Geogenic CO2 affects stabilization of soil organic matter

Abstract

SummaryThe soil on mofette sites is affected by ascending geogenic carbon dioxide (CO2), which partially fills the soil atmosphere. We hypothesized that geogenic CO2 affects the stabilization of soil organic matter (SOM) at lower partial pressures than had been discussed previously for mofette sites. We studied loamy Ah horizons (n = 22; pH 3.4–4) of the soil along a transect on a grassland mofette site in the northwest Czech Republic with CO2 partial pressures (p(CO2)) of up to 0.52. The samples were fractionated by particle size, density and solubility (water‐soluble organic matter (WSOM)), and analysed quantitatively for organic carbon (C) and total nitrogen (N) and qualitatively (13C‐NMR spectroscopy). Soil OM with a narrower C:N ratio accumulated in the clay fraction, but at p(CO2) less than approximately 0.1 the proportion of SOM in the clay fraction relative to total SOM tended to decrease with increasing p(CO2), whereas that of particulate organic matter (POM) fractions increased with increasing p(CO2). We attribute the distribution of SOM among the mineral soil and POM to decreased interactions with minerals of the clay fraction. The formation of iron (Fe) hydroxides, which potentially sorb SOM, was not affected negatively by CO2. The potential reactivity of Fe hydroxides was even positively affected by increased p(CO2). Export of dissolved SOM into the subsoil might increase at mofette sites because of the large amounts of WSOM and decreasing interactions with minerals of the clay fraction. Therefore, our results show negative effects of CO2 on SOM stabilization even at moderate p(CO2).