Characterizing soil ‘road networks’ improves understanding of stream acidification

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First documented following the Industrial Revolution, acid rain has been an important driver of contemporary degradation of land and water ecosystems worldwide. Acid rain results from anthropogenic emissions of SO2 and NOx gases into the atmosphere. It thus made sense that concerted international efforts aimed at reducing atmospheric emissions of acid-forming gases have led to a marked decrease of acid rain events in both North America and Europe by the turn of the 21st century. Management and policy interventions to reduce emissions of acid-forming gases in Asia, however, have been taking place rather slowly. This is problematic because acid rain events could potentially destabilize freshwater ecosystems and undermine biodiversity. But acid rain is just one of several drivers of stream acidification. Many streams are naturally acidic because of local drivers. Chemical weathering of soils and production of acids from decaying vegetation are some examples. The flowpaths – ‘road networks’ under and near the soil surface – that water and all that is dissolved in it take before reaching the stream could also be an important driver. A study led by Sorain J. Ramchunder and others from the National University of Singapore, Mae Jo University in Thailand, and Utrecht University, found that in a small Singaporean catchment: 1) Stream acidification in general is a natural phenomenon (owing to highly weathered and naturally low buffering potential of granite-derived soils and acidic rainfall) 2) Acid rain events because of anthropogenic emissions of sulphur and nitrogen species into the atmosphere could amplify stream acidification 3) Identifying the flowpaths is important for understanding the catchment processes that lead to stream acidification This study underlines the importance of detailed, site-level studies for a nuanced understanding of stream acidification.

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