Page 649 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 19 ecosystem essentials 613
Nitrogen-fixing bacteria, which live principally in the soil and are associated with the roots of certain plants, are critical for bringing atmospheric nitrogen into living organ- isms. Colonies of these bacteria reside in nodules on the roots of legumes (plants such as clover, alfalfa, soybeans, peas, beans, and peanuts) and chemically combine the ni- trogen from the air into nitrates (NO3) and ammonia (NH3). Plants use the nitrogen from these molecules to produce their own organic matter. Anyone or anything feeding on the plants thus ingests the nitrogen. Finally, the nitrogen in the organic wastes of the consuming organisms is freed by denitrifying bacteria, which recycle it to the atmosphere.
To improve agricultural yields, many farmers enhance the available nitrogen in the soil by means of synthetic inorganic fertilizers, as opposed to soil-building organic fertilizers (manure and compost). Inorganic fertilizers are chemically produced through artificial nitrogen fixation at factories. Humans presently fix more nitrogen as synthetic fertilizer per year than is found in all terrestrial sources combined—and the present production of synthetic fer- tilizers now is doubling every 8 years; some 1.82 million tonnes are produced per week, worldwide. The crossover point at which anthropogenic sources of fixed nitrogen exceeded the normal range of naturally fixed nitrogen oc- curred in 1970.
This surplus of usable nitrogen accumulates in Earth’s ecosystems. Some is present as excess nutrients, washed from soil into waterways and eventually to the ocean. This excess nitrogen load begins a water pollution process that feeds an excessive growth of algae and phytoplankton, in- creases biochemical oxygen demand, diminishes dissolved oxygen reserves, and eventually disrupts the aquatic eco- system. In addition, excess nitrogen compounds in air pol- lution are a component in acid deposition, further altering the nitrogen cycle in soils and waterways.
Dead Zones The Mississippi River receives runoff from 41% of the area of the continental United States. It carries agricultural fertilizers, farm sewage, and other nitrogen- rich wastes to the Gulf of Mexico, causing huge spring blooms of phytoplankton: an explosion of primary pro- ductivity. By summer, the biological oxygen demand of bacteria feeding on the decay of the spring bloom exceeds the dissolved oxygen content of the water; hypoxia (oxy- gen depletion) develops, killing any fish that venture into the area. These low-oxygen, or hypoxic, conditions create
dead zones or hypoxic areas that limit marine life. Geo- systems in Action, GIA 19, illustrates dead zones in the Gulf of Mexico and elsewhere. From 2002 on, the Gulf Coast dead zone has expanded to an area of more than 22000 km2 each year. The agricultural, feedlot, and fer- tilizer industries dispute the connection between their nutrient input and this extensive dead zone. The human- caused creation of dead zones in water bodies is cultural eutrophication, discussed later in the chapter.
Similar coastal dead zones occur as the result of nu- trient outflows from more than 400 river systems world- wide, affecting almost 250000 km2 of offshore oceans and seas (Figure GIA 19.2). In Sweden and Denmark, however, a concerted effort to reduce nutrient flows into rivers reversed hypoxic conditions in the Kattegat strait (between the Baltic and North Seas). Also, fertilizer use has decreased more than 50% in the former Soviet Republics since the fall of state agriculture in 1990. The Black Sea no longer undergoes year-round hypoxia at river deltas, as the dead zones in those areas now disap- pear for several months each year.
Dead zones are occurring in lakes as well, such as those that appeared in Lake Erie, one of the Great Lakes, in the 1960s. In 2011, the dead zone in this lake reached its largest extent in recorded history, caused by fertilizers (mainly phosphorus) flowing into the lake combined with slower natural mixing attributed to climate change (see Figure GIA 19.6 and the discussion in Focus Study 19.2).
As with most environmental pollution, the cost of mitigation is cheaper than the cost of continued damage to marine ecosystems. Experts estimate that a 20%–30% cut in nitrogen inflow upstream would increase dissolved oxygen levels by more than 50% in the dead zone region of the Gulf. One government study estimated the level of application of nitrogen fertilizer to be 20% more than soils and plants needed in Iowa, Illinois, and Indiana. The ini- tial step to resolving this issue might be to mandate apply- ing only the levels of fertilizer needed—thus also reaping a savings in overhead costs of agriculture—and, as a second step, to begin dealing with animal wastes from feedlots.
Energy Pathways
The feeding relationships among organisms make up the energy pathways in an ecosystem. These trophic relation- ships, or feeding levels, consist of food chains and food
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Georeport 19.1 Carbon Cycle Response to the Mount Pinatubo Eruption
One month after the 1991 Mount Pinatubo eruption, the second largest volcanic eruption of the 20th century, tempera- tures decreased in the northern Hemisphere and global carbon dioxide levels declined sharply. Scientists initially thought
that the CO2 decrease was caused by a decline in plant respiration linked to cooling temperatures. However, research now suggests that the globally spread atmospheric aerosols from the eruption caused an increase in diffuse light, allowing sunlight to reach more plant leaves (as opposed to direct sunlight that creates shadows). This change increased plant photosynthesis and removed more CO2 from the air. in one deciduous forest, photosynthesis increased by 23% in 1992 and 8% in 1993 under cloudless conditions. Thus, the eruption’s aerosols affected the global carbon cycle, lowering atmospheric carbon levels and enhancing the terrestrial carbon sink. (Please review other effects of the eruption in Chapter 1, Figure 1.11.)