Is chloride a conservative ion in forest ecosystems?
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Chloride (Cl−) has often been assumed to be relatively unreactive in forest ecosystems, and is frequently used as a conservative tracer to calculate fluxes of water and other ions. Recently, however, several studies have detailed cycling of Cl− in vegetation and soils. In this study Cl− budgets are compiled from 32 catchment studies to determine the extent to which Cl− is conserved in the passage through forest ecosystems. Chloride budgets from these sites vary from net retention (input > output) to net release (output > input). In the overall data set, including those sites with very high inputs of seasalt Cl−, there was a strong correspondence between inputs and outputs. However, sites with low Cl− deposition (<6 kg ha−1 year−1) consistently showed net release of Cl−, suggesting an internal source or a declining internal pool. The results indicate that Cl− may be a conservative ion in sites with high Cl− deposition, but in sites with low deposition Cl− may not be conservative. We discuss the possible causes of the Cl− imbalance and reasons why Cl− may not be conservative in ecosystem functions.
KeywordsCatchment Chloride Chlorine Mass balance Soil Tracer Watershed
We are grateful to the many scientists at the catchment and forest sites whose meticulous collection and publication of data made this study possible. Mary Beth Adams of the USDA Forest Service, Colin Neal of the Center for Ecology and Hydrology in the UK, Sirpa Kleemola at Finnish Environment Institute, Gunilla Pihl-Karlsson at Swedish Environmental Research Institute, Reet Talkop at Estonian Environment Information Centre, Milan Vana at Czech Hydrometeorological Institute generously provided unpublished data, and Steve Norton, Lindsey Rustad, and Jake Peters helped us with discussions and interpretations of their data. We thank Chris Evans and Brian Reynolds for reviews that greatly improved the manuscript. Svensson is grateful for financial support from The Swedish Foundation for International Cooperation in Research and Education (STINT). Lovett was partially supported by the US National Science Foundation through grant DEB-0342198 and DEB-0423259 and the Hubbard Brook LTER program. Long-term data provided by Likens for the Hubbard Brook Experimental forest were supported by the US National Science Foundation, including the LTER and LTREB programs, and The Andrew W. Mellon Foundation.
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