Enviromental dynamics of chlorine in water and soil - transport patterns and transformation rates

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Since the 1940s, it has been widely accepted to assume that chloride is conservative in soil and chloride is commonly used as a conservative tracer in budget calculations of other substances within watersheds. However, the past decade of research suggests that chloride participates in a complex biogeochemical cycle. There is strong evidence that a widespread chlorination (consumption of chloride) in soil as a result of microbial activity whereas it is still unknown whether degradation of chlorinated organic matter (release of chloride) is mainly biotic or abiotic. However, it is clear that soil can act both as a sink and a source of chloride, depending on whether the mineralization or formation/volatilization/leaching processes dominate.The project is outlined in three interrelated parts conducted as combined field and laboratory studies with focus on fundamental research questions related to the impact of land-use and environmental variables on the chlorine cycle.The first aim is to quantify major components of the chlorine cycle at a watershed scale. The second aim is to evaluate the ecological role of chlorination by testing three hypotheses: i) reactive chlorine is used to inhibit competing microbes, ii) chlorination is related to microbial defense against O2 stress and iii) chlorination increases the microbial availability of soil organic matter and enhances soil respiration rates.The third aim is to investigate the fate and kinetics of Cl-in in soil by using 36-Cl to test four hypotheses: i) a considerable portion of Cl-in in soil is transformed to Cl-org under aerobic conditions, ii) Cl-in is slowly released from organic matter under anaerobic conditions due to degradation of chlorinated organic matter, iii) prevailing geochemical models of Cl-in behavior are unreliable due to that they do not take transformation of Cl-in to Cl-org in soil into account and, iv) the rate and extent to which radioactive chlorine enters the biosphere is underestimated due to that formation/degradation of Cl-org in soil is not included in the models.