CARBON-ISOTOPE CHARACTERIZATION OF VEGETATION AND SOIL ORGANIC-MATTERIN 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 i sotopic enrichment through decomposition that has been reported for te mperate forests. Soil cores and vegetative samples from the decomposit ion continuum (leaves, new litter, old litter, wood, and roots) were t aken from each of four forest types in the Luquillo Experimental Fores t, Puerto Rico. SOM delta(13)C was enriched 1.6 parts per thousand rel ative to aboveground liner. We found no further enrichment within the soil profile. The carbon isotope ratios of vegetation varied among for ests, 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 dif fered between forests primarily in the top 20 cm where the Colorado fo rest 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 di fferences between forests are likely attriburable to differences in li ght regimes due to canopy density variation, soil moisture regimes, an d/or recycling of CO2. Our data suggest that recalcitrant SOM is not d erived directly from plant lignin since plant lignin is even more C-13 deplered than the bulk vegetation. We hypothesize that the anthropoge nic isotopic depletion of atmospheric CO2 (ca 1.5 parts per thousand i n the last 150 years) accounts for some of the enrichment observed in the SOM relative to the more modern vegetation in this study and other s. This study also supports other observations that under wet or anaer obic soil environments there is no isotopic enrichment during decompos ition or with depth in the active profile.