Soil Organic Matter Characterization by 13C-NMR and Thermal Analysis in Deep Tropical Soil Profiles from the Luquillo Critical Zone Observatory

Abstract:

Tropical forest soils store large quantities of carbon (C) as soil organic matter (SOM), a substantial proportion of which is stored deep (> 30 cm) in the soil profile. Characterization of tropical SOM remains difficult, in part due to the analytical challenges associated high iron and low C concentrations. In this study, we combined solid-state 13C nuclear magnetic resonance (NMR) spectroscopy with analytical thermal analysis (differential scanning calorimetry, DSC; evolved CO2 gas analysis, CO2-EGA) to explore patterns in SOM composition in deep soil profiles from two contrasting soil types at the Luquillo Critical Zone Observatory (LCZO) in northeast Puerto Rico. Prior to 13C NMR, soils were repeatedly demineralized with hydrofluoric acid (HF) to remove paramagnetic compounds and concentrate organic matter. Given the scant information on tropical subsoil OM, we also sought to evaluate the effect of HF acid treatments on tropical subsoil SOM. HF treatments effectively enriched sample C and removed paramagnetic compounds, allowing us to obtain high-quality NMR spectra for low-C subsoils. C:N ratios before and after HF treatment were nearly identical (mean = 16.6 ± 0.8), suggesting that the SOM pool was not substantially fractionated, though C recoveries were low and variable. Thermal analyses confirmed the loss of a substantial fraction of the soil mineral matrix, however, retention of several endothermic regions in post-HF Inceptisol soils indicated that not all minerals were completely solubilized. In addition, important differences in the DSC and CO2-EGA thermograms were observed in comparing samples before versus after HF treatments. These results suggest that the organo-mineral associations were substantially altered, though it is not immediately clear the degree to which alterations in chemical composition versus binding association have changed. In addition to these qualitative changes, quantitative interpretations of 13C-NMR results from low-C and high-Fe soils should be based on spin-counting experiments in conjunction with estimates of signal relaxation parameters.