Chlorination of soil organic matter: The role of humus type and land use

The levels of natural organic chlorine (Cl org ) typically exceed levels of chloride in most soils and is therefore clearly of high importance for continental chlorine cycling. The high spatial variability raises questions on soil organic matter (SOM) chlorination rates among topsoils with different types of organic matter. We measured Cl org formation rates along depth profiles in six French temperate soils with similar Cl deposition using 36 Cl tracer experiments. Three forest sites with different humus types and soils from grassland and arable land were studied. The highest specific chlorination rates (fraction of chlorine pool transformed to Cl org per time unit) among the forest soils were found in the humus layers. Comparing the forest sites, specific chlorination was highest in mull-type humus, characterized by high microbial activity and fast degradation of the organic matter. Considering non-humus soil layers, grassland and forest soils had similar specific chlorination rates in the uppermost layer (0–10 cm below humus layer). Below this depth the specific chlorination rate decreased slightly in forests, and drastically in the grassland soil. The agricultural soil exhibited the lowest specific chlorination rates, similar along the depth profile. Across all sites, specific chlorination rates were correlated with soil moisture and in combination with the patterns on organic matter types, the results suggest an extensive Cl cycling where humus types and soil moisture provided best conditions for microbial activity. Cl org accumulation and theoretical residence times were not clearly linked to chlorination rates. This indicates intensive Cl cycling between organic and inorganic forms in forest humus layers, regulated by humic matter reactivity and soil moisture, while long-term Cl org accumulation seems more linked with overall deep soil organic carbon stabilization. Thus, humus types and factors affecting soil carbon storage, including vegetation land use, could be used as indicators of potential Cl org formation and accumulation in soils.