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The C/N ratio (C:N) or carbon-to-nitrogen 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. 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, which can lead to information about the ecology, climate, and ocean circulation at different times in Earth’s history.
C/N ratios in the range 4-10:1 are usually from marine sources, whereas higher ratios are likely to come from a terrestrial source.  Vascular plants from terrestrial sources tend to have C/N ratios greater than 20.   The lack of cellulose, which has a chemical formula of (C6H10O5)n, and greater amount of proteins in algae versus vascular plants causes this significant difference in the C/N ratio.   
When composting, microbial activity utilizes a C/N ratio of 30-35:1 and a higher ratio will result in slower composting rates. However, this assumes that carbon is completely consumed, which is often not the case. Thus, for practical agricultural purposes, a compost should have an initial C/N ratio of 20-30:1.
Example of devices that can be used to measure this ratio are the CHN analyzer and the continuous-flow isotope ratio mass spectrometer (CF-IRMS).  However, for more practical applications, desired C/N ratios can be achieved by blending common used substrates of known C/N content, which are readily available and easy to use.
Organic matter that is deposited in marine sediments contains a key indicator as to its source and the processes it underwent before reaching the floor as well as after deposition, its carbon to nitrogen ratio. In the global oceans, freshly produced algae in the surface ocean typically have a carbon to nitrogen ratio of about 4 to 10. However, it has been observed that only 10% of this organic matter (algae) produced in the surface ocean sinks to the deep ocean without being degraded by bacteria in transit, and only about 1% is permanently buried in the sediment. An important process called sediment diagenesis accounts for the other 9% of organic carbon that sank to the deep ocean floor, but was not permanently buried, that is 9% of the total organic carbon produced is degraded in the deep ocean.  The microbial communities utilizing the sinking organic carbon as an energy source are partial to nitrogen-rich compounds because much of these bacterium are nitrogen-limited and much prefer it over carbon. As a result, the carbon to nitrogen ratio of sinking organic carbon in the deep ocean is elevated compared to fresh surface ocean organic matter that had not been degraded. An exponential increase in C/N ratios is observed with increasing water depth—with C/N ratios reaching 10 at intermediate water depths of about 1000 meters, and up to 15 in the deep ocean (~ >2500 meters). This elevated C/N signature is preserved in the sediment, until another form of diagenesis, post-depositional diagenesis, alters its C/N signature once again. Post-depositional diagenesis occurs in organic-carbon-poor marine sediments where bacteria are able to oxidize organic matter in aerobic conditions as an energy source. The oxidation reaction proceeds as follows: CH2O + H2O → CO2 + 4H+ + 4e–, with a standard free energy of –27.4 kJ mol–1 (half reaction). Once all of the oxygen is used up, bacteria are able to carry out an anoxic sequence of chemical reactions as an energy source, all with negative ∆G°r values, with the reaction becoming less favorable as the chain of reactions proceeds.
The same principle described above explaining the preferential degradation of nitrogen-rich organic matter occurs within the sediments, as they are more labile and are in higher demand. This principle has been utilized in paleoceanographic studies in order to identify core sites that have not experienced much microbial activity, or contamination by terrestrial sources with much higher C/N ratios. 
Lastly, it should be noted that ammonia, the product of the second reduction reaction, which reduces nitrate and produces nitrogen gas and ammonia, is easily adsorbed on clay mineral surfaces and protected from bacteria. This has been proposed as an explanation for lower than expected C/N signatures of organic carbon in sediments that have undergone post-depositional diagenesis.
Unlike in marine sediments, diagenesis does not pose a large threat to the integrity of the C/N ratio in lacustrine sediments. Though wood from living trees around lakes have consistently higher C/N ratios than wood buried in sediment, the change in elemental composition is not large enough to remove the vascular versus non-vascular plant signals due to the refractory nature of terrestrial organic matter.   Abrupt shifts in the C/N ratio down-core can be interpreted as shifts in the organic source material.
For example, two separate studies on Mangrove Lake, Bermuda and Lake Yunoko, Japan show irregular, abrupt fluctuations between C/N around 11 to around 18. These fluctuations are attributed to shifts from mainly algal dominance to land-based vascular dominance. Results of studies that show abrupt shifts in algal dominance and vascular dominance often lead to conclusions about the state of the lake during these distinct periods of isotopic signatures. Times in which lakes are dominated by algal signals suggest the lake is a deep-water lake, while times in which lakes are dominated by vascular plant signals suggest the lake is shallow, dry, or marshy. Using the C/N ratio in conjunction with other sediment observations, such as physical variations, D/H isotopic analyses of fatty acids and alkanes, and δ13C analyses on similar biomarkers can lead to further regional climate interpretations that describe the larger phenomena at play.