Inoculum handling alters the strength and direction of plant-microbe interactions.

The pooling of soil samples in plant-microbe interaction studies is commonly employed, but the impact of sample handling has rarely been explored experimentally. Concerns have been raised that sample pooling may reduce biological variation leading to inflated type I errors or may alter the magnitude of microbial effects observed, invalidating the results achieved. To assess the impact of inocula pooling on plant-microbe interactions, we examined the reciprocal influence of unpooled and pooled soil microbial inocula on growth of Solidago altissima and Schizachyrium scoparium, with and without inoculum sterilization. Soil pooling had no effect on the variance among replicates in either plant species. However, pooling dramatically altered the magnitude and direction of microbial impacts on plant performance. Pooling of Solidago altissima soil increased the antagonistic effects on growth of both target species. In contrast, pooling of Schizachyrium scoparium soil shifted impacts on Solidago altissima from effectively neutral to slightly positive. Pooling in this system altered both the strength and direction of plant-microbe interactions relative to unpooled soils. Therefore soil mixing should be avoided when the research goal is to determine naturally occurring interaction strengths, even within a single habitat.