Does simulated acid rain increase the leaching of cadmium from wood ash to toxic levels to coniferous forest humus microbes?

Wood ash contains Cd in concentrations not permitted for fertilization use in agriculture (>3 mg kg −1 ). It has been shown that spiking ash with Cd to concentrations of 1000 mg kg −1 induced no further changes in humus microbial activity and community structure as ash alone. To accelerate the weathering process and thus to liberate the spiked Cd from the ash, three treatments – wood ash (A), Cd spiked wood ash (ACd, 1000 mg Cd kg −1 ash), both applied at a fertilization rate of 5000 kg ha −1 , together with a control (C) – were performed in microcosms and incubated in field condition under two types of irrigation – water and simulated acid rain. During the incubation period of one growing season the simulated acid rain plots received a sulfur load of 3.64 g S m −2 , which was 15 times more than the S deposition on the water irrigated plots. The treatments resulted in a mean Cd increase of the humus from 0.23 mg kg −1 of the C treatment to 0.52 and 39.5 mg kg −1 of the A and ACd treatments, respectively. The irrigation had no further effect on the result. The microbial activity, measured as soil basal respiration, and the microbial community structure, measured as humus phospholipid fatty acid and 16S and 18S polymerase chain reaction/denaturing gradient gel electrophoresis patterns, changed only due to the ash (A and ACd treatments) fertilization irrespective of the irrigation. The bacterial biosensor, emitting light in the presence of bioavailable Cd, did not react to any of the treatments. This result shows that Cd in ash was not leached into the humus due to increased deposition of acidified rain.