LIMITED APPLICABILITY OF THE CENTURY MODEL TO HIGHLY WEATHERED TROPICAL SOILS

The CENTURY soil organic matter model was developed to describe nutrie nt dynamics for soils of the North American Great Plains, but is now a pplied to other soil types and climates. Phosphorus transformations in CENTURY are based on empirical relationships, developed for soils whe re N is the most limiting nutrient. This restricts the model's applica bility in P-limited soils, particularly those that differ in reactivit y 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 t emperate zones. Equations used in the model to partition incoming litt er into slowly and rapidly cycling organic matter may not be suited fo r such litter. A problem in validating the model is achieving congruen ce between nutrient fractions that can be measured in the soil and the conceptual pools (state variables) that represent such fractions in t he model. Work on P in tropical soils indicates that parameterizing CE NTURY may require the use of different P fractions than those of the o riginal version. State variables thus will change. The lack of experim ental data on P fractionation in relation to P availability in highly weathered P-deficient soils is a major restriction in formulating an a dequate theoretical framework on P dynamics in tropical soils. The CEN TURY model therefore has limited applicability to highly weathered low -P soils. Various improvements in the P submodel of CENTURY are recomm ended.