Tannins as Regulators of Nitrogen Cycling, Carbon Storage, and GHG Emissions.

This review and synthesis summarizes biogeochemical investigations to elucidate the behavior of tannins as regulators of plant-litter-soil interactions, nitrogen cycling, and carbon storage.  The chemical ecology of tannins and the convergent evolution of tannin-rich plant communities are interpreted in the context of how tannins in plant litter alter the chemical and physical properties of the soil environment.  The implications of tannin biogeochemistry for land management in forestry, agriculture, and restoration ecology are reviewed with emphasis on the potential for mitigating nitrogen loading to surface waters, maximizing carbon sequestration, and minimizing the emission of heat-trapping greenhouse gases (GHGs), such as nitrous oxide and carbon dioxide.
Tannins regulate nitrogen cycling by shifting the dominant vehicle of litter-N release cycling from mineral to dissolved organic forms, minimizing potential N losses from the ecosystem and maximizing litter-N recovery by symbiotic mycorrhizal fungi.  Tannins minimize the production/availability of nitrate, thereby minimizing nitrate leaching to ground water surface water, and minimizing production of nitrous oxide by denitrification.   

Tannins regulate carbon storage by inhibiting decomposition, and providing the substrate to form stable humic acids, which have centuries-long residence time in soil.  Tannins make plant litter unpalatable to detritivores, such as earthworms, and favor the accumulation of a litter layer on the soil surface, sequestering substantial quantities of carbon in the forest floor.   
Tannins regulate metal biogeochemistry through complexation of Al, Mn and Fe, reducing potential Al toxicity and P fixation in soil. Humic substances derived from tannins coat rhizosphere soil surfaces, providing cation exchange capacity to minimize leaching of nutrient cations, and creating a favorable medium for nutrient cycling. Although their adaptive value for antiherbivore defense is in doubt, the alteration of soil conditions and regulation of carbon storage and nitrogen cycling by tannins illustrate how fitness can be influenced by the extended phenotype in plant-litter-soil interactions.