Adding organic matter to restore wetland soils may increase methane generation and is not needed for hydric soil development

Methane (CH4) emissions are a potent contributor to global warming and wetlands can be a significant CH4 source. In a microcosm study we evaluated how the practice of amending soils with organic matter as part of wetland restoration projects may affect CH4 production potential. Organic amendments including hay, manure, biosolids and compost were evaluated at three different levels. Using 1-liter glass microcosms, we measured the production of biogenic gases over 60 days in two soils, a sandy loam (SL) and a sandy clay loam (SCL). Fresh organic amendments increased CH4 production, leading to potentially higher global warming potential and wetland C loss, particularly in sandy soils. Organic amendments increased biogenic gas production in two sequential steady state phases: Phase 1 produced some CH4 but was mostly carbon dioxide (CO2) followed by Phase 2, two to six weeks later, with much higher total gas and nearly equal amounts of CH4 and CO2. The CH4 from the SCL soil ranged from 0.003–0.8 cc/Kg/day in Phase 1 to 0.75–28 in Phase 2 and the SL range from 0.03–16 cc/Kg/day in Phase 1 to 1.8–64 in Phase 2. We had set out to identify an organic amendment that would promote iron (Fe) reduction without excess CH4, but amendments were not needed to produce Fe and make soils hydric. Adding fresh organic matter (hay) resulted in both excess Fe2+ and CH4 whereas composted amendments had little effect. The potential for excess methanogenesis should be taken into account when considering organic matter amendments in mitigation wetlands.