COS 264-5 - CANCELLED - Molecular Composition of Deep Soil Carbon Across Fertility and Precipitation Gradients in Lowland Tropical Forests

Background/Question/Methods

Tropical soils are among the most carbon(C)-rich on Earth, with large stocks of relatively stable C residing in deep subsoils. To explore soil C characteristics and C stock sizes, we assessed a set of distinct lowland tropical forests across soil fertility and rainfall gradients in Panama.Eleven sites were chosen for this project. Samples were analyzed at depth increments of 0-10 cm and 25-50 cm to assess changes in the abundance and molecular composition of soil organic C. Samples underwent sequential acid demineralization first with 10% HCl to remove carbonates and then with a mixture of 10 weight % HCl and 10 weight % HF to remove paramagnetic metals. Soil C was then characterized by 13C Nuclear Magnetic Resonance spectroscopy. Each spectrum was phase-corrected, given a linear baseline, signal regions were integrated to determine the relative abundances of carbon functional groups, and then integrated peak areas were then input into the Molecular Mixing Model described by Baldock et al. (2004). This mixing model estimates the relative abundance of biomolecules present in the sample.

Results/Conclusions

As expected, all soils showed a stark decrease in C content from 0-10cm to 25-50cm and an increase in carbonyl C, indicating increased oxidation of the soil C with depth. O-Alkyl and aromatic regions had the largest peak areas throughout all samples which, the mixing model attributes to carbohydrates and lignin. Mixing model results indicate that carbohydrates are the most abundant organic C in 0-10 cm samples across all sites as well as many of the deeper 25-50 cm samples with high rainfall. More work is still needed to fully characterize soil organic C in these environments. Future work will aim to expand NMR analysis to more sites and depths in addition to performing soil sorption experiments to better quantify dissolved organic carbon concentrations and understand how C is transported and stored in tropical soils.