Biogeochemical cycles

To understand the significance of some of the factors which cause environmental concerns, particularly on a global scale, it is necessary to understand biogeochemical cycles. Biogeochemical cycles are models of the position and behaviour of materials, usually elements or compounds, on a local or global scale under the influence of living organisms, physical earth processes and chemical earth processes. It is possible to distinguish between three types of cycle:

1. Local cycles involve the less mobile elements and have no mechanism of long-distance transport. They are characterised by the absence of leakage from one ecosystem to another. Examples of elements that participate in local cycles are phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), copper (Cu), zinc (Zn), boron (B), molybdenum (Mo), manganese (Mn) and iron (Fe).

2. Global cycles have a gaseous component which allows the element or compound to be transported over great distances in the atmosphere. This includes elements and compounds which may also participate, in a different state, in local cycles. Elements of significance here are carbon (C), oxygen (O), hydrogen (H), sulphur (S) and nitrogen (N), often in a variety of gaseous compounds (e.g. carbon dioxide (CO2), water (H2O), oxides of nitrogen (NOx), methane (CH4), sulphur dioxide (SO2)).

3. Sedimentary cycles, in which particulate material of all sorts is transported by water or sometimes wind.