Quantifying the Effects of Soil Water Content On Elemental Mercury Formation in Non-Sterilized and Sterilized Boreal Soils.

Soils are a source of Hg(0) to the atmosphere however the effects of soil moisture on both biotic and abiotic Hg(0) formation is not well defined. This research quantifies the effect of varying soil moisture (15-80% water filled pore space (WFPS)) and sterilization on the kinetics of Hg(0) formation in soils. Both the cumulative mass of Hg(0) formed in soils (Log cumulative Hg(0) formed = 0.9 ln(WFPS) + 0.6; r² = 0.99; n= 10) and the reduction rate constants (k values) (k = 0.1 ln(WFPS) + 0.3; r² = 0. 6; n= 10) increases log-linearly with increasing WFPS. On average of 90% ± 4% (n = 5) less cumulative Hg(0) formation was measured in sterilized soils compared to non-sterilized soils, highlighting the importance of microbes in mercury reduction process. The mean percentage of total Hg reduced was larger (6 ± 1%) for non-sterilized soils as compared to sterilized soils (0.4 ± 0.1%). Our results highlight two processes contributing to Hg(0) formation in soil: (i) a fast abiotic process that peaks at 45% WFPS which depletes a relatively small pool of Hg(0) and; (ii) a much slower, rate limiting biotic process that generates a large pool of reducible Hg(II).