Toward more robust plant‐soil feedback research

Abstract

Understanding if and how plant-soil biota feedbacks (PSFs) shape plant communities has become a major research priority. In this paper, we draw on a recent, high-profile PSF study to illustrate that certain widely used experimental methods cannot reliably determine if PSFs occur. One problem involves gathering soil samples adjacent to multiple conditioning plants, mixing the samples and then growing phytometers in the mixtures to test for PSFs. This mixed soil approach does not establish that the conditioning plant being present caused the soil biota to be present, the first step of a PSF. Also, soil mixing approximates replacing raw data with averages prior to analysis, a move certain to generate falsely precise statistical estimates. False precision also results from sample sizes being artificially inflated when phytometers are misinterpreted as experimental units. Plant biomass ratios become another source of false precision when individual plant values contribute to multiple ratio observations. Any one of these common missteps can cause still living null hypotheses to be pronounced dead, and risks of this increase with numbers of missteps. If soil organisms truly structure plant communities, then null hypotheses indicating otherwise will not survive proper testing. We discuss conceptual, experimental and analytical refinements to facilitate accurate testing.

Soil biota composition and the performance of a noxious weed across its invaded range

“…The three southernmost sites experienced inhibition of plant growth in live soils sourced from both inside and outside invaded patches, although belowground biomass in the Newmarket treatment, a mid‐latitude site, exhibited neutral effects. However, other work at this site (Nunes and Kotanen ) found negative effects more consistent with other southern Ontario populations. The five northernmost sites experienced improved plant growth in live soils with the exception of plants grown in soils sourced from within invaded patches in Algonquin.…”

“…These interactions are difficult to predict, although further investigation into the taxa responsible for reduced C. arvense performance may be of value for targeted management techniques. For instance, the intentional movement of soil microbial taxa across the invaded range may disrupt plant–microbial community relationships in emerging invasion sites (Lankau and Keymer ). More research into the microbial taxa responsible may assist in identifying the mechanisms involved and the ecosystem consequences of such invasions.…”

“…We examined the homoscedasticity of residuals versus fitted values, as well as the normality of residuals to diagnose the fit of our models. We also performed PERMANOVA using the adonis function from the ‘vegan’ package in R to analyze soil bacterial community composition (Oksanen et al ). PERMANOVA is a non‐parametric method of multivariate analysis of variance, which partitions variation in distance matrices between communities among experimental factors.…”

Toward more robust plant–soil feedback research: Comment

“…), so we urge future PSF studies to justify the use of a given soil handling approach based on the research questions of interest and the study system at hand. We also warn that being coerced into a single type of experimental design, as advocated in the three recent critiques by R&R (Reinhart and Rinella , Rinella and Reinhart , ), will narrow our understanding of the role of PSF in shaping plant communities, and ultimately impede the development of this important field of research.…”

Erratum

“…In Rinella and Reinhart (), several formulas were inadvertently deleted from the final version of record. The full paragraph from the article with the formulas has been reprinted below and has been updated in the version of record, available at https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecy.2146…”