Geographic and vertical patterns of stable carbon isotopes in tropical rain forest soils of Costa Rica

Abstract

Understanding the natural variability in the stable carbon isotope composition of vegetation and soils can aid studies of soil carbon dynamics, and is a prerequisite for using 13C as a natural tracer when vegetation communities change from one photosynthetic pathway to another (e.g. C4 pasture grasses replacing C3 forests). To examine the geographic and vertical patterns of δ 13C under mature rain forest vegetation, we measured the isotopic composition of leaf litter, roots, and mineral soil profiles in 35 plots located in a 140,000-ha study region in northeastern Costa Rica. We hypothesized that soil carbon isotopic composition would be closely coupled to the signature of detrital inputs (the average δ 13C of forest floor and fine-root biomass) and that the increase in δ 13C with depth would be related to abiotic variables that influence soil carbon turnover rates. The correlations between surface soil isotopic composition (0–10 cm) and biophysical factors varied across the landscape. In multiple regression analyses, combinations of soil particle-size distribution and soil C/N ratio explained between 60% and 82% of the variation in surface soil isotopic composition for different regions in the landscape, but the isotopic composition of litter and roots did not explain any additional variation. The isotopic composition of litter, roots, and surface soils (0–20 cm) was fairly similar over large, regional gradients of elevation and edaphic properties (the range of variation was 2.6‰), but diverged considerably at depths below 20 cm (up to 5.8‰ differences). These large regional differences in the isotopic composition of soil C at depth are attributed to variation in soil carbon turnover rates across the landscape.