Limited applicability of the CENTURY model to highly weathered tropical soils

The CENTURY soil organic matter model was developed to describe nutrient dynamics for soils of the North American Great Plains, but is now applied to other soil types and climates. Phosphorus transformations in CENTURY are based on empirical relationships, developed for soils where N is the most limiting nutrient. This restricts the model's applicability in P-limited soils, particularly those that differ in reactivity to P and in organic matter stabilization. Moreover, nutrient ratios in plant litter from nutrient-poor tropical soils may be much wider (e.g., C:P ratios up to 3000) than those commonly found in litter from temperate zones. Equations used in the model to partition incoming litter into slowly and rapidly cycling organic matter may not be suited for such litter. A problem in validating the model is achieving congruence between nutrient fractions that can be measured in the soil and the conceptual pools (state variables) that represent such fractions in the model. Work on P in tropical soils indicates that parameterizing CENTURY may require the use of different P fractions than those of the original version. State variables thus will change. The lack of experimental data on P fractionation in relation to P availability in highly weathered P-deficient soils is a major restriction in formulating an adequate theoretical framework on P dynamics in tropical soils. The CENTURY model therefore has limited applicability to highly weathered low-P soils. Various improvements in the P submodel of CENTURY are recommended.