Steam treatment of surface soil: how does it affect water-soluble organic matter, C mineralization, and bacterial community composition?

Steam soil disinfestation is efficiently used in the field for pre-planting pest control. Providing steam to the soil must have consequences, either beneficial or detrimental for the soil functioning. We set up a laboratory experiment to quantify the soil quality dynamics induced by this agricultural practice. In steamed and control soil, we monitored the size, the activity, and the genetic structure of the bacterial community in the top 2 cm soil every second day over a 10-day period after the treatment. We also characterized the bioavailable organic matter using fluorescence and ATR-FTIR spectroscopy. We showed that steaming induced the release of twice as much dissolved organic carbon in steamed soil as in the control soil. This extra carbon was much less fluorescent and contained a higher proportion of aliphatic compounds (alkyl chains, primary alcohols). After an initial drop in the bacterial community, we observed a tenfold increase in the number of bacteria, a flush in carbon mineralization, and genetic structure modification, which could be related to the newly released carbon. Steam treatment showed strong but quickly reversible impacts on the soil functioning, enabling farmers to sow approximately 1 week after treatment.