Macrofauna and roots reduce methane production and attenuate nutrient recycling in organic-rich fluvial sediments

Organic-rich freshwater sediments display millimetric oxygen and nitrate penetration and are sources of methane to the water column and to the atmosphere via diffusion and ebullition. Radial oxygen loss by submersed aquatic plants and burrow irrigation with O₂ and NO₃^- enriched water by macrofauna can significantly alter the subsurface sediment volume where respiration processes alternative to methanogenesis occur. We tested this hypothesis in perifluvial organic sediments colonized by the submerged phanerogam Vallisneria spiralis and the oligochaete Sparganophilus tamesis. Gas ebullition and diffusive fluxes were measured in microcosms maintained under controlled laboratory conditions over a period of two weeks. Four conditions were reproduced: sediments alone, sediment with oligochaetes, sediment with plants and sediment with plants and oligochaetes. Microcosms with sediments alone released the largest methane volume whereas sediments with plants and macrofauna released the lowest amount. The presence of the oligochaete had comparatively a stronger effect than that of the macrophyte. Simultaneously, the bioturbation activity of the oligochaete enhanced the production of N₂ and the consumption of oxygen and nitrate, suggesting increased rates of aerobic respiration and of denitrification. The presence of plants attenuated net N₂ losses from the benthic system likely due to the competition between assimilative and dissimilative N-related processes.