A trail of isotopes from food webs to ecosystems

Stable isotopes are helpful in relating metabolic processes to ecosystem properties. Animals show characteristic ¹⁵N enrichments relative to their diet, but this enrichment is variable. The cause for this variability is not well known. The soil microbial biomass is likewise ¹⁵N enriched relative to the soil total and soluble N fraction, and this enrichment is also variable. However, we find that this microbial ¹⁵N enrichment is related to C and N availability for soils from semiarid to temperate and tropical climates, grasslands and forests and over 4 million years of ecosystem development. A similar relationship is observed between ¹⁵N enrichment of animals and the C:N ratio of their diet. We will discuss why this relationship exists and how ¹⁵N enrichment reveals metabolic processing throughout food webs, N cycling and is reflected in the stable soil organic matter.