The impact on nutrient cycles from tropical forest to pasture conversion in Costa Rica

Abstract

Changes in nutrient and hydrological cycles caused by land disturbance typically lead to detrimental changes to ecosystems. This study utilized a paired, small-catchment approach to examine the effect of deforestation on soils and streams of the tropical Osa Peninsula, Costa Rica. The first catchment had been cleared for pasture and the second consisted of undisturbed tropical wet forest. Soil concentrations of organic matter, total and soil-available phosphorus (P) were higher in the forested catchment with reductions of >33 % of each in the deforested catchment. The effect of deforestation on stream discharge was a 59 % increase in flow during the wet season and a higher Q5:Q95 (percentile flow) ratio showing that the deforested stream yielded shorter duration, higher magnitude flood peaks. The deforested catchment loss of dissolved inorganic nitrogen increased 95 % over the forested catchment. Soluble reactive phosphorus showed a 43 % higher load in the deforested catchment compared to the forested catchment. The molar N:P ratios were low and both streams were well below the level at which N limitation of lotic algal growth has been reported. It, therefore, appears that N is the limiting nutrient in streams in the study area. Soil nutrient depletion in the deforested catchment, accelerated by a changed hydrological regime, is the likely trajectory of soil–water interactions in this tropical ecosystem. Loss of nutrients and organic matter from terrestrial ecosystems will likely lead to long-term impacts on lowland tropical communities. Should deforestation become widespread along this stretch of the Pacific coastline possible eutrophication of receiving transitional and coastal waters may occur.