WHEN LESS IS MORE: PRIMING OF SOIL ORGANIC CARBON BY PYROGENIC ORGANIC MATTER

Additions of pyrogenic organic matter (PyOM) to soil have been shown to both increase and decrease mineralization of native soil organic carbon (nSOC). This change in mineralization rate is referred to as priming, and may have important implications for carbon (C) turnover in soil. This study identifies several positive and negative priming mechanisms using high-resolution Carbon Dioxide (CO2) measurements of a series of short-term incubation experiments with 13C-labeled PyOM added to a temperate forest topsoil. Irrespective of pyrolysis temperature (200-750°C), addition of more than 2 mg PyOM g-1 soil significantly decreased mineralization of nSOC. Over 35 days, dilution was found to be a relevant process accounting for 20% of negative priming observed at day 7, and 13% at day 35. In comparison, substrate switching accounted for only 1.2% of negative priming at day 7 and 1.1% at day 35. Inhibition did not explain reductions in nSOC mineralization since the microbial biomass did not decrease in comparison to an unamended control. Sorption of dissolved organic carbon (DOC) from soil was responsible for the majority of negative priming observed with PyOM additions based on adsorption isotherm experiments as well as co-location of nSOC on PyOM surfaces shown by NanoSIMS. Maximum sorption potential of soil DOC was 29 times higher for PyOM pyrolyzed at 450°C than for the topsoil, and again tripled with an increase in pyrolysis temperature to 750°C. The contribution of the mechanisms mentioned above changed with time: dilution decreased after 7 days and substrate switching which initially made up less than 1% of negative priming in higher temperature PyOM could not be detected making sorption the dominant negative priming mechanism on monthly time scales. These results have long-term SOC storage implications, as sorption has longer lasting effects than substrate switching or dilution. Understanding the time-scale of sorption and potential desorption will require further long-term studies