Influence of pH and temperature on microbial activity, substrate availability of soil-solution bacteria and leaching of dissolved organic carbon in a mor humus

We investigated how pH and temperature influenced fluxes of dissolved organic carbon (DOC), CO2 and bacterial activity in a laboratory experiment with upper (F) and lower (H) mor humus layers from a limed and an unlimed plot of Norway spruce (Picea abies (L.) Karst). Dolomite lime (8.8 t ha−1) was applied 12 years before the experiment. The microbial activity was manipulated by incubating the mor humus at +4 or +15°C. In a second experiment, the bacterial decomposition of DOC in leachate water from the entire O horizon of the same limed and unlimed plots was investigated. The cumulative amount of respired C from the F and H layers increased in the order unlimed+4°C<unlimed+15°C=limed+4°C<limed+15°C. The cumulative leaching of DOC from the F layer was in the order unlimed+4°C<unlimed+15°C<limed+4°C<limed+15°C. Both liming and temperature influenced the leaching of DOC. In the H layer the cumulative DOC leaching increased in the order unlimed+4°C<limed+4°C<unlimed+15°C=limed+15°C and temperature was the main factor controlling the DOC leaching. The bacterial activity in the limed leachate water (measured as incorporation of 3H labelled thymidine) was higher than in the unlimed leachate water from both F and H layers. This increase in bacterial activity in the limed mor humus was proportionally higher than the corresponding increase in CO2 respiration, which indicated that the bacteria in the soil water were more stimulated by liming than bacteria attached to soil particles and fungi. Chemical fractionation indicated that DOC from the limed treatment was more decomposed than DOC from the unlimed treatment, as the percentage of hydrophilic acids compared to hydrophobic acids was higher in the limed treatment. However, this difference seemed to be of less importance for the substrate availability to the bacteria in the water phase. Instead the bacterial community in the limed mor humus seemed to be more adapted to high pH and had a higher growth rate than the bacterial community in the unlimed humus.