Leaching of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in mor humus as affected by temperature and pH

We investigated how pH and microbial activity influence fluxes and composition of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in a laboratory leaching experiment with mor humus from a limed plot and an unlimed plot of Scots pine in northern Sweden. CaCO3 was applied at a rate of 5 t ha−1 23 yr before sampling. The limed (pH 5.4) and unlimed (pH 4.3) mor humus was incubated at +4°C and +15°C. The biological activity measured as CO2 evolution increased in the following order: unlimed +4°C<limed +4°C<unlimed +15°C<limed +15°C. During the first 36 d, leaching of DOC and DON increased as follows: unlimed +4°C<unlimed +15°C<limed +4°C<limed +15°C, but later on DOC and DON leaching from the unlimed mor humus at +15°C increased markedly. The increase coincided with greater NH4+ leaching. A complementary leaching experiment indicated that Ca2+ ions have a moderating effect on DOC solubility and leaching. pH was more important than temperature for the leaching of DOC and DON at the beginning of the main experiment (d 1–d 36), but temperature was a more important factor after that. The leachates collected at the sixth leaching event were incubated for 24 h at +15°C with [3H]-labeled thymidine and those from the limed treatment showed a higher incorporation of labeled thymidine per amount of DOC than those from the unlimed treatment, which might indicate more accessible substrates for microorganisms. There also was a relatively higher content of hydrophobic acids in the leachates from the limed than from the unlimed treatment, irrespective of temperature.