Forest Humus Type Governs Heavy Metal Accumulation in Specific Organic Matter Fractions

Soils have the potential to accumulate heavy metals and the capacity to do so is strongly related the properties of each soil. Soil organic matter is a key factor in the retention, release, and bioavailability of heavy metals, and here we have determined the accumulation of heavy metals in various types of humus in the Rybnik Forest District in southern Poland. In a novel approach, we analyzed relationships between heavy metals within soil organic matter fractions and evaluated the role of organic fractions in mediating metal mobility. Specifically, we tested whether (i) the type of forest humus determines the heavy metal accumulation; (ii) heavy metals accumulation is associated with soil organic matter fractions; and (iii) heavy metals have an inhibitory influence on biochemical properties especially enzymes activity in different humus types. Four types of humus were sampled (mor, moder, moder-mull, mull), physically fractioned, and a number of chemical and biochemical properties were analyzed. Calculated geo-accumulation index (I-geo) and enrichment factor (EF) confirmed soil pollution with Cd and Pb. The type of humus differed in the accumulation of heavy metals, which is associated to the variable concentration of organic matter remaining at each decay class. We found no relationship between enzymatic activity and heavy metals concentration except for a positive correlation between urease activity and nickel concentration. Considering wider evidence, we propose a biogeochemical link between nickel deposition and the production of soil-borne urease in these forest soils.