SOIL CARBON, NUTRIENTS, AND MYCORRHIZAE DURING CONVERSION OF DRY TROPICAL FOREST TO GRASSLAND

Wildfires and alien grass invasion threaten dry tropical forests throu ghout Central America. Efforts to preserve and restore these forests w ill require a better understanding of how conversion to grassland chan ges key belowground processes and organisms such as soil organic matte r, nutrient cycling, and mycorrhizae. We studied forest, edge, and gra ssland soils from five 60-m transects perpendicular to abrupt forest-g rassland boundaries in Guanacaste Province, Costa Rica. Nutrient conce ntrations, N mineralization dynamics, and mycorrhizal fungal communiti es were compared across vegetation type (forest, edge, and grassland). The dynamics of N mineralization were measured in year-long laborator y incubations, and the diversity of mycorrhizal fungal communities was assessed from populations of soil-borne spores. Soil C, N, and K were lower, while many base cations and micronutrients were higher in gras sland plots than in forest plots. Although differences in the quantity of total soil C and N occurred mainly in the forest-to-edge transitio n, differences in the quality of soil organic matter, as reflected by soil C:N ratios and mineralization rates, occurred in the edge-to-gras sland transition. Beta diversity of mycorrhizal spore communities (mea sured by Sorenson's similarity index) was lower in the grassland plots than in the forest plots, indicating that grass invasion had caused s ome convergence. However, total spore density and alpha diversity of m ycorrhizal spore communities (measured by species richness and Simpson 's diversity index) were not altered by wildfires and grass invasion. These results suggest that persistence and regeneration of forest plan t species in the grasslands may not be constrained to a significant de gree by the lack of mycorrhizal symbionts. These grasslands appear to be sustainable, alternative stable states for these areas. Positive fe edbacks between the alien grassland vegetation and both fire and nutri ent cycling maintain and reinforce this alternative state.