Do soil bacterial communities respond differently to abrupt or gradual additions of copper?

Many experiments that measure the response of microbial communities to heavy metals increase metal concentrations abruptly in the soil. However, it is unclear whether abrupt additions mimic the gradual and often long-term accumulation of these metals in the environment where microbial populations may adapt. In a greenhouse experiment that lasted 26 months, we tested whether bacterial communities and soil respiration differed between soils that received an abrupt or a gradual addition of copper or no copper at all. Bacterial richness and other diversity indices were consistently lower in the abrupt treatment compared to the ambient treatment that received no copper. The abrupt addition of copper yielded different initial bacterial communities than the gradual addition; however, these communities appeared to converge once copper concentrations were approximately equal. Soil respiration in the abrupt treatment was initially suppressed but recovered after four months. Afterwards, respiration in both the gradual and abrupt treatments wavered between being below or equal to the ambient treatment. Overall, our study indicates that gradual and abrupt additions of copper can yield similar bacterial communities and respiration, but these responses may drastically vary until copper concentrations are equal.