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Comparison of lability of soil-derived dissolved organic matter from hillslopes of contrasting forestry history in a Pacific coastal forested watershed Wang, Mengxi


Dissolved organic matter (DOM) is a key aspect of source water quality in forested headwaters and a subject of concern in drinking water treatment due to its potential for adverse chemical reactions. Forest soils contribute a substantial fraction of DOM entering freshwater systems and can be altered by land use practices such as forest harvest, with implications for water quality downstream. This study investigated the relationship between forest clear-cutting and the biodegradability of soil-derived DOM. The potential for water-extractable soil DOM to be metabolized was measured via in-laboratory incubation and quantified via two methods: loss of dissolved organic carbon (DOC) and changes in fluorescence and absorbance properties. Through comparison of clear-cut stands of differing histories with nearby forested hillslopes, a chronology of the extent of DOM alteration with time elapsed since harvest was established. Biodegradable DOC did not differ significantly between forested and clear-cut sites of any age, though the variability was greater for clear-cuts (forested: 9 ± 1% mean ± standard error; clear-cut: 8 ± 2%). DOM character was different in the most recent clear-cut (harvested in 2017), with a greater fraction of high molecular weight, plant-derived compounds resistant to biodegradation. Incubation did not affect forest and clear-cut DOM differently except for the 2017 sites, where the clear-cut sample showed reduced microbial metabolism and a greater degree of humification. These results may be attributable to the prevalence of woody debris at the more recent clear-cuts, a remnant of the logging process. Overall, DOM character pre- and post-incubation were similar, suggesting soil DOM is relatively refractory and may persist in aquatic systems. The higher fraction of terrestrial, refractory DOM, if sustained downstream, would increase coagulant demand during drinking water treatment, but observable differences in water quality are not projected to last beyond 10 years after harvest in regions similar to the study site.

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