《Table 3–Rate of degradation and half-life of dissolved organic matter (DOM) .Degradation and aromat

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《Effect of photo-biodegradation and biodegradation on the biogeochemical cycling of dissolved organic matter across diverse surface water bodies》

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A larger reduction in both a254 and SUVA254，and an associated greater increase in SR values was observed during photobiodegradation relative to biodegradation（Figs.6 and 7）.The difference in intensity of variation in a254，SUVA254，and SRbetween the photo-bio and biodegradation experiments implies that in the complete absence of sunlight，bacterial action alone cannot completely disintegrate larger reactive terrestrial humiclike or fulvic-like DOM components into smaller labile microbial humic-like components.The higher decreasing trend of a254 and SUVA254and corresponding higher increase of SR values suggest greater production of low molecular weight，less aromatic DOM molecules during photo-biodegradation than biodegradation.The a254 and SUVA254values in this study indicate that the locations which receive more terrestrial humic input（B，E，R and L）are more reactive and sensitive to sunlight，hence higher reduction of aromaticity was observed in those water bodies than the agricultural pond.The high amount of less aromatic and low molecular weight microbial components in the pond was not able to undergo further photo-biodegradation.The higher increase in SR values during photo-biodegradation was observed for the water bodies which receive more terrestrial input（B 107.8%±1.5%；L 65.2%±4.5%；E 166.8%±1.7%and R 58.6%±26%）than the agricultural input（P 45%±1.8%）.Increases in SR values indicate a decrease in the mean molecular weight of the DOM compounds in the water sample for all locations.Similar results showing the gradual increase of SR values during photo-biodegradation experiments in similar settings（lake，estuarine，and wetland water samples）were reported previously（Helms et al.，2008；Zhang et al.，2013）.The SR results obtained from our photobiodegradation experiments also suggest that the locations receiving the higher amount of terrestrial humic input（R，B，L and E）are undergoing a more substantial reduction in molecular weight of DOM than the agricultural pond samples.While comparing the difference among the forest dominated watersheds，the estuary samples showed the maximum amount of change of absorption index（a254）during the photobiodegradation experiment（Fig.6）.The larger change in estuary samples could be due to the presence of higher amount of reactive terrestrial humic-like or fulvic-like DOM components derived from forest cover in the watershed compared to other regions（Table 1）.However，the changes in SR during biodegradation do not follow such a trend.Our data suggest that such low molecular weight DOM compounds present in the agricultural pond are very difficult to disintegrate by photo-biodegradation or by biodegradation and can remain in the water column for a longer period.Similar observations were reported from Lake Kasumigaura in Japan（Kawasaki et al.，2013）.