The browning and re-browning of lakes: Divergent lake-water organic carbon trends linked to acid deposition and climate change

Dissolved organic carbon (DOC) concentrations and water colour are increasing in many inland waters across northern Europe and northeastern North America. This inland-water “browning” has profound physical, chemical and biological repercussions for aquatic ecosystems affecting water quality, biological community structures and aquatic productivity. Potential drivers of this “browning” trend are complex and include reductions in atmospheric acid deposition, changes in land use/cover, increased nitrogen deposition and climate change. However, because of the overlapping impacts of these stressors, their relative contributions to DOC dynamics remain unclear, and without appropriate long-term monitoring data, it has not been possible to determine whether the ongoing “browning” is unprecedented or simply a “re-browning” to pre-industrial DOC levels. Here, we demonstrate the long-term impacts of acid deposition and climate change on lake-water DOC concentrations in low and high acid-deposition areas using infrared spectroscopic techniques on ~200-year-long lake-sediment records from central Canada. We show that acid deposition suppressed naturally higher DOC concentrations during the 20th century, but that a “re-browning” of lakes is now occurring with emissions reductions in formerly high deposition areas. In contrast, in low deposition areas, climate change is forcing lakes towards new ecological states, as lake-water DOC concentrations now often exceed pre-industrial levels.

Research on Sensitivity of Soil Acidification in Hebei Plain, North China

Through soil samples collecting and analysis in Hebei plain, we studied acidity characteristics and acidification sensitivity in Hebei plain. Results showed that the main reasons for decline in soil pH value is the atmospheric acid deposition, the excessive application of nitrogen and phosphorus fertilizer, crop base ion absorption. The dissolve of calcium carbonate is the main process plain cushion soil acidification in Hebei plain, its content is the decisive factor of soil acidification sensitivity.

Impacts des dépôts atmosphériques acides sur l'eau des sols forestiers

Impacts of atmospheric acid deposition on soil solutions in forests After a massive input of acidifying components on the environment in the middle of the 20th century, atmospheric acid deposition has decreased as a result of sulphur emission reduction. The continuous acid input might affect the chemistry of soils and drainage waters and accelerate soil acidification. In the framework of the Swiss Long-Term Forest Ecosystem Research (LWF), we examined whether acid deposition has continued to decline in the last ten years in different forest ecosystems and how the chemistry of soil water reacted to the improvement in air quality. Acid deposition decreased significantly at only three out of the nine study sites. Sulphur deposition declined at all sites, but due to the relatively low sulphur load compared to nitrogen deposition, it did not contribute to decrease acid deposition. Chemistry of soil solution remained quite constant since the beginning of the measurements about ten years ago. We did not observe any acidification of soil solution in six out of eight sites. In contrast, changes in soil solution chemistry at two sites showed a rapid acidification. At three sites, the deeper soil layer released large amount of sulphate coupled with base cations, which likely contributed to deplete the soil in nutrients. The analysis of the base saturation in 1039 soil profiles across Switzerland shows a high risk of relatively fast acidification of soil solution in almost 20% of sites.

Comment on “Present-day expansion of American beech in northeastern hardwood forests: Does soil base status matter?”Appears in Can. J. For. Res. 39: 2273–2282 (2009).

In a recent rapid communication, Duchesne and Ouimet (2009. Can. J. For. Res. 39: 2273–2282) reported that the current expansion of American beech ( Fagus grandifolia Ehrh.) in Quebec is mainly caused by soil base cation depletion due to atmospheric acid deposition. They based their conclusions on an examination of the relationships between stem densities in the sapling and tree strata compared against canopy composition and the availability of base cations in 426 sample plots. Here in this comment, we raise some shortcomings with their study and provide a more prudent and complete perspective on the complex dynamics associated with fluctuations in American beech and sugar maple ( Acer saccharum Marsh.).