mass-balance study on chloride fluxes in a large central European catchment during 1900–2010

Using data on long-term monitoring of water quality, mass budgets, and empirical models, we quantified chloride (Cl) leaching from major diffuse and point sources in a large central European catchment (upper Vltava river, Czech Republic) over a 110-year period (1900–2010), with the major aim to evaluate the influence of historical changes in land use and management practices on Cl leaching from agricultural land. The Cl input to farmland in synthetic fertilizers, livestock feed, and atmospheric deposition tripled in the 1950s–1980s (from 23 to 64� kg� ha⁻¹� year⁻¹ on average), and then abruptly decreased to ~14� kg� ha⁻¹� year⁻¹ during 1990–2010. The proportion of drained agricultural land rapidly increased from 4� % in the 1950s to its maximum of 43� % in the 1990s. Until the 1950s, the Cl leaching from agricultural land followed a simple dose–response function. Then, agricultural soils retained on average 16� ±� 4� kg� ha⁻¹� year⁻¹ of Cl during 1959–1985, when the most important changes in land use and management practices occurred, and subsequently became a net Cl source of 11� ±� 3� kg� ha⁻¹� year⁻¹ on average during 1986–2010, when Cl input to soils declined and drainage of new land ceased. Our data suggest that the temporal changes in the Cl storage in agricultural land are associated with changes in Cl concentrations in both permanent soil water and soils. Physico-chemical conditions in freshly drained soils, namely elevated aeration and high concentrations of soil organic matter (SOM), and high Cl inputs probably resulted in a Cl immobilization in soils by formation of organic chlorine (Clₒᵣg) and adsorption that was higher than the Cl production from Clₒᵣg mineralization and desorption. In contrast, Clₒᵣg mineralization and Cl desorption exceeded the Cl retention during the consecutive period of low Cl inputs and decreasing SOM concentrations in agricultural soils. Our study implies that changes in land use and agricultural management can significantly affect dose–response functions even for Cl, which is traditionally considered and modelled as a conservative ion.