Overlooked drivers of the greenhouse effect: The nutrient-methane nexus mediated by submerged macrophytes.

Submerged macrophytes remediation is a commonly used technique for improving water quality and restoring habitat in aquatic ecosystems. However, the drivers of success in the submerged macrophytes assembly process and their specific impacts on methane emissions are poorly understood. Thus, we conducted a mesocosm experiment to test the growth plasticity and carbon fixation of widespread submerged macrophytes (Vallisneria natans) under different nutrient conditions. A refined dynamic chamber method was utilized to concurrently collect and quantify methane emission fluxes arising from ebullition and diffusion processes. Significant correlations were found between methane flux and variations in the physiological activities of V. nantas by the fluorescence imaging system. Our results show that exceeding tolerance thresholds of ammonia in the water significantly interfered with the photosynthetic systems in submerged leaves and the radial oxygen loss in adventitious roots. The recovery process of V. natans accelerated the consumption of dissolved oxygen, leading to increase in the populations of methanogen (153.3 % increase of mcrA genes) and subsequently elevating CH4 emission fluxes (23.7 %) under high nutrient concentrations. Conversely, V. natans increased the available organic carbon under low nutrient conditions by radial oxygen loss, further increasing CH4 emission fluxes (94.7 %). Quantitative genetic and modeling analyses revealed that plant restoration processes drive ecological niche differentiation of methanogenic and methane oxidation microorganisms, affecting methane release fluxes within the restored area. The speciation process of V. natans is incapable of simultaneously meeting improved water purification and reduced methane emissions goals.

Dynamic chamber as a more reliable technique for measuring methane emissions from aquatic ecosystems.

Aquatic ecosystems are the largest natural source of atmospheric methane ("CH4") worldwide. However, the current estimation of CH4 emissions from aquatic ecosystems still has extensive uncertainty due to large spatiotemporal variations in CH4 emissions as well as significant uncertainty in measurement methods. In this study, we initially investigated CH4 fluxes from a simulated eutrophic water body by using static chamber method ("SC") during an incubation period of 36 days. Approximately 23 % of the total flux measurements were unsuccessful because they lacked a linear correlation between the accumulation of CH4 concentrations and enclosure time. CH4 fluxes could be achieved for most measurements. However, 5 min after enclosing, the initial CH4 concentrations measured in the chambers were too high (up to 507.4 ppm) to greatly suppress CH4 emissions from the diffusion process. Therefore, a dynamic chamber method ("DC") was developed to overcome the shortcomings of the SC. To achieve the DC, air samples must be continuously collected at the inlet and outlet of the dynamic chamber at fixed flow rates. In contrast to the SC, effective CH4 flux data could be obtained by the DC for each measurement at different frequencies. The DC measured the diel and daily variations in CH4 fluxes and the displayed CH4 emissions from the simulated water were highly irregular. The displayed emissions had variations up to more than two orders of magnitude. These results implied that the SC measured few intermittent fluxes that were difficult to represent the actual CH4 emissions from eutrophic water. The DC developed in this study considers the temporal variations in CH4 emissions from aquatic ecosystems. Thus, the DC is expected to be applicable in the field flux measurements of CH4 as well as other greenhouse gases to reduce emissions uncertainties.