Controlling Greenhouse Gas Efflux from Managed Wetlands through Iron Cycling.

Efflux of greenhouse gases from agricultural lands contribute appreciably to global climate, and thus managing such systems represents a means for carbon storage.  Managed wetlands, particularly rice paddies, are well suited for altering greenhouse gas production—dominantly carbon dioxide and methane.  Inundated rice paddies restrict oxygen diffusion into soil, thereby linking the metabolic degradation of root exudates and incorporated residual rice straw to slower metabolic (anaerobic) microbial respiration processes.  Anaerobic conditions and limited concentrations of NO₃^- and SO₄^2- results in two dominant terminal electron accepting processes responsible for C oxidation in rice paddies: dissimilatory Fe(III) reduction and methanogenesis.  Iron(III) has the unique characteristic among the electron acceptors that its reduced form can be rapidly oxidized back to Fe(III).  Because methanogenesis is thermodynamically less favorable than Fe(III) reduction, it was our hypothesis that alternating wetting/drying cycles and adding Fe(III) (hydr)oxides to rice paddies would reroute electron flow from methane production.  As a consequence, maintaining iron as the dominant electron acceptor should increase carbon storage (relative to aerobic conditions) while limiting methane production.   Here we conducted a greenhouse microcosm experiment to test this hypothesis, where we adjusted water inundation periods, iron content, and rice growth.  While there was no significant difference in CH₄ emission among the treatments by week 4, late season methane flux was significantly lower in the treatments that were drained and flooded during week 5.  Carbon dioxide flux was also minimized by maintaining flooded soil conditions, while a short, high flux of CO₂ occurred at week 5 for the set of microcosms that were drained and rewetted.  Our results suggest that maintaining Fe(III) as the dominant respiration pathway  can be a viable option for decreasing both CH₄ and CO₂ flux from rice paddies, and consequently enhance C storage in soils.