Bioavailability and isotopic composition of CO2 released from incubated soil organic matter fractions

Faculty/Professorship: Faculty of the Humanities ; Digital Geoarchaelogy  
Author(s): Müller, Carsten W.; Gutsch, Martin; Kothieringer, Katja  ; Leifeld, Jens; Rethemeyer, Janet; Brüggemann, Nicolas; Kögel-Knabner, Ingrid
Title of the Journal: Soil Biology and Biochemistry
ISSN: 0038-0717
Publisher Information: Amsterdam [u.a.] : Elsevier Science
Year of publication: 2014
Volume: 69
Issue: February
Pages: 168-178
Language(s): English
DOI: 10.1016/j.soilbio.2013.11.006
The stabilization of soil organic matter (SOM) is triggered by three main mechanisms: (i) low bioavailability
due to aggregation, (ii) recalcitrance due to the chemical structure, and (iii) association of the SOM
with mineral surfaces. In the present study we used particle size SOM fractions (sand, silt and clay),
derived from the Ah soil horizon from a Norway spruce forest in Southern Germany, to study the effects
of different stabilization mechanisms on the bioavailability of soil organic carbon (SOC) in a one year
incubation experiment. The respired CO2 was hourly recorded, additionally 13CO2 was analysed 20 times
and 14CO2 three times during the incubation experiment. To better differentiate between particulate OM
(POM) and mineral associated OM (MIN), the incubated fractions and bulk soil were separated according
to density (1.8 g cm 3) after the incubation experiment. 13C-CPMAS NMR spectroscopy was used to study
the chemical composition of the incubated samples. We demonstrate a clear increase in SOM bioavailability
due to aggregate disruption, as the calculated theoretical CO2 evolution of the SOM fractions
recombined by calculation was 43.8% higher in relation to the intact bulk soil. The incubated sand
fraction, dominated by POM rich in O/N-alkyl C, showed a prolonged bioavailability of SOC moieties with
mean residence times (MRT) of 78 years. Interestingly, the silt fraction, dominated by highly aliphatic,
more recalcitrant POM, showed low mineralization rates and slow MRT’s (192 years) close to values for
the clay fraction (171 years), which contained a large amount of mineral-associated SOM. The recorded
13/12CO2 signatures showed a high depletion in 13C during the initial stage of the incubation, but an
enrichment of the respired 13CO2 of up to 3.4& relative to the incubated SOM was observed over longer
time periods (after 3 and 4 days for bulk soil and sand, respectively, and after 14 days for silt and clay).
Therefore, we found no evidence for a 13C enrichment of SOM as driven by metabolic isotopic fractionation
during microbial SOM mineralization, but an indication of a change in the isotopic composition
of the C-source over time.
Keywords: Particle size fractionation, 13CO2, 14CO2, Laboratory incubation
Type: Article
Year of publication: 16. May 2014