Thomas Idris Marquand

We present results from two sets of soil mesocosm experiments designed to explore the impact of changes in water table depth on methane (CH ₄ ) and carbon dioxide (CO ₂ ) emissions. In all experiments, soils were initially drained, as they are in situ. The first set of experiments (“short-term experiments”) investigates how CO ₂ and CH ₄ emissions respond to transient changes in water table depth; the second set of experiments (“long-term experiments”) investigates the response of CO ₂ and CH ₄ emissions sustained high water table. In the short-term experiments, the water table was raised then gradually lowered over a month. In the long-term experiments, the water table was raised and maintained for several months. Our short-term experiments show that CO ₂ emissions respond rapidly to changes in water table depth (with lower CO ₂ emissions when the water table is raised and vice versa). Conversely, short-term changes in water table depth do not significantly impact CH ₄ emissions. In the long-term experiments, CO ₂ emission was consistently supressed by the high water table. Soil methane production begins shortly after the water table is raised, but methane emissions from the soil remain low until the onset of ebullition (bubble escape). Raising the water table led to a 30-fold reduction in total CO ₂ -equivalent emissions which subsequently increased 6-fold after CH ₄ ebullition started. Our key findings are that (1) the response time of CO ₂ emissions to raising the water table is much shorter (4 emissions (~2-3 months), and (2) the delayed increase in CH ₄ emissions is driven by a late onset of ebullition which represents a key transition point from low to high CH ₄ emissions. We suggest that periodically raising the water table could reduce greenhouse gas emissions from drained soil without triggering high CH ₄ emissions, allowing conventional arable agriculture during the drained period.

Sulfur isotope fractionation during microbial sulfate reduction is often preserved in the mineral pyrite (FeS ₂ ), which has been used to reconstruct the biogeochemical sulfur cycle and redox geochemistry of the oceans over the Earth history. Understanding what controls the preserved sulfur isotopic composition of pyrite is therefore of paramount importance, but it has been difficult to deconvolve the influence of environmental changes from changes in sedimentation rate. We present a 16‐month record of pore fluid geochemical profiles with in situ sampling apparatus installed in coastal sediments, one of which is dominated by microbial sulfate reduction and the other dominated by bacterial iron reduction. Our data include monthly sulfate (SO ₄ ²⁻ ) and chloride concentrations (Cl ⁻ ), dissolved iron concentrations (Fe ²⁺ ), and the sulfur isotopic composition of dissolved sulfate (δ ³⁴ S _SO4 ) up to 36 cm below the sediment‐water interface. We use a reactive transport model to determine the expressed sulfur isotopic fractionation factor for each month and a Monte Carlo simulation to calculate net sulfate flux into the sediment based on pore fluid profiles from the sulfidic sediment. Net rates of sulfate reduction in the sulfidic sediment vary by three orders of magnitude over the seasonal cycle and are positively correlated with air temperature. The expressed sulfur isotope fractionation factor varies between 20 and 70‰ and reaches the thermodynamic limit in the colder months. Our data suggest that the correlation between temperature and the subsurface microbial sulfur biogeochemical cycle should be considered when interpreting sulfur isotope ratios in pyrite over Earth history.