Geogenic CO₂ affects stabilization of soil organic matter

Abstract

The soil on mofette sites is affected by ascending geogenic carbon dioxide (CO 2 ), which partially fills the soil atmosphere. We hypothesized that geogenic CO 2 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 CO 2 partial pressures (p(CO 2 )) 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 ( 13 C-NMR spectroscopy). Soil OM with a narrower C:N ratio accumulated in the clay fraction, but at p(CO 2 ) less than approximately 0.1 the proportion of SOM in the clay fraction relative to total SOM tended to decrease with increasing p(CO 2 ), whereas that of particulate organic matter (POM) fractions increased with increasing p(CO 2 ). 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 CO 2 . The potential reactivity of Fe hydroxides was even positively affected by increased p(CO 2 ). 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 CO 2 on SOM stabilization even at moderate p(CO 2 ).

Hypoxic and acidic — Soils on mofette fields

“…Beaubien et al (2008) detected an increase of the contents of pedogenic Fe and Mn (hydr)oxides with increasing distance from a mofette, thus increasing availability of O 2 to form oxides of Fe(III) and Mn(IV), which is consistent with the negative correlation between CO 2 concentrations and the amounts of dithioniteextractable Fe , small amounts of dithionite-extractable Al and Si (Stephens and Hering, 2002) and small amounts of oxalate-extractable Mn (Rennert and Pfanz, 2015) in the presence of CO 2 . Thus, the effect of geogenic CO 2 on the contents of pedogenic oxides is expressed by diminished availability of O 2 .…”

“…Under the influence of elevated geogenic CO 2 , the Rennert et al (2011). proportion of microbial C and N of total C and N was lower than in a soil on a reference site (Ross et al, 2001). The amounts of K 2 SO 4 -extractable SOC were larger in soils on a mofette field than those extracted from soil lacking geogenic CO 2 (Rennert and Pfanz, 2015), indicating a rather mobile fraction of SOM. As bioturbation is inhibited on mofette fields, transport via the soil solution may be an important pathway of SOM accumulation in the subsoil.…”

“…Thus, the effect of geogenic CO 2 on the contents of pedogenic oxides is expressed by diminished availability of O 2 . Further, CO 2 has not only a quantitative effect on pedogenic oxides, but also a qualitative one, as ferric (hydr)oxides once formed tend to remain poorly crystalline and/or small, as deduced from a positive correlation between the ratio of oxalate-to dithionite-extractable Fe with the CO 2 partial pressure (Rennert and Pfanz, 2015). Similarly, Casanova et al (1999) detected two-line ferrihydrite rich in Al, Si and Ca precipitated in the vicinity of a CO 2 spring in the Massif Central, France.…”

“…The study also showed an increase of the infrared absorption bands typical of lignin with increasing [CO 2 ]. Rennert and Pfanz (2015) detected that the fraction of total SOM in the clay fraction decreased with increasing [CO 2 ] and that of POM increased in soils along a CO 2 gradient. This effect was pronounced at [CO 2 ] b10%.…”

“…Knowledge on processes and properties gained there might be transferrable to smaller-scale processes in soils with impeded gas exchange, e.g., in aggregates of compacted soils. This is the more likely, as stabilization of SOM is already affected by CO 2 at [CO 2 ] N 10-20% (Rennert and Pfanz, 2015), which is in a range of concentrations that may prevail in soils even in the absence of geogenic CO 2 (Geisler, 1973). However, these smallscale effects of increased [CO 2 ] on soil properties and thus functions have not been studied yet.…”