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FOREST, RANGE & WILDLAND SOILS

Molecular-Scale Characterization of Hot-Water-Extractable Organic Matter in Organic Horizons of a Forest Soil

^a Civil and Environmental Engineering, Syracuse Univ., Syracuse, NY 13244
^b Centre for Forestry and Horticultural Research, School of Science, Faculty of Environmental Sciences, Griffith Univ., Nathan, QLD 4111, Australia

^* Corresponding author (abalaria{at}syr.edu).

Hot-water-extractable organic matter (HWEOM) has been shown to be highly correlated with microbial biomass in forest soils. We conducted elemental and ¹³C solid-state nuclear magnetic resonance spectroscopic analyses to assess the composition and structural chemistry of HWEOM and its variations with soil depth within O horizons in a forest site in New Hampshire. The HWEOM fraction exhibited a higher H/C ratio and higher O-alkyl C proportion than the soil from which it was extracted. It also had a 30 to 40% lower C/N ratio than the whole soil. The relative proportion of O-alkyl C in the HWEOM increased with soil depth in the forest floor, while alkyl C decreased, contrary to the pattern observed for the whole soil. The spectral and elemental properties of HWEOM present in these acidic Spodosols support the hypothesis that HWEOM is largely a mixture of carbohydrates and proteins. We estimate that while HWEOM includes both labile C fractions and microbial biomass, microbial biomass can account for no more than 40% of the C extracted by hot water.

Abbreviations: CPMAS, cross polarization with magic angle spinning • DD, dipolar dephasing • DOC, dissolved organic carbon • HBEF, Hubbard Brook Experimental Forest • HWEOC, hot-water-extractable organic carbon • HWEOM, hot-water-extractable organic matter • NMR, nuclear magnetic resonance • SOM, soil organic matter