Notes on the Carbon: Nitrogen (C: N) Ratio in Soil

This article provides notes on the Carbon: Nitrogen (C: N) ratio in soil.

It is the intimate relationship between organic matter and nitrogen contents of soils. The ratio of the weight of organic carbon to the weight of total nitrogen in a soil or organic material is known as C: N ratio. The importance of C: N ratio in controlling the available nitrogen, total organic matter and the rate of organic materials decomposition is recognized in developing appropriate soil management practices.

The C: N ratio of soil is one of its characteristic equilibrium values, the figure for humus being roughly 10:1 although values from 5: 1 to 15: 1 are generally found in most arable soils. 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.

As a rule microbial protoplasm contains 5 to 15 parts of carbon to 1 part of nitrogen, but 10: 1 is a reasonable average for the predomi­nant aerobic flora. A change in the microbial population brought about by anaerobiosis or the accumulation of fractions resistant to further decomposition can modify the C: N equilibrium value of humus.

Such C: N ratio of soils can vary with the climates i.e. rainfall, temperature etc. The C: N ratio is generally lower in warmer (arid soil conditions) regions than that of cooler ones (humid soil conditions) in-spite of having the same rainfall under both the soil conditions. The ratio is also narrower for sub-soils as compared to surface soil horizons.

The carbon and nitrogen (C: N) ratio in plant material is variable and ranges from 20: 1 to 30: 1 to legumes and farm yard manure to as high as 100: 1 in certain straw residues. On the other hand C: N ratio of the bodies of micro-organisms is not only more constant but much narrower between 4: 1 and 9:1. Bacterial tissue in general is somewhat richer in protein than fungi and consequently has a narrow C: N ratio.

Therefore, it is usually found that most of the applied fresh organic materials in soils carry large amounts of carbon with relatively very small amounts of total nitrogen. As a result, the value of C: N ratio is wide and the values of C: N ratio for soils is in between those of higher plants and the microbes.

The C: N ratio in soil organic matter is important for two major reasons:

(i) Keen competition for available nitrogen results when organic residues of high C: N ratio are added to soils and

(ii) Because this C: N (10: 1) is relatively constant in soils, the maintenance of carbon and hence soil organic matter is dependent to no small degree on the level of soil nitrogen.

So the C: N ratio obviously has practical implications on the availability of nitrogen in soils as well as in plants. As for example, large amount of fresh organic materials having wide C: N ratio (50: 1) is incorporated into the soil under favourable soil conditions for decomposition.

A rapid change will found. The heterotrophic micro-organisms—bacteria, fungi and actinomycetes become active and increases their population with the production of large amounts of CO₂.

Under these conditions, nitrate nitrogen (NO₃-N) disappears from the soil because of the urgent needs by the micro-organisms. And for the time being, little or no nitrogen is available to plants. As the decomposition precedes, the C: N ratio of the organic materials decreases with the loss of carbon and conservation of nitrogen.

As the decay process proceeds, both carbon and nitrogen are now subject to loss as CO₂ and nitrates respectively. It is only a question of time until their percentage rate of disappearance from the soil becomes more or less the same, i.e. the percentage of the total nitrogen being removed equals the percentage of the total carbon being lost.

At this point the C: N ratio becomes more or less constant 10: 1 to 12: 1 always being somewhat greater than that of the ratio in microbial tissue.

Since carbon and nitrogen are reduced to almost a definite ratio, the amount of soil nitrogen largely determines the amount of organic carbon present when stabilization occurs. Thus, the greater the amount of nitrogen present in the original organic material, the greater will be the possibility of an accumulation of organically bound carbon.

Since a definite ratio (1: 1.7) exists between the organic carbon and the soil humus, the amount of organic matter to be maintained in any soil is largely conditional on the amount of organic nitrogen present. The ratio between nitrogen and organic matter is thus constant (organic matter: nitrogen, 20: 1 for most soils).