Large but variable methane production in anoxic freshwater sediment upon addition of allochthonous and autochthonous organic matter

An important question in the context of climate change is to understand how CH₄ production is regulated in anoxic sediments of lakes and reservoirs. The type of organic carbon (OC) present in lakes is a key factor controlling CH₄ production at anoxic conditions, but the studies investigating the methanogenic potential of the main OC types are fragmented. We incubated different types of allochthonous OC (alloOC; terrestrial plant leaves) and autochthonous OC (autoOC; phytoplankton and two aquatic plants species) in an anoxic sediment during 130 d. We tested if (1) the supply of fresh alloOC and autoOC to an anoxic refractory sediment would fuel CH₄ production and if (2) autoOC would decompose faster than alloOC. The addition of fresh OC greatly increased CH₄ production and the δ¹³C-CH₄ partitioning indicated that CH₄ originated exclusively from the fresh OC. The large CH₄ production in an anoxic sediment fueled by alloOC is a new finding which indicates that all systems with anoxic conditions and high sedimentation rates have the potential to be CH₄ emitters. The autoOC decomposed faster than alloOC, but the total CH₄ production was not higher for all autoOC types, one aquatic plant species having values as low as the terrestrial leaves, and the other one having values as high as phytoplankton. Our study is the first to report such variability, suggesting that the extent to which C fixed by aquatic plants is emitted as greenhouse gases or buried as OC in sediment could more generally differ between aquatic vegetation types.