Abstract
Abstract. 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, composted yard waste, and wood mulch were evaluated at three different levels. Using 1 L glass microcosms, we measured the production of biogenic gases over 60 d in two soils designated by texture: 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, and CH4 production was more pronounced in SL. We observed biogenic gas production in two sequential steady-state phases: Phase 1 produced some CH4 but was mostly carbon dioxide (CO2), followed by Phase 2, 2 to 6 weeks later, with higher total gas and nearly equal amounts of CH4 and CO2. If this is generally true in soils, it may be appropriate to report CH4 emissions in the context of inundation duration. The CH4 from the SCL soil ranged from 0.003–0.8 cm3kg-1d-1 in Phase 1 to 0.75–28 cm3kg-1d-1 in Phase 2 and from SL range from 0.03–16 cm3kg-1d-1 in Phase 1 to 1.8–64 cm3kg-1d-1 in Phase 2. Adding fresh organic matter (e.g., hay) increased concentrations of ferrous iron (Fe2+), whereas in some cases composted organic matter decreased both Fe2+ concentrations and CH4 production. Methanogenesis normally increases following the depletion of reducible Fe; however, we observed instances where this was not the case, suggesting other biogeochemical mechanisms contributed to the shift in gas production.
Highlights
The ecological benefits of wetlands are well documented, including their role as carbon (C) sinks to stabilize global climate (Mitsch et al, 2015)
In Experiment 1, we evaluated Fe and CH4 production by varying organic matter (OM) type and dose and soil type (SL versus sandy clay loam (SCL))
Some CH4 was produced almost immediately upon inundation (Table 2a), but after the breakpoint (40 d in both the sandy loam (SL) and SCL soils), there is a large increase in CH4 as well as an average 4.7 × ± 1.9 increase in total gas production (Table 2b)
Summary
The ecological benefits of wetlands are well documented, including their role as carbon (C) sinks to stabilize global climate (Mitsch et al, 2015). Created or restored wetlands may effectively sequester C, it may take hundreds of years to offset their radiative forcing due to methane (CH4) emissions (Neubauer, 2014) With such a large number of human-made wetlands and their potential to increase global warming, it is vital to consider factors that may contribute to CH4 emissions. Organic amendments such as straw, wood mulch, manure, and biosolids, mixed into the soil, are thought to accelerate C storage by enhancing the conversion of plant-derived compounds to microbial residues (Richardson et al, 2016). Largely aliphatic C from cell membrane lipids, can accumulate in soil and are not directly accessible by methanogens (Chen et al, 2018) Plants contribute both above- and belowground organic matter (OM).
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