Types of Biogeochemical Cycles
There are two basic types of biogeochemical cycles, gaseous and sedimentary. In the gaseous type of biogeochemical cycle there is a prominent gaseous phase. Cycling of carbon and nitrogen represents gaseous biogeochemical cycles. In sedimentary cycles the main reservoir is the lithosphere from which the nutrients are released largely by weathering of rocks. The sedimentary cycle is exemplified by phosphorus and sulphur. When we describe biogeochemical cycle we often say that a cycle is perfect or imperfect. A perfect nutrient cycle is one in which the nutrients are replaced as fast as they are used up. Most gaseous cycles arc generally considered perfect. In contrast, sedimentary cycles are considered relatively imperfect, as some nutrients are lost' from the cycle into the soil and sediments and become unavailable for immediate cycling i.e., there are more stages in which short-term or long-term stagnation occurs. Most significant of the stagnation stages is sedimentation in oceans and deep continental lakes. So if portions of nutrients such as phosphorus or sulphur are lost they are unavailable to organisms for comparatively, longer periods. Human beings have so speeded up the movement of many nutrients that the cycles tend to become imperfect or rather acyclic resulting in too much of nutrients at one stay or too little at another.
Factors or processes which promote nutrient loss from the compartments of biogeochemical cycles to the reservoir can impoverish ecosystems over long run. For example, continuous cultivation and cropping without the use of fertilisers is bad for the soil. Small particles and nutrients wash with runoff waters or leach down to groundwater and rivers through subsoil to the sea, where they may get buried with sediments which may eventually be incorporated into rocks. Agriculture, forestry operation (e.g. deforestation) and other activities can profoundly affect the rates of nutrient cycling. For instance, burning of fossil fuels contributes towards the build up of carbon dioxide in the atmosphere. We will learn more of this as we discuss each cycle individually.