I have had a significant amount of experience with prairie soils in the north central US (mostly mollisols). In my evaluation of cropland soils, grassland soils, and wetland soils, I have found the C:N ratios almost always at about 11:1 +/- 1 which is at the 10:1 value. This is assuming that the soils are in equilibrium with their management and environment. This can vary, usually when additional plant/animal biomass is added but due to the microbial equilibrium reestablishing shortly after the biomass is added, returns to near the equilibrium level.

An interesting sidelight of evaluating soil C:N ratios is their usefulness in evaluating environmental pollution or contamination. Contamination from hydrocarbons will widen the C:N ratio due to the excess C while fertilizer contamination will narrow the C:N ratio due to excess N. the rebound from each of these types of contamination take some time to rebound and evaluating C:N on a periodic basis can be used to evaluate effectiveness of remediation.

"This critical ratio (10: 1) is a reflection of the dynamic equilibrium that results from the dominating presence of a microbial population, the ratio being similar to the average chemical composition of microbial cells." - Shreeja D

Thomas' response was on the ball but there are exception all the way up to peat

The ratio would always steer around 10.

The C: N ratio is closely linked to the microorganisms that use the C chains as an energy source. Fungi prefer to consume residues with high C: N ratios (for example, a residue of grasses usually has values ​​of 60-80) while bacteria prefer to consume materials with lower C: N ratios (of 30-40 like legume residues ). In other ranges of relationships C: N both types of microorganisms coexist. Mushroom mycelia have a mean C: N of 12 and bacteria cells a mean C: N of 8. When the vegetable waste has a ratio above these values ​​it indicates that there are chemical bonds between carbon molecules with energy capable of being used by microorganisms. These are the ones that possess the enzymes capable of cutting those junctions and take advantage of that energy.

The long chains of C shorten and reach CO2 that is released as gas into the atmosphere while the N is recycled many times before becoming part of a residue with C: N close to 10 that we usually know as stabilized soil organic matter.

Decomposition of organic materials in soil takes place over a period of time, and different kinds of organisms participate in the process. The first group of microorganisms live on the easily decomposable sugars, starches, and cellulose. As the energy supply from these materials gets depleted, these organisms die off and become part of the organic matter, effectively narrowing its C: N ratio. Another group of fungi and bacteria then will dominate utilizing the dead fungal tissue and the remaining organic residues.

Microbial decomposition tends to narrow the C: N ratio of the products towards a limiting value of 4:1 to 9:1, but the C: N ratio of soil seldom reaches this value. The organic matter will stabilize in humus of a C: N ratio that is characteristic of the particular soil, but a ratio of 9:1 to 12:1 is common in soils. This is usually taken as 10:1 for ordinary purposes.

Microorganisms have a C:N ratio of 10:1. If the soil has a wider C: N ratio, microorganisms will take soil available N for their own cell synthesis and proliferation, thereby depriving the soil of its N.and causing N immobilization. Thus, a C:N ratio of 10:1 ratio is considered ideal for soil. Organic residue with wider C:N ratio (Straw) on addition to soil take much longer to decompose. While composting substrate mixture, C:N ratio is highly important to initiate decomposition.

I have had a significant amount of experience with prairie soils in the north central US (mostly mollisols). In my evaluation of cropland soils, grassland soils, and wetland soils, I have found the C:N ratios almost always at about 11:1 +/- 1 which is at the 10:1 value. This is assuming that the soils are in equilibrium with their management and environment. This can vary, usually when additional plant/animal biomass is added but due to the microbial equilibrium reestablishing shortly after the biomass is added, returns to near the equilibrium level.

An interesting sidelight of evaluating soil C:N ratios is their usefulness in evaluating environmental pollution or contamination. Contamination from hydrocarbons will widen the C:N ratio due to the excess C while fertilizer contamination will narrow the C:N ratio due to excess N. the rebound from each of these types of contamination take some time to rebound and evaluating C:N on a periodic basis can be used to evaluate effectiveness of remediation.

I agree on above answers, this is just about the way to evaluate correct humification process of crop soil as we can read in relevant litterature; however some variations may occur naturally depending on local conditions.

I just would like to add the choice of soil improvers should take care about this fact ; the nearer C/N to 10 ( eg 15 instead of 40/60), will give better results: it means an humification process has started properly....And losses of weight of organic matter will be reduced.

The C:N ratio of 10:1 can be considered as the equilibrium state in respect of carbon and nitrogen cycles triggered and activated by soil biota. It is time dependent process as the climate has profound influence on bioconversion of organic matter.

Isn't incredible, still, that this soil biota, with all its diversity, leads to this ratio, even if microorganisms may be the most important players in this transformation.

I'm in awe. :)

Yes. It is truly incredible making humans subdued and humble before the soil sheltered living organisms. Thank you Thomas.

I like to look at all this as analogous to the concept of climax in any type of ecological succession. Being the soil an ecosystem itself, its communities and their metabolism tend to reach a climax C/N ratio of 10:1, which supports the highest level of microbial diversity and health (as reflected above ground) by the diversity of the plants community, like on a good quality prairie.

10-20 c/n ratio is good indicator of fertile soil.

Hi Thomas

The responses above indicate that C:N can be quite variable and the 10 figure is really only applicable to fertile soils, such as arable soils that would have originally supported broadleaf forests. On more siliceous parent materials and more podzolic-trending profiles under heathland and conifers, the litter become more recalcitrant and slower to decompose and C:N can be >20

Please contact me on 07706 126696 when you are on campus and we can talk this over

Regards

Ian Baillie

Knowing a C:N ratio of manures and crop residues may be useful as an indicator of potential availability of N. Overall, materials with a C:N ratio 30:1 will tend to immobilize N. So, it is practically difficult to find exactly a C:N ratio of 10:1 in agricultural soils. So, the range is important whether a C:N ratio of a particular soil type is suitable for microorganisms, N availability and plant growth.

The C / N ratio in the organic matter of arable ( cultivated ) surface ( Ap) horizons commonly ranges from 8 /1 to 15 / 1, the median being between 10 / 1 and 12 / 1 .

Imagine the topsoil (horizon A) as a gigantic digestor. Here is where biological activity is maximal in decomposing organic matter. At the end of the process the latter becomes stable to support life a step further, while its concurrent mineralization uptakes nutrients to support above soil life. Thus, I am not surprised that eventually the C/N ratio would tend to reach a constant value. Amazing indeed!

Bruno Borsari has provided a nice reply. I am sending a brief supplementary. It is the microorganisms which help in decomposition or mineralization of soil organic matter. Most of these microbial tissues have C:N ratio around 10:1.Therefore, decomposed soil organic matter also tends to exhibit C:N ratio around 10:1. Since soil organic matter is the major contributor to soil C:N ratio, most of the soils with stable organic matter content shows C:N ratio around 10:1.

The C/N ratio of soil microbial tissues is 5:1 to 10:1, the average value is 8:1. Mounting evidence indicates that organic matter derived from microbial metabolic residues might contribute as much as over 80% of the soil organic carbon stock (Liang & Balser, 2010). Therefore, I am not surprised that the average C/N ratio of SOM in the surface layer of arable land is around 10:1.

Liang, C. & Balser, T.C. 2010. Microbial production of recalcitrant organic matter in global soils: implications for productivity and climate policy. Nature Reviews Microbiology, 9, 75.

The addition of organic matter serves to meet the hunger needs of soil sheltered life. Therefore, it is very essential that OM served to them has to be nutritious, balanced, palatable, healthy for them to grow and develop both in quality and quantity, thus facilitating them to do their best in improving and maitaining the soil fertile and productive. At no time, soil has to be considered as waste bin to dispose dangerous and polluted industrial wastes which enter the food chain and ultimately endanger the health and nutrition of plants,animals and humans. As for C/N ratio, it is at 10:1 that carbon and nitrogen attain equilibrium status in soils of tropical regions of the world unlike in temperate zones of the world where it could go wider.

One planet, one soil, one destiny

Soils of non-tillage environment tends to have higher carbon : nitrogen ratio due to non-disturbances from frequent tillage, accumulation organic and less disturbances are responsible for most soils having C:N ratio up to 10.

I agree with the points made above. C:N ratio varies between soil and land use/management types along gradients of nutrient enrichment. It is, expressed as DOC:DON, nonetheless a very powerful predictor of stream DOC:DON ratios across sites from oligotrophic to eutrophic status. See our new article for details:

Yates, C. A., Johnes, P. J., Owen, A. T., Brailsford, F. L., Glanville, H. C., Evans, C. D., Marshall, M. R., Jones, D. L., Lloyd, C. E. M., Jickells, T., Evershed, R. P. (2019) Variation in dissolved organic matter (DOM) stoichiometry in freshwaters: Assessing the influence of land cover and soil C:N ratio on DOM composition. Limnology and Oceanography. Early online 6 May 2019. doi: 10.1002/lno.11186.

Very good and interesting discussion