NMR Spectroscopic Assessment of Recalcitrant Soil Organic Matter in Iowa Mollisols.

Both the concentrations and the stocks of soil organic carbon vary across the landscape. Do the amounts of recalcitrant components of SOM vary with landscape position?

We studied four soils in central Iowa, two developed in till and two developed in loess. There were well-drained and poorly drained soils. We collected surface-horizon samples and fractionated the organic matter from the entire soil samples as well as from the clay fractions. We treated the samples with four 10% HF treatments at 60°C to dissolve minerals.

We used solid-state nuclear magnetic resonance spectroscopy to assess the organic matter in these soils. The techniques included: ¹³C CP/TOSS (cross polarization / total suppression of spinning sidebands) (combined with 40-µs dipolar dephasing to generate subspectra of unprotonated carbons and mobile groups like CH₃) and quantitative ¹³C DP/MAS (direct polarization / magic-angle spinning), with and without 60-µs of recoupled dipolar dephasing. We used ¹³C chemical shift anisotropy to separate signals of anomeric carbons from those of aromatic carbons.

Quantitative solid-state NMR allowed us to conclude (1) that the HF treatment had little impact on the organic C functional groups in the samples and (2) that organic matter in the clay fraction of poorly drained soils had more aromatic C than that of the clay fraction of the well-drained soils. The non-protonated aromatic C was 70 – 80 % of the total aromatic C (35 – 43 % of total organic C).