Effect of the microbial diversity on the adaptation of the soil nitrifying community to zinc

Abstract

Background and aims After long term exposure the nitrification is not affected by increased trace metal concentrations in contrast to short term exposure. It is shown that this is related to the development of a tolerant community. In addition, we showed that the Zn adaptation rate (6-12 months) increases if the growth rate of the nitrifying community in soil is stimulated. Here, we assess the role of microbial diversity on the adaptation rate and extent. Methods Gamma-irradiated sterilized soil was inoculated (5 wt%) with 13 different soil samples with different amoA DGGE profiles. The soil inocula were, for 5 treatments, selected mixtures of 3-20 soil samples or, for 8 treatments, single soil samples, all non-metal contaminated soils. Soils were amended with 150 mg NH4+ kg-1 to stimulate the growth during the initial incubation period at 25 °C. When the nitrification was restored, soils were either or not spiked with ZnCl2 (1500 mg Zn kg-1) and leached 1 week later. Potential nitrification rate and the Zn tolerance were determined 4 months after spiking. The ammonium oxidizing community was determined by PCR-DGGE of the bacterial amoA gene. Results Two months after inoculation, the nitrification in the inoculated soils had recovered and no detectable nitrification was measured in the sterile control soil. The initial Zn tolerances (Zn EC50 values) prior to exposure to Zn were within a factor 6 among all treatments, illustrating that the initial tolerance is relatively similar among soils compared to differences previously found in tolerant communities. The DGGE analysis revealed a different amoA DGGE profile for the 13 treatments. Currently, soils have received 150 mg NH4+ /kg to stimulate growth and are incubated at 25 °C. Conclusions The experimental set-up was successful to produce one soil with 13 different treatments of soil microbial diversity but similar initial Zn tolerance. The effect of this diversity on adaptation to Zn will be reported at the conference.status: accepte