Soil pooling often, but not always, alters the impacts of plant-microbe interactions on plant growth

The study of plant-microbe interactions has become a foundational research focus of plant ecology. A recurring issue for this area is whether soil pooling introduces bias when quantifying the local soil microbial interactions experienced by plants.

To address this issue, we exposed nine herbaceous species to field-collected soil inocula that were either pooled or kept separate in a greenhouse study with autoclaved controls. We determined whether pooling altered the strength, direction, or variance of plant-microbe impacts on plant growth and whether pooling changed which species were most responsive to soil microbes.

Sample pooling, compared to individual samples, significantly altered the strength of plant-microbe interactions in four of nine herbaceous species, but the direction of the effect varied. Pooling reduced microbial inhibition of growth in two species, reversed the direction from negative to positive in another, and generated a positive interaction in the fourth. The patterns of microbial impacts on growth across species were strongly altered, leading to no correlation between results generated by pooled and unpooled soils. Contrary to expectations, variances were typically higher in pooled soils. These differences nearly disappeared in autoclaved soils, suggesting soil microbes as the driving factor.

While neither universal nor predictable, sample pooling to generate experimental inocula altered the strength, direction, and variance of microbial impacts on plant growth in several species. This effect occurred often enough that researchers must carefully weigh whether pooled samples are appropriate in addressing their research questions.