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

Abstract Terrestrial inputs of dissolved organic matter ( DOM ) make a significant contribution to the carbon pool of headwaters and the reactivity of this pool depends on its source and diagenetic state, being influenced by photochemical and biological processes. The main goal of this study was to characterise the composition and reactivity of soil and leaf litter DOM from a native forest of Nothofagus pumilio (Nothofagaceae), and from natural stream water, evaluating the effect of degradation processes. Photo‐ and biodegradation laboratory experiments were conducted using DOM leached from soil and leaf litter, while the impact of photodegradation alone was also analysed through laboratory assays using stream water. The effects of photo‐ and biodegradation were evaluated through changes in the concentration of dissolved organic and inorganic carbon ( DOC and DIC , respectively) and optical DOM proxies (absorbance and fluorescence). In the initial characterisation, DOM from soil and water leachates showed naturally high humification, aromaticity and lignin content compared with the DOM of leaf litter leachate rich in non‐humic compounds. Photo‐ and biodegradation increased humification of the DOM . DOM from leaf litter leachate was more bioavailable than DOM from soil leachate, 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 characterised by the loss of labile protein‐like compounds and an increase in refractory humic‐like components. Long‐rod shaped bacteria were more abundant in leaf litter leachate, suggesting their preference for labile DOM , whereas cocci dominated in the humic and more biorecalcitrant DOM from soil leachate. This study showed a continuum of DOM humification, with decreasing DOM reactivity from leaf litter leachate towards soil leachate and stream water. Soil leachate DOM was probably the main source of stream water DOM , as reflected by their similar signatures and close positioning in the reactivity continuum, although carbon mineralisation was much lower in soil leachate than stream water.