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Source: Berner and Berner (1987).

Ammonia is also released by microbial activity in the soil. Much is reabsorbed by plants, which also absorb nitrate deposited on leaves. Ammonia in the air can neutralize some of the atmospheric sulfate, the other major acid rain component. Rainfall can also contain significant quantities of dissolved organic nitrogen (DON). The source of the DON may be from ocean spray tranported to the land.

Overall, rainfall contributes about 17 Tg N/yr in the form of nitrate to the land, plus 15 Tg N/yr as ammonia and 9 Tg N/yr as DON. Dry deposition contributes another 16 Tg/ yr, mostly nitrate, for a total flux to land of 57 Tg/yr. The oceans receive a total of at least 20 Tg/yr.

The uncertainty in estimates for global nitrogen fixation and denitrification will need to be reduced before it can be determined if fixation and denitrification balance each other (Figure 14.7 and Table 14.3). The biological fixation on land includes an anthropogenic flux of 44 Tg/yr due to human cultivation of leguminous crops.

The nitrogen cycle is an interesting example of the Gaia hypothesis at work. Lovelock points out that one of the effects of life is the maintenance of chemical disequilibrium in the environment. If you turned off all biotic reactions, chemical equilibrium would eventually be achieved among the various species in the environment. In the case of nitrogen, the atmospheric O2 would combine with its N2 to form nitrate, which would be dissolved in the oceans. It is commonly understood that the oxygen in the atmosphere is maintained there by photoautotrophs. What is less well known is the fact that most of the nitrogen is in the atmosphere instead of the ocean, because of denitrifiers. Without life, Earth's atmosphere might resemble that of Mars, which is 95% CO2.

Industrial fixation of nitrogen by the Haber-Bosch process has altered human ecology significantly. Each year, 175 million tons of nitrogen flow into the world's croplands, half of which is assimilated by cultivated plants. Synthetic fertilizers provide about 40% of all nitrogen taken up by these crops. The crops furnish about 75% of all nitrogen consumed by humans (the rest comes from fishing and from grazing cattle). Thus, about one-third of the protein in humanity's diet depends on synthetic nitrogen fertilizer. This has undoubtedly facilitated the tripling of the world's population that occurred in the twentieth century.

14.2.5 Sulfur Cycle

Roughly 90% of the world's supply of sulfur (about 1016 metric tons) is held in sediments and rocks and the remaining fraction is largely found dissolved within our oceans, which

Dust 20

Wet and dry deposition 84

Volcanism 10

Industrial

Aerial transport to sea

Aerial transport to land 20

Sea Biogenic Volcanism salt gases 10 144 43

Dust 20

Wet and dry deposition 84

Volcanism 10

Industrial

Aerial transport to land 20

Sea Biogenic Volcanism salt gases 10 144 43

Weathering and erosion 72

Pyrite Hydrothermal

39 sulfides

39 96

Figure 14.9 Global sulfur cycle. (Based on Krebs, 1994.)

Weathering and erosion 72

Pyrite Hydrothermal

39 sulfides

39 96

Figure 14.9 Global sulfur cycle. (Based on Krebs, 1994.)

in turn represents the dominant reservoir for the majority of sulfur being cycled within our biosphere. The cycle reactions for sulfur are depicted schematically in Figure 13.25, and the global cycle in Figure 14.9.

Sulfur is essential for life largely because it is a component of two of the amino acids, cysteine and methionine. The sulfur cycle may be the cycle that is most affected by human activities. Human emissions of CO2 and nitrogen are about 5 to 10% of natural emissions. However, human activity releases about 160% as much sulfur as nature does.

Like nitrogen, sulfur cycles through a number of oxidation states. The most oxidized form, sulfate, serves as an alternative electron acceptor in respiration. It is generally used only if both oxygen and nitrate are absent, since it is less energetically favorable to organisms. Sulfate is the form of sulfur taken up by the primary producers, and thereby introduced into the food chain.

The most reduced inorganic form is sulfide. The toxic volatile compound hydrogen sulfide is commonly associated with anaerobic environments that have large amounts of organic matter, such as marsh sediments, anaerobic digesters in wastewater treatment plants, and petroleum deposits. In these the organic matter serves as a reducing agent, first reducing oxygen to CO2, any nitrate is denitrified, and then all sulfate is reduced to H2S. Sulfide is also found in deep groundwater aquifers, having been reduced by either soil organic matter or by ferrous iron. If ferrous iron is present, the sulfide will form the nonvolatile, low-solubility precipitate iron sulfide (FeS). Sulfide is also used by photoauto-trophic bacteria as an electron donor in place of H2O in a process similar to photosynthesis. Sulfur here plays the role of oxygen, and instead of O2 being produced, elemental sulfur is the product. In aerobic environments, reduced forms of sulfur are rapidly oxidized to sulfate by chemoautotrophic bacteria. Thus, the sulfur cycle is closely linked to the carbon cycle.

The sulfur cycle is also linked to the phosphorus cycle in aquatic systems (Section 14.2.6). When iron sulfide is oxidized in aquatic sediments, sulfate is released and phosphorus precipitates with the iron, becoming unavailable to organisms. Under anoxic conditions, the iron is reduced, releasing the phosphorus to the water, and the iron precipitates again as a sulfide.

Because reduced forms of sulfur are often associated with deposits of fossil fuels and mineral ores, combustion and smelting of these resources often results in emissions of sulfur dioxide to the atmosphere. This is rapidly oxidized to sulfate. In the absence of alkaline species, this is present in the form of sulfuric acid aerosols. These are easily washed out of the atmosphere by precipitation, which as a result has greatly increased acidity and reduced pH. The result is called acid rain or acid precipitation. Acid precipitation has several ecological impacts on aquatic and terrestrial ecosystems where it falls. These are detailed in Section 15.7.

The ocean is also a significant source of atmospheric sulfur, with a biological origin. Marine phytoplankton produces volatile dimethyl sulfide [(CH3)2S]. In the air this oxidizes rapidly to sulfate, much of which is washed back into the sea.

14.2.6 Phosphorus Cycle

The phosphorus cycle differs from those of carbon, nitrogen, and sulfur in several ways: There are fewer steps, there is no change in oxidation state, there is no significant atmospheric component, and it tends to cycle locally. For these reasons, it is easier to study.

Phosphorus is present in three main forms: free or orthophosphate; polyphosphate, which is a polymer of orthophosphate; and organic phosphate. Although present in living things in much smaller quantities than carbon or nitrogen, its importance is clear from biochemistry, as phosphate forms the backbone of the DNA molecule and is central to energy metabolism in cells. Phosphate is the most common limiting nutrient in aquatic ecosystem. Humans disturb the phosphorus cycle in aquatic systems by discharging was-tewater containing phosphate. This stimulates excessive cyanobacter, algae, and plant growth, which later die and deplete the water of oxygen. Aquatic systems with high nutrient loading are called eutrophic (Section 15.2.6).

Figure 14.10 shows one particular phosphorus cycle based on measurements in a salt marsh using radioactive tracers. Note that the various compartments do not have to be in steady state, as defined by equation (14.3). Each compartment may be importing or exporting phosphorus from the ecosystem. An energy flow diagram was also done for this ecosystem. An interesting conclusion stemmed from the observation that the filterfeeding mussels were more important for their role in nutrient cycling than for energy processing. In other words, they were more important for recycling phosphorus than as a food source for other organisms.

Sediments store and release phosphate to the water, depending on oxygen concentrations in aquatic systems. When oxygen is available, phosphate is absorbed by ferric hydroxide. When oxygen is limiting, the ferric iron is reduced to ferrous form, and the phosphate is released. This occurs seasonally in temperate-zone lakes.

Figure 14.10 Phosphorus cycle from a Georgia salt marsh. Reservoirs are in mg P/m2, fluxes are in mg P/m3 ■ day. Uptake by Spartina and release from detritus vary seasonally as shown. (Based on Odum, 1987.)

14.2.7 Cycles of Other Minerals

Numerous other minerals are required by living things, including potassium, calcium, and magnesium. Others are considered micronutrients, such as iron, chlorine, manganese, boron, zinc, copper, and molybdenum. Yet others, such as silicon, sodium, and cobalt, are required only by some organisms. Some are toxic, such as mercury and arsenic.

Iron is the sixth most common mineral element in the lithosphere (at approximately 5%), surpassed only by silica, calcium, magnesium, aluminum, and sodium. With an estimated total mass of 2 x 1017 metric tons of iron, there is roughly 10 times more iron than carbon in the lithosphere. However, the presence of iron in the biosphere is several thousand times lower than that of the macroelements, and its atmospheric fraction would be nil were it not for dust entrained by the wind. Its low solubility under aerobic conditions limits its availability. The two main forms are ferric (Fe3+) and ferrous (Fe2+). The solubility of ferric iron at pH 6.0 in waters of moderate alkalinity is about 0.13 mg/L. As a result, it is a limiting nutrient in some ecosystems, such as the open ocean. Ferrous iron is much more soluble, at about 20 to 30 mg/L, but this form is converted to ferric in the presence of oxygen. Ferrous iron may be found in deep groundwaters or wetland sediments where oxygen is depleted. When oxygen is present at pH levels above 6, ferrous iron is oxidized abiotically. At lower pH levels this occurs very slowly. But chemolithotrophic bacteria such as Thiobacilus ferrooxidans and Gallionella ferruginea can take over, harvesting energy in the process. Others, such as Sphaerotilus and Leptothrix perform the reaction but seem not to obtain energy from it.

One of the most thoroughly studied ecosystems in the world is the Hubbard Brook Experimental Forest in New Hampshire. A calcium budget developed for this forest found that it receives 3 kg/ha • yr from rainfall and 5 kg/ha • yr from weathering of bedrock. The output from the watershed via streamflow carries the sum of these, 8 kg/ha • yr. The biota and abiotic reservoirs exchange 50 kg/ha • yr between themselves. This shows how the biological community recycles the mineral many times faster than the overall throughput of the ecosystem.

Human activity has greatly affected the transport of many minerals. It has even created new ones. Radioactive strontium-90 is created by fallout from nuclear explosions. It behaves like calcium. Radioactive cesium-137 behaves like potassium and is rapidly recycled by organisms. Industrial activities have resulted in the release to the environment of otherwise scarce minerals, including mercury and chromium. Acid rain has increased leaching of aluminum into aquatic ecosystems, affecting fish life. Aluminum in soil water also decreases absorption of magnesium by plant roots. Recall that magnesium atoms are part of the chlorophyll molecule. Aluminum is added in the treatment of drinking water in the form of alum, although most is then removed.

14.2.8 System Models of Cycles

The cycling of nutrients among compartments can be modeled mathematically using simple rate expressions. Assuming steady-state nonreactive (conservative) systems, the expressions become even simpler. We illustrate with an example involving nitrogen balances, first as a linear steady-state system and then as a nonlinear system in both steady-state and dynamic conditions. The models can be used to explain and predict the distribution of nutrients in ecosystems.

Consider the system in Figure 14.11 which represents major nitrogen components in an aquatic system. The four compartments are: particulate nitrogen, which represents phytoplankton; dissolved organic nitrogen (DON), which is excreted by the phyto-plankton; ammonia, produced by mineralization of the DON; and nitrate, produced by nitrification. Notice that the phytoplankton can utilize both nitrate and ammonia nitrogen.

The concentration of each species is represented by X and the flux between compartments by J. Each flux is assumed to be proportional to the concentration of the source species (first order kinetics). For example, the uptake of nitrate is assumed to be proportional to nitrate concentration: Jj = kj Xj. This makes the system linear. The constant of proportionality is k. The system is set up by writing the mass balance equations for each

compartment based on equation (14.3):

Nitrate : Particulate N DON : Ammonia :

These equations form four equations with four unknowns. However, they are not independent equations. That is, one equation could be derived by combining the others. To see how this happens, write the equations for two compartments with two fluxes. You will see that the two mass balances are identical. Another way to see that the equations as given do not provide a useful solution is to recognize that one possible solution is to set all the concentrations to zero. In fact, an infinite number of solutions are possible, depending on the total concentration in the system. Thus, to find a unique solution we must specify the total concentration, M; then the individual X's will represent fractions of the total:

If we replace one of the four mass balance expressions, say the nitrate equation, with this equation, we will have our four independent expressions. To solve we express them in matrix form:

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