| The majority of carbon in the terrestrial biosphere is stored in soil organic matter (SOM). Decomposition of SOM from tropical ecosystems is a major source of increasing atmospheric CO{dollar}sb2.{dollar} There is a great deal of uncertainty in estimating carbon emissions from specific tropical soils, and information on loss of soil carbon under varying management practices is limited. Tropical volcanic ash store more carbon than tropical soils formed in more crystalline mineral materials. Despite their small distribution, their exceptionally high carbon content makes them potentially large sources of atmospheric CO{dollar}sb2.{dollar}; In the volcanic ash soils studied, replacement of tropical forests with pasture promotes translocation of organic matter in Fe/Al-humus complexes from surface horizons to subsurface horizons. The carbon translocated is old carbon with long ({dollar}>{dollar}2000 yr) turnover times. With additions of organic matter to surface horizons by pasture vegetation, there is a net increase in carbon in pasture soils, relative to forest soils.; Replacement of tropical forests with sugar cane results in losses of SOM, transformations of iron oxide minerals from crystalline to weakly-crystalline, disassociation of Fe/Al-humus complexes, and formation of weakly-crystalline aluminosilicate minerals. The mineral formation and transformations are facilitated by additions of calcium silicate fertilizer, by compaction of soils with agricultural machinery, and by associated changes in soil solution chemistry and redox conditions. The new mineralogy is similar to soils exposed to greater weathering (higher mean annual precipitation). As the more weathered soils have lower carbon contents, the mineralogical changes may also indicate a reduced ability of the cane soil to store carbon upon reforestation.; Because of the high SOM of these soils, 1.36 Pg of carbon is stored in the 480,000 ha study region on the island of Hawaii. The regional change in soil carbon since conversion of forest to pasture and sugar cane is approximately {dollar}-{dollar}0.05 Pg, or a 4% loss. The change is small because losses of carbon from soils under sugar cane are mostly offset by gains in carbon under pasture. These results suggest that on volcanic ash soils, regional land use can be managed to minimize losses in soil carbon storage. |
