Biogeochemical cycles or organic and abiotic phase deal with the cycling of nutrients in the ecosystem
Note: the law of conservation of mass; matter is neither created nor destroyed.
The cyclic movement of chemical elements of the biosphere within organisms and the environment is referred to as the biogeochemical cycle (Vernadskii 1934). Movement through the biotic community can be viewed in terms of the food chain.
The flow of chemical elements through the food chains is the organic phase of the biogeochemical cycle. The biogeochemical cycles include the Abiotic phase.
The abiotic phase is very important to the ecosystem as a major reservoir of all nutrients. The flow of the food chain in the abiotic phase is much slower than the one in the organic or biotic phase.
There is food two classes of abiotic phases in the biogeochemical cycles:
1. Sedimentary phase ( found in all cycles)
2. Atmospheric phase (found in some cycles)
In some cycles such as the nitrogen cycle, the atmospheric phase is more important than the sedimentary phase, and in some others such as the phosphorus cycle atmospheric phase does not exist. But in sulphur cycle, both phases are present but their importance depends on other environmental factors. Biogeochemical cycles that have a dominant atmospheric phase are called atmospheric reservoirs e.g. phosphorus cycle.
Biogeochemical cycles are also described as:
1. Gas cycle: the main reservoir is the atmosphere and ocean. Carbon and nitrogen are representatives of such
2. Sedimentary cycle: the main reservoir is the lithosphere (soil, rock other sediments) from which elements are released by weathering e.g. phosphorus and sulphur.
In this cycle mentioned above they have:
1. Both have abiotic and biotic factors
2. Both are driven by the flow of energy.
3. Both are tied to the water hydrological cycle
Water determines the structure and function of an ecosystem apart from its being vital for life. All elements depend on water for cycling as it provides a solvent medium for their uptake. The hydrological cycle is an important medium through which nutrients are introduced into autotrophic plants.
Water vapour gathers to form a cloud and moves with the wind over the earth. These vapour precipitate in form of rain, snow, hail, dew etc. over the surface of the earth.
To be used biologically, the free molecular nitrogen must be fixed. It must be split into free nitrogen atoms, thus N2—2N.
The free nitrogen atom has to combine with hydrogen to form NH3 with the release of an electron. 2N+3H2 àNH3+energy
Note: Nitrogen combines with hydrogen to form ammonia to release energy.
Nitrogen Fixation is in two ways:
1. High energy fixation
2. Biological fixation
High energy fixation: high energy involved in high energy fixation are those such as cosmic radiation and lightning that provide the high energy needed to combine nitrogen with oxygen and hydrogen of H2O resulting in ammonia and nitrate. The materials produced are carried by rainwater.
Biological fixation: it is the most significant fixation. Some bacteria, fungi and blue-green algae can extract molecular nitrogen from the atmosphere. Free-living bacteria that are responsible for fixing nitrogen are nitrifying bacteria, acetobacter and clostridium. These contributed to soil fertility.
Nostoc and Anabaena are important blue-green algae found in free land, fresh water and marine water and they help in nitrogen fixation.
Nitrogen fixation by a symbiotic and non-symbiotic microorganism, in the soil and water, is one source of nitrogen.
Another source is an organic protein of dead organic materials decomposed by a group of microorganisms to produce amino acids and ammonia.
Ammonia is released in the atmosphere or retained in the soil to be absorbed by plants as ammonia salt
The conversion of NH3 or NH4+ salt to NH3– is called Nitrification.
In the first step, Nitrites (NO2+) are formed and converted into NO3–.
The conversion of NH3 to nitrate is done by Nitrosomonas
The bacteria Nitrobacter convert nitrite to Nitrate shown in the below equation
NH3 + 3O2 à 2NO + 2H2O + 2H+ (Nitrosonomas)
2NO2– + O2 à 2NO3– (Nitrobacter)
Nitrate forme can be taken up by plants at the begging of the food chain.
Certain conditions nitrate is not produced in the nitrogen cycle but it is degraded into gaseous nitrogen (n2) nitrogen and ammonia.
The degradation of nitrogen is known as Denitrification. Bacteria Pseudomonas is important in this process.
- Sulphur is an essential part of protein and ammonia acid
- Sulphur can exist in a number of states namely:
- Element sulphur
- Sulphur monoxide
- Sulphur dioxide
Note: Of all these sulphur, sulphide, sulphates are the most important in nature