The influence of diet on the distribution of nitrogen isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant nitrogen isotopic composition. 
The isotopic composition of the nitrogen in an animal reflects the nitrogen isotopic composition of its diet. The δ^(15)N values of the whole bodies of animals are usually more positive than those of their diets. Different individuals of a species raised on the same diet can have significantly different δ^(15)N values. The variability of the relationship between the δ^(15)N values of animals and their diets is greater for different species raised on the same diet than for the same species raised on different diets. Different tissues of mice are also enriched in ^(15)N relative to the diet, with the difference between the δ^(15)N values of a tissue and the diet depending on both the kind of tissue and the diet involved. The δ^(15)N values of collagen and chitin, biochemical components that are often preserved in fossil animal remains, are also related to the δ^(15)N value of the diet. 
The dependence of the δ^(15)N values of whole animals and their tissues and biochemical components on the δ^(15)N value of diet indicates that the isotopic composition of animal nitrogen can be used to obtain information about an animal's diet if its potential food sources had different δ^(15)N values. The nitrogen isotopic method of dietary analysis probably can be used to estimate the relative use of legumes vs non-legumes or of aquatic vs terrestrial organisms as food sources for extant and fossil animals. However, the method probably will not be applicable in those modern ecosystems in which the use of chemical fertilizers has influenced the distribution of nitrogen isotopes in food sources. 
The isotopic method of dietary analysis was used to reconstruct changes in the diet of the human population that occupied the Tehuacan Valley of Mexico over a 7000 yr span. Variations in the δ^(15)C and δ^(15)N values of bone collagen suggest that C_4 and/or CAM plants (presumably mostly corn) and legumes (presumably mostly beans) were introduced into the diet much earlier than suggested by conventional archaeological analysis.

Influence of Diet On the Distribtion of Nitrogen Isotopes in Animals

https://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_6/b_fdi_47-48/010011800.pdf

Physical control of soil organic matter dynamics in the tropics

Factors controlling humification and mineralization of soil organic matter in the tropics

The feeding ecology of earthworms – A review

δ13C values of soil organic carbon and their use in documenting vegetation change in a subtropical savanna ecosystem

The effects of endogeic earthworms on the soil organic matter (SOM) dynamics of moist tropical soils are: (i) a sharp increase of mineralization during digestion; (ii) the presence in fresh casts of large amounts of mineral nutrients which are reorganized in microbial biomass at the scale of days to weeks depending on soil properties; (iii) a subsequent blocking of mineralization at the scale of months to years in the compact structure of ageing casts (i.e. older than 1–2 weeks). It is hypothesized that, at the larger scale of soil profile and years, activities of earthworms will result in an acceleration of SOM turnover and the accumulation of labile rather than passive organic matter.

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Small-scale and large-scale effects of endogeic earthworms on soil organic matter dynamics in soils of the humid tropics

The ability of earthworms to assimilate soil organic pools of different ages was investigated in field conditions through natural 13C labelling. Provided that 13C natural abundance of earthworm tissues is determined by their diet, the assimilation of soil organic matter by earthworms was estimated by measuring 13C/12C ratio of tissues of earthworms sampled in soils containing organic pools with differing 13C/12C ratios. Earthworms were collected in two locations where the vegetation shifted from a C3 to a C4 type (France), or from a C4 to a C3 type (Ivory Coast) 3 to 20 years ago. The results show that irrespective of their ecology (litter or soil-feeding), earthworms mainly feed on recent soil organic pools and assimilate soil organic matter with the same age distribution as the overall decomposers in the soil.

Agnés Martin

Papers

Small-scale and large-scale effects of endogeic earthworms on soil organic matter dynamics in soils of the humid tropics

Earthworm diet related to soil organic matter dynamics through 13C measurements.