Wardle and Nilsson 1997

Wardle DA, Nilsson M-C. 1997. Microbe-plant competition, allelopathy and arctic plants. Oecologia 109: 291-293.

These authors critique Michelsen et al. (1995), a study that came to several important conclusions regarding the interactions between Arctic plants and the soil microbial communities. This is a very negative review of that paper, in which these authors question almost all of the conclusions of Michelsen et al. (1995).

These authors make two main criticisms. First, they question the measures of soil microbial activity used by the earlier paper. Second, they question the conclusions regarding the allelopathy of Empetrum hermaphroditum. Soil microbial activity was measured by Michelsen et al. (1995) in two ways: soil respiration, and soil ergosterol content. Neither approach is necessarily informative about one of Michelsen et al.’s (1995) main claims, that soil microbial biomass was increased by the addition of plant leaf extracts. There are a number of studies, many of them with Nilsson as a co-author, in which a lack of association between microbial biomass and soil respiration was demonstrated. Furthermore, ergosterol is presented by Michelsen et al. (1995) as an indicator of fungal biomass, but previous work by Newell and colleagues (e.g. Newell and Fallon, 1991; Newell 1992) showed that ergosterol is not a reliable indicator of biomass nor is it useful as a proxy measure of soil fungal activity; the ratio of ergosterol to fungal biomass is highly variable.

From the reference list in this short paper, it appears that Wardle and Nilsson had, by early 1997, completed a considerable body of work regarding the allelopathic and other ecological interactions of E. hermaphroditum in sub-arctic environments. The conclusion by Michelsen et al. (1995) that the chemicals released by this plant have a greater impact on microbial communities than potential surface-plant competitors is not supported by this work by Wardle, Nilsson, and their colleagues.

The conclusion of Michelsen et al. (1995) that currently has the most direct bearing on my own work is that key plant traits often possessed by prostrate shrubs in tundra ecosystems such as a high root: shoot ratio and storage of nutrients such as nitrogen in the roots allow those plants to escape from or outcompete soil microorganisms. This conclusion was not addressed by these authors, but given the devastation inflicted upon the other conclusions, my confidence in the utility of Michelsen et al. (1995) in addressing issues of interactions between Cassiope tetragonal and soil microbial communities has been shaken.