Changes in carbon stability and chemistry along deep tropical soil profiles at the Luquillo Critical Zone Observatory

Background/Question/Methods

Tropical forests are the largest terrestrial carbon (C) sink, and tropical forest soils contribute disproportionately to the deep soil C pool. However, the mechanisms controlling C stabilization in tropical soils remain understudied. The goal of this study was to determine how C quantity, quality and stability changes with soil depth along the first 1.4 meters of tropical soil profiles from the Luquillo Critical Zone Observatory (LCZO) in northeast Puerto Rico. We excavated pits at four sites representing two soils formed on contrasting parent materials (sedimentary volcaniclastic, Oxisol; igneous quartz diorite, Inceptisol) at two elevations corresponding with different forest types (Tabonuco, lower elevation; Colorado, upper elevation). At six soil depths (0, 20, 50, 80, 110 and 140 cm) we measured soil C, nitrogen (N) and phosphorus (P) concentrations. For the same samples, we performed a 90-day laboratory incubation and calculated average soil respiration rates (R₂₅, µg CO₂-C g^-1 soil day^-1) as an index of biological C stability. We used differential scanning calorimetry (DSC) to evaluate the energy density (E_density, J mg^-1 C) and thermal characteristic of soil C. Finally, we performed solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy to assess differences in soil C chemistry depth, soil and forest type.

Results/Conclusions

Soil C, N, extractable P, R₂₅ and E_density all declined significantly with depth (P < 0.01). The strong positive relationship between R₂₅ and E_density (R²= 0.58, P < 0.01) indicates C bioavailability scales directly with the energy density. There were no differences in R₂₅ or E_density across the two soil or forest types. ¹³C-NMR analyses indicates higher alkyl : O-alkyl ratios and an enrichment in aliphatic and proteinaceous C with depth, compared with greater aromatic and carbohydrate signals in surface soils. Overall, our study finds that subsoil C is less energy-rich, less bioavailable and more microbially processed compared with surface C. Taken together, these findings support the notion that subsoil C, while present in very low concentrations, may be particularly stable, representing a long term C-sink for highly-weathered tropical soils.