Signatures in the soil: Soil charcoal and phosphorus distribution patterns along an elevational gradient in a Costa Rican tropical rainforest

Fire history and soil phosphorus were investigated in old-growth tropical rainforests from the Continental Divide to the Caribbean lowlands along Volcan Barva in Costa Rica. The charcoal age and abundance, and phosphorus distribution were measured at multiple soil depths in one ha plots at 250 m elevational intervals from 60 m to 2600 m. Soil charcoal is used as a proxy record of ancient fires in these old growth forests. Charcoal was present at every elevation and its mass (to 1 m soil depth) ranged from as much as 1027 g m-2 at 300 m to as little as 20 g m-2 at 1750 m. Charcoal samples were radiocarbon dated and the dates ranged from 23,240 yr B.P. at 1750 m to 140 yr B.P. at 2600 m elevation. These forests have regenerated multiple times as a consequence of anthropogenic or naturally induced fires. Interestingly, none of the highest elevation forest charcoal samples (>2300 m) were older than 170 yr indicating that the forests near the Continental Divide may be relatively young stands that re-established since the last volcanic eruption.; Landscape patterns of soil phosphorus (P) in old-growth forests along the elevational transect were studied with the specific objective of contrasting wet season versus dry season soils. Both total soil P and the labile P pool (plant available) were larger during the dry season than the wet season and both increased with increasing elevation from 60 m to 2600 m indicating a phosphorus availability gradient. PCA (principal components analysis) clusters derived from Hedley soil P fractions of wet season samples show clearly defined phosphorus availability gradient for the elevational transect. In addition, the three PCA clusters, available P, short-term occluded, and long-term occluded P, correspond closely to the pools in the P transformations model described by Tiessen et al. (1984). When labile P from surface soils at sites along the transect is compared with nitrogen mineralization from a previous study on this transect (Mans et a1.1988), N availability and P availability cross around 1000 m indicating that P limitation at lower elevations yields to N limitation at higher elevations.