Molecular Probing of DOM Indicates a Key Role of Spruce-Derived Lignin in the DOM and Metal Cycles of a Headwater Catchment: Can Spruce Forest Dieback Exacerbate Future Trends in the Browning of Central European Surface Waters?

Abstract

Peatlands of the Northern Hemisphere and Central European coniferous forests experience significant environmental change. The resultant browning of surface waters, that is, elevated concentrations of dissolved organic matter (DOM) and metals, is of interest in the context of the global C cycle, peatland and forest management, and water treatment. In an attempt to identify the causes of this process in the Harz Mountains (Central Germany), we studied the spatiotemporal variations in DOM molecular composition (thermally assisted hydrolysis and methylation combined with GC–MS) and metal concentrations in headwater stream samples. We found strong relationships between DOM and metals and seasonal variations in the DOM quality and tentatively DOM–metal binding mode: during summer base flow, DOM and metal concentrations are low, and all elements other than the alkali and alkaline earth metals (Ca, Mg, Sr, K, and Na) are positively correlated to DOM, whereas during spring and autumn (high discharge), only metals with strong affinity for DOM (Fe, As, Cu, Cr, Pb, and Ti), but not weakly binding ones (Al, Cd, La, Mn, Ni, Zn, and Zr), are correlated to DOM, indicative of selectivity in DOM–metal interactions. The products of polyphenols are the key ingredients of the DOM–metal complexes. We argue the importance of spruce lignin-derived vanillic acid moieties, which are involved in weak (all seasons) and strong, multidentate and/or colloidal, binding (spring and autumn) of metals. Considering the ongoing spruce forest dieback and climate change acceleration, it is tempting to conclude that spruce necromass and forest soils may release vast amounts of lignin-derived DOM and associated metals to headwater streams. This would have significant implications for forest soil C stocks and the management of connected drinking water reservoirs.

Role of Direct and Sensitized Photolysis in the Photomineralization of Dissolved Organic Matter and Model Chromophores to Carbon Dioxide

“…DOM is largely comprised of phenols, which are key contributors to the photochemical and photophysical characteristics of DOM. − Phenols are characterized by their hydroxyl group (−OH), which enhances their electron-donating capacity and ability to form • OH . Carboxyl groups (−COOH) are likely integrated into many of phenolic moieties within DOM and largely influence DOM’s solubility, especially in polyaromatic compounds …”

Discharge and Water Quality of the River Moselle from 1990 to 2020 as Related to Climatic Changes and De-Industrialization

“…Another topic will be the increased brownification of surface waters that has also been observed in recent years in Central Europe. As has been suggested by researchers [56], this phenomenon is due to the stronger release of dissolved organic compounds (DOC) and associated pollutants, e.g., heavy metals in forest regions that are prone to drought and calamities.…”

Inferring the Molecular Basis for Dissolved Organic Matter Photochemical and Optical Properties

“…Phenols, typically derived from vascular plant debris, are commonly found in DOM and are one of the most studied DOM constituents. − Phenolic compounds consist of one or more aromatic rings with one or more hydroxyl groups (−OH) and play an important role in mediating photochemical reactions (i.e., scavenging ROS) because of the strong antioxidant properties . Some phenolic compounds can be directly photodegraded; however, phenols containing electron-donating substituents may exhibit enhanced photostability .…”