The Geography of Organic Decomposition Rates

Abstract Decomposition of organic matter is a critical link in the cycling of nutrients through vegetated ecosystems. The primary controls of decay rates are climatic energy and moisture and the chemical-physical nature of the organic matter. A model was developed which uses these controls to predict the geography of decomposition rates for Canada and the United States. Annual actual evapotranspiration (AE) expresses the climatic controls, and the lignin concentration of the organic matter expresses chemical-physical controls. A preliminary model for predicting decay rates was validated using recent field studies. The model showed no consistent pattern of regional under- or overprediction and was therefore used to produce maps of litter that is easy (5 percent initial lignin concentration), intermediate (15 percent), and difficult (30 percent) to decompose. A fourth map presents the geographic variability of lignin's control over decay rates. Decomposition rates in arid areas could not be predicted because of the confounding influence of arthropod activity, especially termites. At a continental scale of analysis climate overwhelms all other controls of decomposition rates, with AE accounting for about three-fourths of the explained variance in decomposition rates. As geographic scale is reduced, the chemical-physical nature of the organic matter and other controls becomes more evident.