Characteristics of soil organic matter in a limnic histosol of the alpine morainic system

Histosols represent a large carbon reservoir and their preservation is important to limit the emission of CO2 and the related ecological consequences. In these soils, C storage is governed by waterlogging, bacterial activity, and type of vegetation from which soil organic matter (SOM) originated. In limnic peats, moreover, calcium could play some roles on SOM dynamics. The characteristics of organic fractions composing a Limnic Histosol subjected to intermittent periods of oxidation have been studied to understand the main factors influencing SOM dynamics and accumulation. Soil organic matter was constituted mainly by the unextractable fraction (88.0% of organic C). Dissolved organic matter was in low concentrations (1.30%), formed mainly by undecomposed labile material. Fulvic and humic acids (HAs) were highly humified and rich in O-containing groups, probably because of the periodic oxidizing conditions. Humic acids (concentration, 5.38%), further fractionated by dialysis in three fractions on a molecular weight basis, were constituted mainly by the heaviest fraction (greater than 50 kDa; concentration, 5.36%). Humin was composed of a hydrophobic fraction, derived probably from lipids and waxes, and of a hydrophilic fraction, with features closer to those of HA. Both fractions contained surprisingly large amounts of carboxyl and phenolic groups and trapped a large percentage of Ca, which is unextractable even after strong acid treatments. This suggests that humin was actually formed by smaller molecules that strongly interact with Ca through their polar groups, forming larger aggregates. The large percentage of unextractable organic material and the strong interaction with Ca could preserve SOM from microbial degradation and, hence, limit the mineralization even in the absence of water, preserving a C sink that is presently storing a large amount of C in both organic and carbonate forms.