Nutrient cycling in ecosystems and global biogeochemical cycles involves chemical and biological transformations through both production and decomposition. The hydrologic cycle is not only crucial in making water available to organisms but also in transporting nutrients within ecosystems. Carbon moves through biological systems and returns to the atmosphere through respiration. Burning of fossil fuels increases the carbon dioxide and sulfur in the atmosphere, which is implicated in global warming and acidic conditions in lakes and oceans. The nitrogen cycle is dominated by biotic transformations controlled by bacteria, whereas phosphorus and sulfur cycles are dominated by geochemical processes.
learning outcomes
You should be able to:
Compare and contrast the roles of different water bodies (rivers, lakes, oceans, glaciers, underground aquifers) in the hydrologic cycle, including approximate residence times of water molecules.
Describe and evaluate data on the effect of fossil-
List the important forms of nitrogen moving through the nitrogen cycle, and explain the importance of microbes in this cycle.
Describe the basics of the phosphorus cycle, including speed of movement through various parts of the cycle.
Explain the pools and fluxes of the sulfur cycle, and describe differences in the cycling of sulfur on land and in oceans.
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The residence time of a water molecule is short in organisms, soil, and rivers and is much longer in lakes, glaciers, and oceans. Why?
The residence time of water depends on the rate at which it moves from one pool to another. Organisms, soil, and rivers all have short water residence times because they have relatively small volumes of water compared with lakes, glaciers, and oceans, and thus the rate of water movement from one pool to the next is much faster.
Why is the concentration of CO2 rising in the atmosphere and oceans? How does this rise in CO2 affect climate and ocean chemistry?
The concentration of CO2 is rising in the atmosphere and oceans because of fossil-
Slightly less than half of the CO2 emitted by burning of fossil fuels has been absorbed by the oceans. CO2 reacts with water to form carbonic acid (H2CO3). As levels of carbonic acid rise, the pH of seawater drops. This increase in acidity can have negative effects on many marine organisms that have calcium carbonate skeletons.
Explain how the nitrogen cycle is dominated by biological processes whereas the phosphorus and sulfur cycles are dominated by geochemical processes.
Nitrogen enters the biotic system as atmospheric N2 and is fixed by bacteria into ammonia (NH3). Ammonia is rapidly transformed into ammonium (NH4+), which can then be used by plants and bacteria. Nitrifying bacteria can transform ammonium to nitrate (NO3–), another form of nitrogen that plants and bacteria can use. Denitrifying bacteria take nitrate and convert it back to N2 and N2O gases, which are then released back into the atmosphere. Collectively, this microbial processing of nitrogen is very fast and accounts for about 95 percent of all natural nitrogen flux on Earth, making it a mostly biologically driven cycle.
Unlike nitrogen, phosphorus and sulfur are found in rocks and deep-
As you have seen, biogeochemical cycles are intimately involved in how ecosystems function. Just as human alterations of those cycles are affecting ecosystems worldwide, the resulting changes could have important effects on humans as well.