Assessing the recycling of chlorine and its long-lived 36Cl isotope in terrestrial ecosystems through dynamic modeling

Abstract

It is unclear to what extent chlorine (Cl) and its long-lived isotope 36Cl are recycled in different terrestrial environments in response to time-variable inputs. A new version of a dynamic compartment model was developed to examine the transformation and transfer processes influencing the partitioning and persistence of both Cl and 36Cl in forest ecosystems. The model’s performance was evaluated by comparing simulations and field observations of scenarios of stable Cl atmospheric deposition and of global 36Cl fallout. The model reproduced Cl storage in soil reasonably well, despite wide heterogeneity in environmental conditions and atmospheric deposits. Sensitivity analysis confirmed that the natural production of organochlorine in soil plays a major role in Cl build-up and affects long-term Cl dynamics. The timeframe required for the soil organochlorine pool to reach equilibrium in a steady-state system was several thousands of years. Interestingly, root uptake flux, a predominant pathway of the inorganic cycle, was found to affect both inorganic and organic pools in soil, highlighting the importance of plant–soil interactions in Cl dynamics. Model outputs agreed well with local 36Cl measurements, and demonstrated that 90% of the 36Cl found in soil may have come from bomb-test fallout. The pattern of estimated 36Cl/Cl ratios showed that soil 36Cl was not in equilibrium with 36Cl levels in rain input in the post-bomb period. Complete recovery of a natural isotopic ratio in drainage water will need a time close to the residence time of organic 36Cl in soil: i.e., 800 years. A simple dynamic model concept was found to be suitable to illustrate the plant-soil interactions combining both the inorganic and organic Cl cycles acting over different time scales.