Relative to their own requirements, animals and microbes live in a carbon-rich, nitrogen-poor world

Grass plant development (Cynodon at left, Zoysia at right) after treatment with an N-only fertilizer.

I came across an interesting example as I was reading the ‘Internal Structure’ chapter in Joshua Schimel’s Writing Science. The example is a quotation from Nitrogen and Nature by Vitousek et al., and it has some relevance to the idea that carbon supplements applied to turfgrass soils would stimulate soil microbes.

Here’s the quotation, with emphasis mine.

“Organisms use essential elements at characteristic ratios, and these ratios differ systematically among different groups of organisms. Element ratios are widely used in the analysis of marine ecosystems. Their application is usually less explicit in terrestrial ecology, but they provide the basis for using critical element ratios to predict element mineralization or immobilization during decomposition. One general feature of terrestrial ecosystems is that C:element ratios in plants, especially trees, are much wider than those in other organisms as a consequence of plants’ use of C-based compounds (cellulose, lignin) to provide structure. For N in particular, soil bacteria generally have a C:N ratio near 6, while plants often have C:N ratios > 100. Even the leaf litter produced in forests on infertile soils can have C:N ratios in excess of 100.

Consequently, relative to their own requirements, animals and microbes live in a C-rich, N-poor world. Animal nutrition and growth are often constrained by the N content of their food, and protein deficiency is widespread. This difference in stoichiometry can sustain N limitation to animals even where plants are not limited by N supply. Microbes also encounter little N (relative to their requirements) in the plant litter they decompose, and so they retain the N they obtain from their substrate and acquire more directly from inorganic pools in the soil. As a result, N cycling from organic matter back to biologically available forms lags behind the decomposition of plant litter.”

Let’s take a typical putting green with 1.4% organic matter in the top 10 cm (4 inches) of the rootzone. That organic matter contains about 12,000 kilograms of carbon in 1 hectare, or 10,700 pounds of carbon in 1 acre.

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