Characterization and reactivity continuum of dissolved organic matter in forested headwater catchments of Andean Patagonia

GARCÍA, ROBERTO DANIEL; DIÉGUEZ, MARÍA DEL CARMEN; GEREA, MARINA; GARCÍA, PATRICIA; REISSIG, MARIANA

FRESHWATER BIOLOGY (PRINT)

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2018 vol. 63 p. 1049 - 1062

0046-5070

1. Terrestrial inputs of dissolved organic matter (DOM) make a significant contribution to the carbon (C) pool of headwaters. The reactivity of this pool depends on its source and diagenetic state and is influenced by both photochemical and biological processes. 2. The aim of this study was to characterize the reactivity and transformation of soil and leaf litter DOM from a native Nothofagus pumilio forest. Photo- and biodegradation laboratory experiments were conducted using DOM leached from soil (SL) and leaf litter (LL), while the impact of photodegradation alone was also analyzed through laboratory assays using stream water (SW). The effects of photo- and biodegradation were evaluated through changes in the concentration of dissolved organic and inorganic C (DOC and DIC, respectively) and optical DOM proxies (absorbance and fluorescence). 3. In the initial characterization, SL and SW showed naturally high humification, aromaticity and lignin content compared to LL, which was rich in non-humic compounds. Four fluorescent DOM components were validated through PARAFAC modeling: three humic (C1, C2 and C4) and one non-humic component (C3). C4 was recorded exclusively in LL. 4. Photo- and biodegradation increased humification in the DOM leachates and stream water, with a stronger effect on LL, and led overall to loss of reactivity. LL was more bioavailable than SL, as reflected by the higher growth of bacteria, DOC consumption and DIC production. In general, biodegradation increased DOM molecular weight, aromaticity and lignin content. Changes in fluorescent DOM (FDOM) showed a trend characterized by the loss of labile protein-like compounds and an increase in refractory humic-like components. Microbial communities growing in the different leachates showed some particularities; short-rod and long-rod shaped bacteria were more abundant in LL, suggesting their preference for labile DOM, whereas cocci dominated in the humic and more biorecalcitrant SL. 5. On the whole, this study showed a continuum of DOM humification/reactivity, with increasing DOM humification and degree of C mineralization (DOC:DIC ratio); overall decreasing reactivity from LL towards SL and SW was observed. Soil DOM probably makes the main contribution to stream DOM, which is reflected in their similar signatures and close positioning in the reactivity continuum, although C mineralization was much lower in SL than SW.