Applications from food web and ecosystem theory - Ecological Applications: Toward a Sustainable World - page 259

Farm animals are responsible for further problems of nutrient loss from the land.
Cattle, pigs and poultry are the three major nitrogen contributors in agriculture
feedlots. The nitrogen-rich waste from poultry can be dried to provide a readily
transportable, inoffensive and valuable fertilizer for crops and gardens. By contrast,
cattle and pig excreta is 90% water and smells bad. A unit of 10,000 pigs produces
as much pollution as a town of 18,000 people. In many parts of the world, the law
increasingly restricts the discharge of animal waste into waterways. The simplest
practice converts pollutant into fertilizer and returns the material to the land as
semisolid manure or sprayed on as slurry, diluting its concentration to something
similar to a more 'primitive' and sustainable type of agriculture. When it comes to
nitrate pollution of waterways, one of the biggest culprits is farm specialization
where forage crops are grown in one area, but stock is fattened on the other side of
the country. This means that fertilizer must be used to make up the shortfall
when plants are reaped and transported to the stock, whose excreta can hardly be
shipped all the way back to the farm of origin. In the USA, for example, only 34%
of the nitrogen excreted in animal waste is returned to cropped fi elds (Mosier et al.,
2002) - much of the rest eventually fi nds its way into streams and rivers. A change
in practice to one where animal feed crops and stock fattening occur in the same
area would certainly reduce nutrient loss to waterways. Of course, there is an even
more pervasive problem that intimately involves us all - the nutrients released as
human excreta are rarely taken back to be placed on the land where they
originated.

9.7.3 Constructing
wetlands to manage
water quality

The excess input of nutrients from sources such as agricultural runoff and sewage
has caused many 'healthy' oligotrophic lakes (low nutrients, low plant productivity
with abundant water weeds, and clear water) to switch to a eutrophic condition where
high nutrient inputs lead to high phytoplankton productivity (sometimes dominated
by bloom-forming toxic species), making the water turbid, eliminating large plants
and, in the worst situations, leading to anoxia and fi sh kills. This process of cultural
eutrophication of lakes has been understood for some time. But it was only recently
that people noticed huge 'dead zones' in the oceans near river outlets, particularly
those draining large catchment areas such as the Mississippi in North America and
the Yangtze in China. The nutrient-enriched water fl ows through streams, rivers
and lakes, and eventually to estuary and ocean where the ecological impact may be
huge, killing virtually all invertebrates and fi sh in areas up to 70,000 km 2 in extent.
The United Nations Environment Program (UNEP) has reported that 150 sea areas
worldwide are now regularly starved of oxygen as a result of decomposition of algal
blooms fueled particularly by nitrogen from agricultural runoff of fertilizers and
sewage from large cities (UNEP, 2003). Ocean dead zones are typically associated
with industrialized nations and usually lie off countries that subsidize their agri-
culture, encouraging farmers to increase productivity and use more fertilizer.
The only way to alleviate problems in the world's oceans is by careful management
of terrestrial catchment areas to reduce agricultural runoff of nutrients and to treat
sewage to remove nutrients before discharge (known as tertiary treatment). The
vegetation zones between land and water, such as wetland areas (consisting of
swamps, ditches and ponds) and riparian forest along the banks of streams, can also
be benefi cial because the plants and microorganisms remove some of the dissolved
nutrient as it fi lters through the soil.