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

The stabilization of soil organic matter (SUM) is triggered by three main mechanisms: (i) low bioavailability due to aggregation, (ii) recalcitrance due to the chemical structure, and (iii) association of the SUM with mineral surfaces. In the present study we used particle size SUM 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 (CO2)-C-13 was analysed 20 times and (CO2)-C-14 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. C-13-CPMAS NMR spectroscopy was used to study the chemical composition of the incubated samples. We demonstrate a clear increase in SUM bioavailability due to aggregate disruption, as the calculated theoretical CO2 evolution of the SUM 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 (CO2)-C-13/12 signatures showed a high depletion in C-13 during the initial stage of the incubation, but an enrichment of the respired (CO2)-C-13 of up to 3.4 parts per thousand relative to the incubated SUM 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 C-13 enrichment of SUM as driven by metabolic isotopic fractionation during microbial SUM mineralization, but an indication of a change in the isotopic composition of the C-source over time. (C) 2013 Elsevier Ltd. All rights reserved.