Factors controlling humification and mineralization of soil organic matter in the tropics

The first part of this review focuses on the chemical composition and morphological features that characterize primary and secondary organic resources for humification. The chemical pathways of decomposition and humification of SOM in tropical soils are discussed referring mainly to the chemical structural changes identified by using both solid-state C-13 nuclear magnetic resonance spectroscopy (C-13 NMR) of bulk soil samples and liquid-state C-13 NMR Of chemically isolated SOM fractions. The stabilization effects and mechanisms exerted on SOM by clay minerals and sesquioxides in tropical soils are also reviewed. Successively, relevant aspects of organic matter mobilization and dissolved organic matter dynamics in temperate versus tropical ecosystems are examined. In the second part of the review, general and specific aspects of mineralization processes in relation to the chemistry of main SOM pools (labile versus stable SOM components) in the tropics are discussed. Amounts, distribution, and forms of nutrients in SOM, nutrient release from organic versus inorganic sources, nutrient cycling in natural and cultivated soils, and the contribution of SOM to cationic nutrition in tropical soils are reviewed. The final part of the review focuses on the main chemical factors that control CO2 evolution and denitrification processes during SOM mineralization in tropical areas. (C) 1997 Elsevier Science B.V.