Environmental Engineering Reference
In-Depth Information
The oxygen cycle
After nitrogen, oxygen is the most abundant uncombined element in the atmosphere. Molecular
oxygen is also dissolved in the hydrosphere, combined with minerals forming oxides in the
lithosphere, and locked in water molecules. The oxygen cycle describes the movement of
oxygen within and among the four Earth spheres.
Oxygen is linked to the carbon cycle because it is a by-product of photosynthesis and the
essential component of respiration for aerobic organisms. Photosynthesis by plants and algae
(Equation 2.2) is the most important sources of oxygen that is released to the atmosphere and
the oceans. The production of oxygen by terrestrial and marine photosynthetic organisms is
about 7 × 10
15
moles per year and the total oxygen accumulated in the atmosphere is about
38 × 10
18
moles (Lasaga and Ohmoto, 2002). However, evidence shows that oxygen
concentration in the atmosphere is decreasing in part as a result of the burning of fossil fuels
and carbonaceous materials (Keeling and Shertz, 1992; Sirignano et al., 2010). Other major
oxygen sinks are respiration, decay of organic matter where oxygen is consumed in the process
and carbon dioxide released, formation of mineral oxides through weathering, and the burial
of calcium carbonate in ocean sediments.
Besides photosynthesis, an additional mechanism for the production of oxygen is
photolysis, which generates molecular oxygen by breaking molecular water with UV radiation.
The phosphorus cycle
Ionic phosphorus is an essential nutrient for metabolic pathways of living organism and is a
limiting element in most soils and in freshwater environments (Tilman and Lehman, 2001).
Phosphorus undergoes movement primarily throughout the hydrosphere, lithosphere, and
biosphere but not throughout the atmosphere, although small amounts of phosphorus can be
transported by the atmosphere as dust particles. The most important mechanism that provides
phosphorus to ecosystems is aqueous transfer.
Even when present in soils, phosphorus is unavailable because it is prone to form insoluble
complexes with other minerals (Vance, 2001). The phosphorus cycle starts in soils where
weathering releases phosphorus by several mechanisms and ultimately is taken in by plants
and incorporated into molecules. Some of the mechanisms of phosphorous release include:
Acidification of soils by biochemical respiration that produces carbon dioxide that then is
converted into carbonic acid, which dissolves phosphorus.
●
Organic acids released by plant roots dissolve phosphorus.
●
Symbiotic fungi association with plant roots makes phosphorus available to plants. Plants
then absorb the phosphorus and incorporate it into molecules that are essential for their
metabolisms (Filippelli, 2008).
●
When plants die, decomposition releases the carbon-bound phosphorus to the soil by the
action of bacteria and fungi (Filippelli, 2008). Soil erosion transports particulate and dissolved
phosphorus to rivers that eventually move it to oceans. The high pH of ocean water makes
phosphorus even more insoluble, so its ultimate fate is precipitating with sediments to the bot-
tom. Once it becomes part of sediments, phosphorus can be recycled back to the continent by
subduction followed by volcanism and uplift (Compton et al., 2000), which may take millions
of years. Oceans have other inputs of phosphorus besides what is brought by rivers. These
inputs are the result of low-temperature weathering, high-temperature submarine hydrothermal
weathering, and submarine low-temperature seawater basalt exchange (Froelich et al., 1982).
