Soil organic matter – decomposition rates

In previous posts we discussed how the organic carbon is transferred to the ground and what exactly happens in the organic matter during the decomposition procedure.

 But how fast is it decomposed?

 Carbon Flow in Terrestrial Reservoirs (Credit: David Bice)

The organic matter influences the microbial activity in the soil, acting as a carbon source for the microbes (Agren and Bosatta, 1996).As the organic matter is transferred from the plants to the soil, the different substances are decomposed with different speed depending on their structures (Stockmann et al., 2011). For example, leafs are decomposed faster than the wood.

Biogeochemical cycles (2014 UCAR)

Many studies (Meentemeyer, 1978; Parton et al., 2007; Melillo et al., 1982) have concluded that the main factors contribute in the decomposition rate, are:

1. Climate
2.  Humidity
3. Microbiology of the soil 
4. Quality of the organic matter

The climate and the humidity of the soil affect the microorganisms, thus the decomposition of the organic matter. In general, the quality of the organic matter can be characterized by the C/N (Carbon/Nitrogen) ratio. To be more specific, the higher the C/N ratio, the lower the decomposition rate of the organic matter (Melillo et al., 1982; Gupta and Germida, 1988). That shows the importance of the relation between the nitrogen and the carbon cycle, giving the ‘title’ of the limiting factor for the microbial activity.

 Carbon and Nitrogen cycles (2014 UCAR)

 

The increase of the CO₂ emissions helps the greenhouse effect rising the global temperature (see 3^rd post-Human influence in the Atmosphere!). The decomposition of the soil carbon is affected by the temperature because there is a positive feedback to the microbial activity due to temperature increase. This means that the terrestrial carbon is released back to the atmosphere (Stockmann et al., 2011).

A general view of the carbon movements in the terrestrial ecosystem is illustrated below:

 Terrestrial Sequestration Diagram. (Source: EPA)

See you very soon!