COS 186-6 - Tropical Forest Soil Carbon Depth Distributions, Organic Chemistry, and Age Variation across the Panamanian Lowlands

Abstract: Tropical forests hold some of the largest carbon (C) stocks globally, however, there is substantial variation in soil organic carbon (SOC) stocks among tropical forests. Subsoil C stocks ( > 20 cm) are poorly characterized in the tropics, making it difficult to predict how climate change might impact these large stocks of C. We hypothesized that: (H1) tropical forests on relatively clay-rich, infertile soils have larger subsoil C stocks relative to more phosphorus and base-rich soils; (H2) the large SOC stocks in infertile tropical soils are characterized by more complex organic chemistry (e.g. aromatics), contributing to longer-term storage and older radiocarbon ages. To address these hypotheses, we analyzed SOC stock sizes, biomolecular composition, and radiocarbon age from 0 – 100 cm depths in 13 distinct lowland forests of Panama across five USDA soil orders and a rainfall range of 1860 – 3300 mm MAP.

We found that SOC stocks decreased and became older with depth across sites. Solid-state ¹³C NMR spectroscopy showed that Alkyl/O-Alkyl ratios, which indicate degree of decomposition, increased with depth. Soil order and depth were significant predictors of ¹³C NMR biomolecular group abundances. Specifically, alkyl C, representative of lipids and waxes, was significantly greater in surface soils in Oxisols and Ultisols related to less-weathered soil orders, and N-alkyl C, representative of proteins, declined with depth. Resin extractable phosphate stocks to 1m depth were significantly negatively correlated with alkyl C across depths (R²=0.32 at 25-50 cm, n=21), indicating that more fertile soils contained fewer waxy compounds. Radiocarbon age (∆14C) varied significantly with soil order. Oxisols had relatively older SOC and Mollisols had younger SOC than other soils. There was a significant positive relationship between resin extractable phosphate stocks to 1m depth vs. ∆14C at 50 -75 cm (R²=0.55, n=8). More phosphorus-rich soils had younger SOC at depth. Biomolecular composition of SOC from 0-10 cm was a significant predictor of ∆14C at depth, with a positive relationship for alkyl C (R²=0.72 at 25-50 cm, n=10; R²=0.55 at 50-75 cm, n=9) and a negative relationship for aromatic C (R²=0.65, n=10). Higher abundances of alkyl C in surface soil corresponded to younger C at depth, while higher abundances of aromatic C in the surface corresponded to older C at depth. This study indicates large spatial variation in SOC stocks, biomolecular composition, and age among tropical forests, with larger, older, and more waxy C groups stored in more strongly weathered tropical soils.