Question

Explain why the C:N ratio of a soil is important. How does the C:N ratio of added organic residue affect soil processes, especially decomposition?

Answer 1

A carbon-to-nitrogen ratio (C/N ratio or C:N ratio) is a ratio of the mass of carbon to the mass of nitrogen in a substance. It can, amongst other things, be used in analysing sediments and compost. A useful application for C/N ratios is as a proxy for paleoclimate research, having different uses whether the sediment cores are terrestrial-based or marine-based. Carbon-to-nitrogen ratios are an indicator for nitrogen limitation of plants and other organisms and can identify whether molecules found in the sediment under study come from land-based or algal plants.^[1]Further, they can distinguish between different land-based plants, depending on the type of photosynthesis they undergo. Therefore, the C/N ratio serves as a tool for understanding the sources of sedimentary organic matter

Soil organic C is known to be protected by three main processes in soils - chemically, physically, and biochemically. Among the biochemical characteristics of an organic material, C/N is among the ones that show a great influence on its decomposition rate. A material with lower C/N ratio, like soybean residue, compared to a corn residue for example (higher C/N), would be much easier to be decomposed by the microorganisms, being known as a more labile material. But an interesting fact showed recently by a group of researchers is that the stoichiometry of the organic material being decomposed is determinant on how much CO2 would be released during decomposition. The microorganisms have a fixed nutrient ratio (C/N/P) and, during the decomposition process they impose its own stoichiometry to the transformed material. It means that, a diet rich in C causes microorganisms to release more C as CO2 into the atmosphere as the microbes try to maintain their healthy C/nutrients ratio.