Changes in soil carbon and nitrogen after contrasting land-use transitions in northeastern Costa Rica

Northeastern Costa Rica is a mosaic of primary and secondary forests, tree plantations, pastures, and cash crops. Many studies have quantified the effects of one type of land-use transition (for example, deforestation or reforestation) on soil properties such as organic carbon (C) storage, but few have compared different land-use transitions simultaneously. We can best understand the effects of land-use change on regional and global ecosystem processes by considering all of the land-use transitions that occur in a landscape. In this study, I examined the changes in total soil C and nitrogen (N) pools (to 0.3 m) that have accompanied different land-use transitions in a 140,000-ha region in northeastern Costa Rica. I paired sites that had similar topography and soils but differed in recent land-use history. The following land-use transitions were represented: 12 conversions of primary forests to banana plantations, 15 conversions of pastures to cash crops, and four conversions of pastures to Vochysia guatemalensis tree plantations. The conversion of forests to bananas decreased soil C concentrations and inventories (Mg C ha^-1) in the surface soil by 37% and 16.5%, respectively. The conversion of pastures to cash crops reduced soil C concentrations and inventories to the same extent that forest-to-banana cropping did. Furthermore, young Vochysia plantations do not appear to increase soil C storage, at least over the 1st decade. When data from all land-use transitions were pooled, the difference in root biomass and leaf litter pools between land-use pairs explained 50% of the differences in soil C concentrations and 36% of the differences in soil C inventories. Thus, reduced productivity or C inputs to the soil is one mechanism that could explain the losses in soil C pools with land-use change. In this landscape, losses of soil C due to cultivation are rapid, whereas reaccumulation rates are slow. Total soil N pools (0-10 cm) were also reduced after the conversion of forests to banana plantations or the conversion of pastures to crops, despite fertilization of the cropped soils. This suggests that the added N fertilizer is not retained but instead is exported via produce, N gas emissions, and hydrologic processes.