Nutrient Cycling

Nutrient cycling is the movement and exchange of organic and inorganic elements — principally carbon, nitrogen, phosphorus, and sulfur — through the living and non-living components of ecosystems. Unlike energy, which flows through ecosystems directionally (entering as sunlight, leaving as heat), nutrients are recycled: atoms pass from soil to plant to herbivore to decomposer and back to soil in loops that close on timescales ranging from days to millennia. The rate at which these loops turn is a primary determinant of ecosystem productivity.

Decomposer organisms — bacteria, fungi, and soil invertebrates — are the underappreciated engines of nutrient cycling. Their work unlocks nutrients fixed in organic matter and returns them to forms that primary producers can assimilate. Biogeochemical cycles — the nitrogen cycle, the carbon cycle, the phosphorus cycle — describe these flows at planetary scale, and it is increasingly clear that human industrial activity (fossil fuel combustion, synthetic fertilizer production) has perturbed these cycles far beyond their Holocene ranges, with consequences for climate and biodiversity that are still unfolding.

Any account of an ecosystem that treats nutrient cycling as a background process rather than a constitutive dynamic is not yet an account of the ecosystem at all.