Biogeochemical cycles

Biogeochemical cycles describe the chemical and physical transformation of elements on earth. Many key processes in these cycles are mediated by biological organisms, hence the bio in biogeochemical. Biogeochemical cycles are closed (to a good approximation). Elements are, for the most part, neither destroyed nor created.

Biogeochemical cycles describe the pathways by which energy from the sun is assimilated by living organisms and stored as chemical energy. They are essential for recycling material. Without them, life would run out of stuff it needs. While there are, for the most part, no net changes in the amount of an element, fluxes in and out of compartments are changing due to the activities of humans. A key question for modern scientists is to gauge how significant these changes are.

When we study biogeochemical cycles, we focus on the elements that are essential for life. Hydrogen, carbon, oxygen, nitrogen, sulfur and phosphorous are the elements that constitute the bulk of all living tissue and these are the elements we focus on when we study biogeochemistry.

In this class, we are going to study primarily the nitrogen, carbon, and sulfur cycles. Phosphorous does not cycle significantly on human timescales. Hydrogen is closely connected to both carbon and oxygen.

The key things to be aware of in studying any biogeochemical cycle are the following:

• Major reservoirs • Fluxes • Oxidation states of elements in each compartment (biogeochemical cycles primarily represent changes in oxidation states from one compartment to the next) • Chemical properties of various molecular species • Magnitude of human impacts

We’ll begin by looking at the nitrogen cycle. The major reservoir for nitrogen is the atmosphere, where it is found as the relatively inert gas N2. Key to its use by most living things is breaking the N-N triple bond, which is known as “nitrogen fixation”. Nitrogen fixation is accomplished by nitrogen-fixing bacteria that contain a metalloenzyme known as nitrogenase, by lightening, and by human industry. Most organisms can only assimilate “fixed” nitrogen – nitrogen in the form of ammonia or nitrate. The most dramatic change in the nitrogen cycle is the amount of nitrogen humans are now fixing through industrial processes. It is estimated that human activities have probably doubled the amount of nitrogen that is fixed per year. Humans have also altered the concentration of nitrogen-containing compounds in the atmosphere. Nitrous oxide (N2O) concentrations are increasing at a rate of about 0.2-0.3% per year. Nitrous oxide is both a green house gas and a cause of stratospheric ozone depletion. Nitric oxide (NO) concentrations are also increasing. About 80% of all NO emissions are caused by humans. Nitric oxide contributes to photochemical smog and acid rain. An increase in the amount of fixed nitrogen in the environment changes the composition of plants. Many ecosystems have historically been nitrogen-limited. In some areas, nitrogen may no longer been limiting. Many aquatic ecosystems are visibly burdened by the high growth of plants and bacteria. Nitrate in ground water can cause problems for infants who drink the water because inside their bodies they convert the nitrate to nitrite and nitrite binds to iron in hemoglobin, preventing oxygen from binding, which leads to suffocation. Nitric acid is contributing to the acidification of aquatic ecosystems.