Changes in Carbon Chemistry and Stability Along Deep Tropical Soil Profiles at the Luquillo Critical Zone Observatory

Abstract:

Tropical forests are the largest terrestrial carbon (C) sink, and tropical forest soils contribute disproportionately to the poorly-characterized deep soil C pool. The goal of this study was to evaluate how carbon chemistry and stability change with depth in tropical forest soils formed on two contrasting parent materials. We used soils from pits excavated to 140 cm depth that were stratified across two soil types (Oxisols and Inceptisols) at the Luquillo Critical Zone Observatory in northeast Puerto Rico. We used ¹³C nuclear magnetic resonance (NMR) spectroscopy to characterize soil C chemistry and differential scanning calorimetry (DSC) coupled with evolved gas analysis (CO₂-EGA) to evaluate the thermal stability of soil C during ramped combustion. Thirty-four samples with an initial C concentration ≥1% were chosen from discrete depth intervals (0, 30, 60, 90 & 140 cm) for ¹³C NMR analysis, while DSC was performed on 122 samples that included the NMR sample set and additional samples at 20, 50, 80 and 110 cm depth.

Preliminary ¹³C NMR results indicate higher alkyl : O-alkyl ratios and an enrichment of aliphatic and proteinaceous C with depth, compared with greater aromatic and carbohydrate signals in surface soils. The energy density of soil C (J mg^-1 C) also declined significantly with depth. In Oxisols, most CO₂ evolution from combustion occurred around 300ºC, while most CO₂ evolution occurred at higher temperatures (400-500ºC) in Inceptisols. Our findings suggest soil C is derived primarily of plant biomolecules in surface soils and becomes increasingly microbial with depth. Soil matrix-mediated differences in C transport and preservation may result in differences in C chemistry between the two soil types and a more thermally labile C pool in the Oxisols. We suggest that energy-poor substrates, combined with potentially stronger organo-mineral interactions in subsoils, may explain the long-term stability of deep C in highly weathered tropical soils.