Essay on the Carbon Cycle

Here is an essay on the carbon cycle.

Carbon is an essential element that can be cycled from inorganic to organic form. It is pre­sent in nature as inorganic CO₂ and in living and dead organisms as various organic matters. The cyclic conversion is maintained between in­organic and organic form (Fig. 2.33).

CO₂ becomes available in nature by micro­bial decomposition of organic substance, burning of fuels such as coal and petroleum; and respiration in plants and animals. Carbonates present in rocks and sediments are also con­tribute in this respect.

The inorganic CO₂ are converted to orga­nic carbon by different types of autotrophs i.e., photolithotrops and chemolithotrophs. After the fixation of carbon into organic compounds, it can move in heterotrophic organisms. Later on, the degradation of organic compounds takes place in soil and CO₂ goes back in the atmo­sphere.

The decomposition of organic matters takes place by both aerobic and anaerobic condition. During aerobic degradation, 60-90% of carbon of organic residue is utilised as a source of energy, releasing CO₂ in atmosphere. But in anaerobic condition, about 95-98% carbon is released as methane (CH₄). Methane again goes back to nature as CO₂ through oxidation.

Decomposition of Organic Matters in Soil:

Organisms like bacteria including actinomycetes and also fungi of different groups such as Zygomycotina, Ascomycotina, Basidiomycotina and Deuteromycotina play a major role in the decomposition of organic matters in the soil.

The decomposition process starts while the plants are at the senescent stage and it conti­nues throughout their life-cycle. The different types of microbes decompose at different level of decomposition. Most of the work in this respect has been done on fungi.

Different steps of decomposition — as done by fungi — are given:

i. The primary colonisers are weak parasites and/or saprophytes of sugar fungi i.e., the members of Mucorales etc., which colonise in senescent tissues.

ii. The secondary colonisers grow in dead tissues. In the first phase, members of Ascomycotina and their imperfect forms decompose cellulose and in this stage sugar fungi also become active. Finally, members of Basidiomycotina appear and decompose lignin.

The plant parts comprise chiefly of cellulose, hemicellulose, pectic substances and lignin.

These are degraded by the following fungi:

i. Cellulose:

Cellulose is broken down into sim­ple sugars. The cellulose decomposers are comprised of Ascomycqtina and Deuteromycotina, represented by various species of the genera Aspergillus, Penicillium, Fusarium, Chaetomium, Trichoderma, Phoma, Cladosporium etc.

ii. Hemicellulose:

The fungi involved in the breakdown of hemicelluloses are the species of different genera such as Aspergillus, Penicillium, Chaetomium, Fusarium, Tricho­derma, Glomerella, Alternaria etc.

iii. Pectic Substances:

Microbial breakdown of pectic substances are caused by pectinases, produced by different microbes like Aspergillus, Penicillium, Rhizoctonia, Rhizopus, Monilia etc.

iv. Lignin:

It is the hardest chemical of woody parts of the plants, degraded by different members of Agaricales and Aphylloporales under Basidiomycotina.

In addition to fungi, both aerobic (bacilli and pseumonads) and anaerobic (methanogens) bacteria and also Actinomycetes are involved in the decomposition of organic matters.