Organochlorine turnover in forest ecosystems: The missing link in the terrestrial chlorine cycle

Abstract

Research in the last 20 years has shown that chlorine undergoes transformations between inorganic and organic forms as part of a complex biogeochemical cycle in terrestrial systems. Natural organochlorine production appears to be associated with the decomposition of plant material on the soil surface, though the chlorine cycle budget implies that a proportion of natural organochlorine enters soil through plant litter and atmospheric deposition as well. Organochlorine compounds may form through biotic and abiotic pathways, but the rates and magnitude of production in the field remain undefined. We have performed a time‐dependent trace of chlorine concentration through forest ecosystems, revealing distinct fractions of naturally produced organochlorine in plant biomass. Aliphatic organochlorine constitutes an intrinsic component of healthy leaves that persists through senescence and humification of the plant material, making a substantial contribution to the pool of soil organochlorine. Plant leaves also contain soluble aromatic organochlorine compounds that leach from leaf litter during early decay stages. As decay progresses, high concentrations of insoluble aromatic organochlorine accrue in the humus, through de novo production as well as adsorption. The rates of aromatic organochlorine production and degradation vary seasonally and conversely. This study presents the first unambiguous evidence that there exist multiple pools of chlorinated organic matter in the soil environment and that leaf litter deposition makes a significant and refractory contribution to the soil organochlorine pool, providing key insights into the biogeochemical chlorine cycle.

Experimental Evidence of Large Changes in Terrestrial Chlorine Cycling Following Altered Tree Species Composition

“…22 The difference in chlorination rates between the field and laboratory (i.e., between long-term net and short-term gross rates) can be explained by the relatively rapid degradation of a large proportion of the Cl org , 11 and by the fact that mass balance calculations in field studies reflect the cumulative net increase of Cl − Cycling. This study also provides information regarding how to interpret throughfall and stemflow, and regarding uptake and crown leaching, and associated internal cycling of Cl in plants.…”

Natural Niche for Organohalide-Respiring Chloroflexi

“…In terrestrial environments, the transformation of chloride into organochlorine compounds occurs in part via the activity of the chloroperoxidase enzyme (38), resulting in organochlorine levels often exceeding those of chloride in surface soils (36,37). With soil depth, chlorine speciation changes from predominantly organic to inorganic, suggesting that natural organochlorines in soil organic matter may undergo biogeochemical dechlorination processes as well (29). The hypothesis that organohalide-respiring Chloroflexi may use natural organochlorines as electron acceptors in uncontaminated environments has been discussed in recent literature (2,7,19,23,24), though the association between organohalide-respiring Chloroflexi and natural organochlorine in natural terrestrial environments has not been investigated.…”

Novel Firmicutes Group Implicated in the Dechlorination of Two Chlorinated Xanthones, Analogues of Natural Organochlorines

“…T he biogeochemical importance of naturally occurring chlorinated compounds (commonly referred to as organochlorines or organochlorides) has been garnering attention, as these compounds are important components of the chlorine and carbon cycles (1,2,3,4,5). More than 4,400 natural organohalogens have been identified (6), and the production of these compounds is becoming well understood (1,2,7,8,9).…”