Biogeochemical processes along a nutrient gradient in a tropical ombrotrophic peatland

Biogeochemical properties, including nutrient concentrations, carbon gas fluxes, microbial biomass, and hydrolytic enzyme activities, were determined along a strong nutrient gradient in an ombrotrophic peatland in the Republic of Panama. Total phosphorus in surface peat decreased markedly along a 2.7 km transect from the marginal Raphia taedigera swamp to the interior sawgrass swamp, with similar trends in total nitrogen and potassium. There were parallel changes in the forest structure: basal area decreased dramatically from the margins to the interior, while tree diversity was greatest at sites with extremely low concentrations of readily-exchangeable phosphate. Soil microbial biomass concentrations declined in parallel with nutrient concentrations, although microbes consistently contained a large proportion (up to 47%) of the total soil phosphorus. Microbial C:P and N:P ratios and hydrolytic enzyme activities, including those involved in the cycles of carbon, nitrogen, and phosphorus, increased towards the nutrient-poor wetland interior, indicating strong belowground nutrient limitation. Soil CO2 fluxes and CH4 fluxes did not vary systematically along the nutrient gradient, although potential soil respiration determined on drained soils was lower from nutrient-poor sites. Soil respiration responded strongly to drainage and increased temperature. Taken together, our results demonstrate that nutrient status exerts a strong control on above and below-ground processes in tropical peatlands with implications for carbon dynamics and hence long term development of such ecosystems.