Cadmium-containing wood ash in a pine forest: effects on humus microflora and cadmium concentrations in mushrooms, berries, and needles

Abstract

The cadmium (Cd) concentration of wood ash (1–30 mg·kg–1) is higher than allowed for agriculture fertilizers (3 mg·kg–1). Therefore, the objectives of this field study were to test if the Cd of wood ash has the potential to affect the coniferous forest humus microflora and if Cd enters the human food chain. These objectives were tested with ash (A) and Cd-spiked ash (ACd, 400 mg Cd·kg–1) at a fertilization rate of 3 t·ha–1. Microbial community structure, respiration, needle litter decomposition, growth rates and Cd tolerance of bacteria, and the bioavailability of Cd were measured. Also, Cd concentrations of humus, soil percolation water, mushrooms, fruits and leaves of berries, and needles were determined. The amount of Cd in the percolation water or bioavailable Cd, measured with a bacterial biosensor, and Cd tolerance of bacteria did not increase, although the ACd treatment increased the amount of humus total and extractable Cd. Only the ACd and not the A treatment caused Cd concentration increment in Lactarius rufus and berries of Empetrum nigrum. In spite of the high Cd concentration of the spiked ash, it did not have harmful effects on humus microorganisms during this 4-year study. Thus, wood ash is safe to use as a fertilizer in forests.