Abstract

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.

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