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The amount and quality of inputs into soil organic matter will be altered by both climate and landuse change. The increase in growth of plants caused by increasing CO2 concentration implies not only potential increases in yields but also increases in plant residues. Simulation models using doubled CO2 levels predict global net primary productivity (NPP) to increase by 16.3%, over half of which will occur in the tropics. For tropical ecosystems increases in NPP will be dominated by the effects of elevated CO2, with water and nitrogen availability and temperature playing a less significant role. Phosphorus limitation may determine whether the potential for increased plant growth will be realized. The distribution of C3 and C4 species in the tropics could be affected by landuse change and estimates of yield increases will be dependent on their proportions. The allocation of photosynthate to the root will increase under elevated CO2, resulting in increased fine root dry weight and root length. Root sink strength and the turnover of roots and associated symbionts are critical knowledge gaps. Carbon: nitrogen ratios in tissues will increase resulting in decreased decomposition rates. The concentration of secondary compounds will be affected more by nitrogen limitations than a direct CO2 effect. Changes in lignin, tannin and polyphenol levels are more important in the decomposability of tropical litters than changes in the C : N ratios. Decomposition models will have to be altered to take into account changes in plant composition. The role of models in predicting the effects of management practice on long-term fertility is addressed.