Increases in organic carbon and nitrogen concentrations in boreal forested catchments - Changes driven by climate and deposition

Brownification, caused by increasing dissolved organic carbon (DOC) concentrations is a threat to aquatic ecosystems over large areas in Europe. The increasing concentrations of DOC in northern boreal streams and lakes have attracted considerable attention with proposed important drivers such as climate, deposition and land-use, and complex interactions between them. Changes in total organic N (TON) concentrations have received less attention, even though carbon and nitrogen losses are highly related to each other. We used long-term (1990-2019) monitoring records of 12 small data-rich headwater forested catchments in a large gradient of climate and deposition. We found that total organic carbon (TOC) concentrations were significantly increasing in almost all study catchments. The mean air temperature and change in sulphate concentrations had a strong, significant correlation to TOC change-%. Both explained, alone, more than 65% of the change in TOC concentrations, and, together, up to 83% of the variation. Sulphur deposition has already decreased to low levels, our results indicate that its importance as a driver of TOC leaching has decreased but is still clearly detected, while the impact of climate warming as a driver of TOC leaching will be even more pronounced in the future. A positive correlation was found between drainage-% and increases in TON, suggesting also importance of land management. TON trends were tightly connected to changes in TOC, but not directly linked to decreasing S deposition. (C) 2021 The Authors. Published by Elsevier B.V.

Automated summary

Brownification, caused by increasing dissolved organic carbon (DOC) concentrations, poses a threat to aquatic ecosystems in Europe. Changes in total organic carbon (TOC) concentrations are significantly influenced by mean air temperature and sulphate concentrations, with climate warming expected to have a more pronounced impact on TOC leaching in the future. The increase in drainage percentage is positively correlated with increases in total organic nitrogen (TON), highlighting the importance of land management in the context of water quality.