A nitrogen budget for late-successional hillslope tabonuco forest, Puerto Rico

Nitrogen budgets of late successional forested stands and watersheds provid e baseline data against which the effects of small- and large-scale disturb ances may be measured. Using previously published data and supplemental new data on gaseous N loss, we construct a N budget for hillslope tabonuco for est (HTF) stands in Puerto Rico. HTF stands are subject to frequent hurrica nes and landslides; here, we focus on N fluxes in the late phase of inter-d isturbance forest development. N inputs from atmospheric deposition (4-6 kg N/ha/yr) are exceeded by N outputs from groundwater, gaseous N loss, and p articulate N loss (6.3-15.7 kg N/ha/yr). Late-successional HTF stands also sequester N in their aggrading biomass (8 kg N/ha/yr), creating a total bud get imbalance of 8.3-19.7 kg N/ha/yr. We surmise that this imbalance may be accounted for by unmeasured inputs from above- and belowground N-fixation and/or slow depletion of the large N pool in soil organic matter. Spatial a nd temporal variability, especially that associated with gaseous exchange a nd soil organic matter N-mineralization, constrain the reliability of this N budget.