Physical control of soil organic matter dynamics in the tropics

Management of soil organic matter (SOM) is esential to sustaining the quality and productivity of soils around the globe. This appears to be particularly true in the tropics where there is a greater proportion of nutrient poor, highly weathered soils that are more susceptible to losses of SOM. Developing management practices that promote the maintenance and storage of SOM in the tropics depends on understanding the factors that control SOM dynamics. This paper describes the role that soil physical properties (mineralogy, texture, and structure) play in regulating the accumulation and loss of SOM in tropical soils. Two different approaches are presented here. The first approach explores relationships between total SOM and soil physical properties in the tropics. These include effects of climate and mineralogy on latitudinal gradients in SOM, interactions between texture and mineralogy as determinants of SOM storage and relationships between SOM and the structural stability of soils. The second approach describes characteristics of SOM associated with different physical constituents of the soil, with particular attention to particle-size fractions and aggregated particles of different sizes. In each case we summarise findings on the distribution of SOM among fractions and characterise its biochemical composition, bioavailability and turnover. Evidence for and against the physical protection of organic matter from microbial attack in tropical soils is also given. Wherever possible, we compare and contrast the findings for tropical soils with those of temperate soils. The influence of landuse management on physical control of SOM dynamics is discussed as an overriding factor with each aproach. (C) 1997 Elsevier Science B.V.