Chlorination and dechlorination rates in a forest soil — A combined modelling and experimental approach

Much of the total pool of chlorine (Cl) in soil consists of naturally produced organic chlorine (Cl_org). The chlorination of bulk organic matter at substantial rates has been experimentally confirmed in various soil types. The subsequent fates of Cl_org are important for ecosystem Cl cycling and residence times. As most previous research into dechlorination in soils has examined either single substances or specific groups of compounds, we lack information about overall bulk dechlorination rates. Here we assessed bulk organic matter chlorination and dechlorination rates in coniferous forest soil based on a radiotracer experiment conducted under various environmental conditions (additional water, labile organic matter, and ammonium nitrate). Experiment results were used to develop a model to estimate specific chlorination (i.e., fraction of Cl⁻ transformed to Cl_org per time unit) and specific dechlorination (i.e., fraction of Cl_org transformed to Cl⁻ per time unit) rates. The results indicate that chlorination and dechlorination occurred simultaneously under all tested environmental conditions. Specific chlorination rates ranged from 0.0005 to 0.01 d⁻¹ and were hampered by nitrogen fertilization but were otherwise similar among the treatments. Specific dechlorination rates were 0.01–0.03 d⁻¹ and were similar among all treatments. This study finds that soil Cl_org levels result from a dynamic equilibrium between the chlorination and rapid dechlorination of some Cl_org compounds, while another Cl_org pool is dechlorinated more slowly. Altogether, this study demonstrates a highly active Cl cycling in soils. - Highlights: • Chlorination and dechlorination rates in soil were revealed by a radiotracer method. • Chlorination was hampered by nitrogen addition. • Both Cl⁻ and many Cl_org compounds are highly reactive in soils. • Some formed Cl_org seem to be refractory.