@gayathree, due to ideal soil moisture condition during incubation, there will be more soil biological activity/ microbial population resulted additional organic matter in soil. In general, ideal soil moisture leveland air condition enhances greater biomass production which also produce more potential food for soil biota.

Thank you very much sir J. C. Tarafdar . Really helpful.

Increase in organic mater may be due to increase in micobial activity with increasing temperature or decay of small parts of plant residues in the soil then converion to organic carbon.

decrease in orcanic carbon also occures with increase in tempreture within incubation.

Could be due to the increasing microbiology activity in soil

Professor Dr Akram Othman Esmail Thank you very much sir for the elaborated answer

Aishah Ramadan Mohamed Thank you very much for the answer.

Most recent researches proves that a sharp increase of OC observed in the second period of soil incubation (7-15 days) due to microbial activity, resulting in mineralizing organic compounds in soils and thus increasing the levels of OC.

I am agree with @ Jamal Elfaki

Some small parts of roots ,or otherand parts of plants and microorganisms may exist in the soil sample they may be decay to organic form during incubation with by best regards.

Please consult this article

Article Effect of Sugarcane Straw and Goat Manure on Soil Nutrient T...

First of all, which method do you use? Is it a variant of Walkley-Black methods or else? If it is C analyser there will be no or negligible changes in the C content. If it is walkley-black you can get either decrease or increase depending on your incubation conditions. This method has quite a lot limitations and very large range of recovery rate depending on the soil properties. You can check the limitations from the Nelson and Sommers 1996 in Methods of soil analysis Part 3-chamical methods. Or you can make a quick literature study by using walkley-black, soil organic carbon, recovery rate key words. I hope you will find a satisfying answer in the out coming recent research papers.

Thank you very much

Veli is correct. Mineralisation of existing C through microbial activity cannot increase the total amount present: that would require net C fixation.