Soil–solution partitioning of <scp>DOC</scp> in acid organic soils: results from a <scp>UK</scp> field acidification and alkalization experiment

<jats:title>Summary</jats:title><jats:p>Dissolved organic carbon (<jats:styled-content style="fixed-case">DOC</jats:styled-content>) is an important component of the global carbon (<jats:styled-content style="fixed-case">C</jats:styled-content>) cycle and has profound impacts on water chemistry and metabolism in lakes and rivers. Reported increases of <jats:styled-content style="fixed-case">DOC</jats:styled-content> concentration in surface waters across <jats:styled-content style="fixed-case">E</jats:styled-content>urope and <jats:styled-content style="fixed-case">N</jats:styled-content>orthern <jats:styled-content style="fixed-case">A</jats:styled-content>merica have been attributed to several drivers, including changing climate, changing land‐use to eutrophication and declining acid deposition. The latter of these suggests that acidic deposition suppressed the solubility of <jats:styled-content style="fixed-case">DOC</jats:styled-content>, and that this historic suppression is now being reversed by reducing emissions of acidifying pollutants. We studied a set of four parallel acidification and alkalization experiments in organic matter‐rich soils, which, after three years of manipulation, have shown distinct soil solution <jats:styled-content style="fixed-case">DOC</jats:styled-content> responses to acidity change. We tested whether these <jats:styled-content style="fixed-case">DOC</jats:styled-content> concentration changes were related to changes in the acid/base properties of <jats:styled-content style="fixed-case">DOC</jats:styled-content>. Based on laboratory determination of <jats:styled-content style="fixed-case">DOC</jats:styled-content> site density (<jats:styled-content style="fixed-case">S.D</jats:styled-content>. = amount of carboxylic groups per milligram <jats:styled-content style="fixed-case">DOC</jats:styled-content>) and charge density (<jats:styled-content style="fixed-case">C.D</jats:styled-content>. = organic acid anion concentration per milligram <jats:styled-content style="fixed-case">DOC</jats:styled-content>) we found that the change in <jats:styled-content style="fixed-case">DOC</jats:styled-content> soil–solution partitioning was tightly related to the change in degree of dissociation (α = <jats:styled-content style="fixed-case">C.D.:S.D</jats:styled-content>. ratio) of organic acids (<jats:italic>R</jats:italic><jats:sup>2</jats:sup> = 0.74, <jats:italic>P</jats:italic> < 0.01). Carbon turnover in soil organic matter (<jats:styled-content style="fixed-case">SOM</jats:styled-content>), determined by soil respiration and <jats:styled-content style="fixed-case">β‐D</jats:styled-content>‐glucosidase enzyme activity measurements, also appears to have some impact on <jats:styled-content style="fixed-case">DOC</jats:styled-content> leaching, <jats:italic>via</jats:italic> constraints on the actual supply of available <jats:styled-content style="fixed-case">DOC</jats:styled-content> from <jats:styled-content style="fixed-case">SOM</jats:styled-content>; when the turnover rate of <jats:styled-content style="fixed-case">C</jats:styled-content> in <jats:styled-content style="fixed-case">SOM</jats:styled-content> is small, the effect of α on <jats:styled-content style="fixed-case">DOC</jats:styled-content> leaching is reduced. Thus, differences in the magnitude of <jats:styled-content style="fixed-case">DOC</jats:styled-content> changes seen across different environments might be explained by interactions between physicochemical restrictions of <jats:styled-content style="fixed-case">DOC</jats:styled-content> soil–solution partitioning and <jats:styled-content style="fixed-case">SOM</jats:styled-content> carbon turnover effects on <jats:styled-content style="fixed-case">DOC</jats:styled-content> supply.</jats:p>