Abstract
Methane (CH4) is one of the key greenhouse gases (GHGs) in the atmosphere with current concentration of 1859 ppb in 2017 due to climate change and anthropogenic activities. Rivers are of increasing concern due to sources of atmospheric CH4. However, knowledge and data limitations exist for field studies of subtropical agricultural river catchments, particularly in southern China. The headspace balance method and the diffusion model method were employed to assess spatiotemporal variations of CH4 diffusive fluxes from April 2015 to January 2016 in four order reaches (S1, S2, S3, and S4) of the Tuojia River, Hunan, China. Results indicated that both the dissolved concentrations and diffusive fluxes of CH4 showed obvious spatiotemporal variations. The observed mean concentration and diffusive flux of CH4 were 0.40 ± 0.02 μmol L−1 and 41.19 ± 2.50 µg m−2 h−1, respectively, showing the river to be a strong source of atmospheric CH4. The CH4 diffusive fluxes during the rice-growing seasons were significantly greater than the winter fallow season (an increase of 80.26%). The spatial distribution of CH4 diffusive fluxes increased gradually from (17.58 ± 1.42) to (55.56 ± 4.32) µg m−2 h−1 due to the organic and nutrient loading into the river waterbodies, with the maximum value at location S2 and the minimum value at location S1. Correlation analysis showed that the CH4 diffusive fluxes exhibited a positive relationship with the dissolved organic carbon (DOC), salinity, and water temperature (WT), while a negative correlation occurred between CH4 diffusive fluxes and the dissolved oxygen (DO) concentration, as well as the pH value. Our findings highlighted that a good understanding of exogenous nutrient loading in agricultural catchments will clarify the influence of human activities on river water quality and then constrain the global CH4 budget.
Highlights
Methane (CH4) is a powerful greenhouse gas (GHG), which has a 28 times greater global warming potential (GWP) than carbon dioxide (CO2) over a 100-year time period [1], accounting for approximately 20% of the radiative forcing added to the atmosphere [2]
Compared with previous studies by Koné et al (2010) [44], Teodoru et al (2014) [45], and Borges et al (2018) [34], CH4 concentrations in this study were significantly greater than the above results, which may mainly be due to the high nutrient loading (DOC and NH4+–N), sedimentation, and algae blooms impacted by agricultural fertilizer applications being able to stimulate methanogenesis in rivers draining into rice paddy catchments [8,16,47]
Similar to the results from Stanley et al (2016) [7] and Zhang et al (2016) [40] reported previously, our findings showed that CH4 diffusive fluxes were at their lowest values in reach S1, their highest values in reach S2, and the values decreased in a downstream direction in reaches S3 and S4
Summary
Methane (CH4) is a powerful greenhouse gas (GHG), which has a 28 times greater global warming potential (GWP) than carbon dioxide (CO2) over a 100-year time period [1], accounting for approximately 20% of the radiative forcing added to the atmosphere [2]. The current global concentration of atmospheric CH4 (~1859 ppb) reached 257% of the preindustrial level (~722 ppb) due to increased emissions from anthropogenic sources and, at present, is increasing by 7 ppb percent year [4]. Terrestrial freshwater has been well documented as a major source of atmospheric CH4. The global CH4 emission rate from global freshwater is estimated to be 26.8–103.3 Tg yr−1, accounting for a significant share of 50% of anthropogenic CH4 emissions to the atmosphere [5,6,7]. Recent studies have shown that CH4 emissions from freshwater are the most uncertain component of this current estimate of terrestrial freshwater systems, primarily due to the small amount of data, limited geographic distribution of measurements, and the great diversity of hydrology and climates across terrestrial freshwaters [3,8], in the riverine types, which vary in terms of different CH4 emissions under different river systems [9]. More comprehensive data from riverine areas are required to accurately represent, within global CH4 budgets, the continental CH4 fluxes associated with terrestrial rivers
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