CARBON-ISOTOPE CHARACTERIZATION OF VEGETATION AND SOIL ORGANIC-MATTER IN SUBTROPICAL FORESTS IN LUQUILLO, PUERTO-RICO

We examined natural abundances of C-13 in vegetation and soil organic matter (SOM) of subtropical wet and rain forests to characterize the isotopic enrichment through decomposition that has been reported for temperate forests. Soil cores and vegetative samples from the decomposition continuum (leaves, new litter, old litter, wood, and roots) were taken from each of four forest types in the Luquillo Experimental Forest, Puerto Rico. SOM delta(13)C was enriched 1.6 parts per thousand relative to aboveground liner. We found no further enrichment within the soil profile. The carbon isotope ratios of vegetation varied among forests, ranging from -28.2 parts per thousand in the Colorado forest to -26.9 parts per thousand in the Palm forest. Isotope ratios of SOM differed between forests primarily in the top 20 cm where the Colorado forest was again most negative at -28.0 parts per thousand, and the Palm forest was most positive at -26.5 parts per thousand. The isotopic differences between forests are likely attriburable to differences in light regimes due to canopy density variation, soil moisture regimes, and/or recycling of CO2. Our data suggest that recalcitrant SOM is not derived directly from plant lignin since plant lignin is even more C-13 deplered than the bulk vegetation. We hypothesize that the anthropogenic isotopic depletion of atmospheric CO2 (ca 1.5 parts per thousand in the last 150 years) accounts for some of the enrichment observed in the SOM relative to the more modern vegetation in this study and others. This study also supports other observations that under wet or anaerobic soil environments there is no isotopic enrichment during decomposition or with depth in the active profile.