Molecular-scale characterization of hot-water-extractable organic matter in organic horizons of a forest soil

Hot-water-exuactable 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.