Changes in nutrient cycling during tropical deforestation

The ecology of tropical land rehabilitation and reforestation will benefit from a clarification of how specific types of land use can affect ecosystem processes, and to what extent such processes can be recovered during reforestation. The general objective of my research was to investigate changes in nutrient cycling during natural and silvicultural managed reforestation of humid tropical forests that were previously affected by pasturing and cropping activities.; I used selected sites representing chronosequences after pasture abandonment to natural succession in Puerto Rico to study the changes in nutrient cycling during natural regeneration. I found relatively minor changes in the re-distribution of soil C, N, and P within different pools during natural post-pasture reforestation, while net N mineralization, as indexed by 10-day aerobic incubations, increased with years after pasture abandonment. An increase in N supply capacity without a change in fractions of C, N, and P, suggests that nutrient stock in the mineral soil may not be the main factor controlling either N mineralization or secondary forest succession. Other factors, not investigated in this study, seem to be more important in limiting forest regeneration in this region.; I also investigated nutrient dynamics in sites in Hawaii that were silviculturally reforestated by the establishment of plantations of fast-growing tropical trees species. In this study, I examined the differences between two fast- growing tropical species on their effect on soil properties and N cycling. I used pure stands of Eucalyptus saligna (Sm.) and the N{dollar}sb2{dollar}-fixing Albizia falcataria (L.) Fosberg (=Paraserianthes falcataria(L.) Nielsen). After thirteen years of site occupancy, soils under the Albizia stands contained larger pools of total C and N and larger pools of inorganic N. Net N mineralization rates also were larger in soils under Albizia. Soil microbial biomass on Albizia soils was dominated by bacteria, while it was dominated by fungi on Eucalyptus soils. Results from this study showed the potential benefit of using N{dollar}sb2{dollar}-fixer species for restoration purposes. When natural processes are inadequate for forest land rehabilitation, the use of managed silvicultural regeneraton is an alternative method to re-establish soil fertility and ecosystems functions.