Biogeochemical cycles, Hydrological cycle, Nitrogen Fixation, Ammonification, Nitrification, Denitrification and Sulphur Cycle

Biogeochemical cycles

Biogeochemical cycles or organic and abiotic phase deal with cycling of nutrient in the ecosystem

Note: law of conservation of mass; matter is neither created nor destroyed.

The cyclic movement of chemical elements of the biosphere within organisms and environment is referred to as the biogeochemical cycle (Vernadskii 1934). Movement through the biotic community can be viewed in terms of food chain.

The flow of chemical elements through the food chains is the organic phase of the biogeochemical cycles. The biogeochemical cycles include Abiotic phase.

Abiotic phase is very important to ecosystem as a major reservoir for all nutrients. Flow of food chain in the aboitic phase is much slower than the one in the organic or biotic phase.

There are food two classes of abiotic phase in the biogeochemical cycles:

1.    Sedimentary phase ( found in all cycles)

2.    Atmospheric phase (found in some cycles)

In some cycle such as nitrogen cycle, atmospheric phase is more importance than sedimentary phase and in some other such as 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 dominant atmospheric phase are called atmospheric reservoir e.g. phosphorus cycle.

Biogeochemical cycles are also described as:

1.   Gas cycle: the main reservoir is atmosphere and ocean. Carbon and nitrogen is representative of such

2.   Sedimentary cycle: the main reservoir is 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 flow of energy.

3.   Both are tied to water hydrological cycle

Hydrological cycle:

Water determines the structure and function of an ecosystem apart its being vital for life. All elements depend on water for cycling as it provides solvent medium for their uptake. Hydrological cycle is an important medium which nutrients are introduced into autotrophic plants. 

Diagram of Hydrological Cycle

Hydrological Cycle

Water vapour gathers to form cloud and move with the wind over the earth. These vapour precipitate inform of rain, snow, hail, dew etc. over the surface of the earth.

Nitrogen Fixation

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 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 lightening that provide high energy needed to combine nitrogen with oxygen and hydrogen of H₂O resulting in ammonia and nitrate. The materials produced are carried by rain water.

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, azetobacter and cloustridium. These contributed to the 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.

Ammonification

Nitrogen fixation by symbiotic and non symbiotic microorganism, in the soil and water is one source of nitrogen. Another source is organic protein of dead organic materials decompose by group of microorganisms to produce amino acid and ammonia.

Ammonia is release in the atmosphere or retained in the soil to be absorbed by plant as ammonia salt

Nitrification

The conversion of NH₃ or NH₄⁺ salt to NH₃^- is called Nitrification.

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 below equation

NH₃ + 3O₂ à 2NO + 2H₂O + 2H⁺ (Nitrosonomas)

2NO₂^- + O₂ à 2NO₃^- (Nitrobacter)

Nitrate forme can be taken up by plant at the begging of food chain.

Denitrification

Under certain condition nitrate is not produce in nitrogen cycle but it is degraded in to gasous nitrogen (n₂) nitrogen and ammonia

Degradation of nitrogen is known as Denitrification. Bacteria Pseudomonas is important in thi process.

Sulphur Cycle

Sulphur is an essential part of protein and ammonia acid

Sulphur can exist in a number of states namely:

i.        Element sulphur

ii.       Sulphur monoxide

iii.      Sulphate

iv.      Sulphite

v.       Sulphur dioxide

vi.      Sulphide

Note: Of all these sulphur, sulphide, sulphaate are the most important in nature