difficulties.
Eutrophication leads to the proliferation of aquatic plants, especially algae, and cyanobacteria,
organisms that derive nutrients directly from the water, rather than through roots attached to a substrate.
A eutrophic lake or pond can usually be recognized by its surface covering of green algae. The life
cycles of such organisms are short and as they die their remains sink and are decomposed by aerobic
bacteria, whose populations increase in proportion to the food supply available to them. The bacteria
obtain the oxygen they need from that dissolved in the water, and under eutrophic conditions the
amount they remove exceeds the amount being introduced, so the water is depleted of dissolved
oxygen. A common measure of water pollution is its ‘biochemical oxygen demand’ (BOD), calculated
from the reduction in the amount of dissolved oxygen in a water sample incubated in darkness for 5
days at a constant 20°C; it is also a measure of bacterial activity.