Bromine accumulation in acidic black colluvial soils

Abstract

Recent investigations showed that bromine is incorporated to soil organic matter (SOM), its content increasing with humification. But few research was done on its long-term accumulation and the role played by pedogenetic processes, as those involved in organic matter stabilization. We investigated bromine content and distribution in four deep, acidic, organic-rich, Holocene soils from an oceanic area of Western Europe. Bromine concentrations (93–778 lgg􀀂1) in the silt + clay (<50 lm) fraction were on average 3-times higher than those (17–250 lgg􀀂1) in the fine earth (<2 mm), the former containing almost all bromine (90 ± 5%). Inventories were between 148 and 314 g m􀀂2, indicating a rather large variability in a small area, and total estimated retention was low (6–16%). The degree of SOM bromination, expressed as the Br/C molar ratio, varied between 0.03 and 1.20 mmol Br/mol C. The ratio was highly correlated (n = 23, r2 0.88, p < 0.01) with the age of the SOM for the last 􀀁12 ka. Partial least squares modeling indicates that bromine concentration depends on the amount of organic matter stabi-lized as aluminium-OM associations, and to a lesser extent on soil acidity (pH) and iron-OM associations. Thus, at scales of thousands of years, bromine accumulation in acidic soils is linked to the pool of metal–clay-stabilized organic matter.