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

Organochlorine molecules (Clorg) are surprisingly abundant in soils and
frequently exceed chloride (Cl−) levels. Despite the widespread abundance of Clorg and
the common ability of microorganisms to produce Clorg, we lack fundamental knowledge
about how overall chlorine cycling is regulated in forested ecosystems. Here we present
data from a long-term reforestation experiment where native forest was cleared and
replaced with five different tree species. Our results show that the abundance and
residence times of Cl− and Clorg after 30 years were highly dependent on which tree
species were planted on the nearby plots. Average Cl− and Clorg content in soil humus
were higher, at experimental plots with coniferous trees than in those with deciduous
trees. Plots with Norway spruce had the highest net accumulation of Cl− and Clorg over
the experiment period, and showed a 10 and 4 times higher Cl− and Clorg storage (kg
ha−1) in the biomass, respectively, and 7 and 9 times higher storage of Cl− and Clorg in the
soil humus layer, compared to plots with oak. The results can explain why local soil
chlorine levels are frequently independent of atmospheric deposition, and provide
opportunities for improved modeling of chlorine distribution and cycling in terrestrial ecosystems.