The solubility of carbon inputs affects the priming of soil organic matter

We investigated the extent water soluble and insoluble (hexane soluble) plant residue fractions in influence the priming of soil organic carbon (SOC). Carbon-13-labeled water-soluble and insoluble plant materials were added to an organic or mineral soil at three moisture levels and incubated for 57 days. Plant material decomposition and resulting priming effect (PE) was quantified. Water-soluble additions were not always mineralized at higher rates than insoluble material and their mineralization rates varied greatly across soil type and moisture content. Regardless of solubility, less than 33 % of the added carbon was mineralized to CO2 by the end of the experiment, but a positive PE was observed in both soils. In general, water-soluble material caused a greater PE than insoluble organics, though the difference was not always significant throughout the experiment. Both additions induced greater PE in mineral soil than in an organic soil on a soil organic carbon basis. Moreover, the PE of soluble substrates generally increased when soil moisture increased, which was not observed for the PE of insoluble substrates. Solubility alone is insufficient in protecting organic substrates against microbial decomposition, with soluble substrates causing only a short and pulsed response to priming of SOC, and may therefore have a less pronounced impact on soil processes and final C balance than insoluble substrates in the long-term.