Evaluation of the importance of sediment organic matter composition for CH4 production by microcosm tests with and without addition of natural sources (cyanobacterial biomass and riparian pasture) in two subtropical, eutrophic reservoirs.

The biochemical composition of sediments, which depends on the origin of the organic matter (OM), is decisive in methane (CH4) production. This study aimed to determine the CH4 produced under anaerobic conditions from different substrates: native reservoir sediments and sediments with the addition of complex OM from Microcystis spp. blooms and terrestrial plants (pasture), alongside the biochemical characterization of the substrates used. The biochemical composition of the sediments explained the differences in CH4 production rate (µmol/g OM/day). Positive correlations were found between CH4 production rate and proteins (PRT) (r = 0.695), lipids (LIP) (r = 0.582), TN (r = 0.605), and biopolymeric carbon (BPC) (r = 0.784). Principal component analysis showed an association of CH4 production rate with PRT, LIP, TN, and BPC concentrations, sharing the same direction of the vectors. The addition of Microcystis bloom and pasture to the sediments significantly increased the rate and production of CH4 compared to native sediments, with higher values for bloom addition. In the studied reservoirs, cyanobacteria biomass is a very important source of organic matter to the sediments and a support for methanogenesis. Nonetheless, OM from surrounding vegetation, mainly pastures, could also play a significant role during events that increase reservoir levels, generating important CH4 emission "hot spots" at the periphery of the reservoirs.