Lysis of soil microbial cells by CO2 or N2 high pressurization compared with chloroform fumigation

The classical chloroform fumigation-incubation (CFI) and fumigation-extraction (CFE) methods are nowadays among the most used for determining soil microbial biomass, although the chloroform lysing of microbial cells is not always complete. Here, we have tested a physical method, used for sterilizing foods but never in soil, based on N-2 or CO2 high pressurization (N2HP or CO2HP, respectively) to cause microbial cell lysis. The N2HP and CO2HP were tested on two soils differing for their organic matter content, one agricultural (AGR) and one forest (FOR), and firstly were compared with the CFI. The CO2 extra-flush from both soils during 10-d incubation by N2HP was lower than that by CFI method, whereas that by CO2HP was greater. Then, the lysis by CO2HP was compared with that by the CFE method by varying CO2 pressure and duration. The CO2HP, at proper conditions, was more efficient than CFE method to cause the lysis of soil microbial cells. Moreover, both CO2 pressure value and duration were important in increasing the extractable organic C compared to the CFE. The most successful combination of high CO2 pressure and duration was 4.13 MPa and 32 h. However, we cannot exclude that CO2HP might have caused the release of soil organic C not ascribable to living organic matter. Further studies using C-13 and/or N-15-labeled microbial cells should assess the release of abiotic organic C.