Michelsen et al. 1995

Michelsen A, Schmidt IK, Jonasson S, Dighton J, Jones HE, Callaghan TV. Inhibition of growth, and effects on nutrient uptake of arctic graminoids by leaf extracts – allelopathy or resource competition between plants and microbes? Oecologia 103: 407-418.

These authors conducted an experiment to examine the potential allelopathic effects of plant leaf extracts among a few species of arctic tundra plants. Three arctic/subarctic plants suspected of releasing phytotoxic compounds upon their competitors including Cassiope tetragonal were harvested and their leaves and branches ground up to make leaf extracts. Three species of arctic graminoids (Carex bigelowii, Festuca vivipara, Luzula arcuata) were then treated with these extracts while growing in either sterilized or non-sterilized soil in greenhouses in southern Sweden. All of the graminoids, the soil they grew in, and two of the leaf extract-providing plants came from a montaine subarctic hillside in northern Sweden; the third leaf extract came from Betula pubescens ssp. tortuosa, a birch, were collected from individuals growing near the tree line at 450m altitude near Abisko Scientific Research Station, above the Arctic Circle in a subarctic ecosystem.

The experimental design was factorial, with two soil types (sterilized vs. non-sterilized) and four leaf-extract treatments including a control of distilled water. In addition to growth of the graminoids, measurements were made of the chemicals in the leaf extracts and soils, nutrient uptake by excised roots, soil ergosterol content, and soil respiration. Excised roots take up nutrients in a manner directly correlated to the nutrient-limitation status of the plant; more phosphorus-starved plants, for example, have roots that more rapidly take up phosphorus when offered. Soil ergosterol content is a measure of fungal biomass, while soil respiration was taken as a measure of total microbial activity.

Sterilized soil had higher extractable nutrients, probably as a result of the breakdown of microbial cells during autoclaving. Nitrate levels were negligible, both in soils and in leaf extracts; nitrate was a component of the dilute nutrient water used to maintain the plants while growing. Some mycorrhizae were found, but they covered less than 1% of the roots in non-sterilized soil, and had no impact on other measured parameters.

The highest growth of all three test graminoids was recorded in sterilized soil with no extract added (i.e. distilled water added instead of leaf extract solution). Plants growing under these conditions experienced no inhibition from the materials of other plants, and did not compete with soil microbes for nutrients, at least during the early stages of the experiment before microbes recolonized the sterilized soils. Recolonization was much faster by prokaryotes than by fungi, as measured by the contrast in soil respiration rates and soil ergosterol contents. Recolonization also varied between leaf extract treatments, with a negative correlation between microbial activity and plant growth; strongly growing plants were able to outcompete colonizing microbes, while poorly growing plants were further inhibited by colonizing and rapidly growing microbes.

All three leaf extracts significantly reduced growth of all three graminoid species. However, it is not clear that allelopathy alone was responsible for this effect. The results of this study indicate that competition between plants and microbes also played a major role. In particular, the components of the leaf extracts, especially labile carbon and (in the case of the Betula extract) phosphorus, appeared to stimulate the microbial community, increasing competitive pressure on the plants. Added nitrogen, for example, appears not to have much benefited the plants, as their roots were N-limited when grown in non-sterilized soil even though the leaf extracts included high concentrations of inorganic nitrogen.

The susceptibility of plants to this combined effect of allelopathy and microbial competition probably varies by species. Plant traits of particular importance are probably the root: shoot ratio, in which plants with more robust roots are less harmed, and the storage of nutrients such as nitrogen in plant roots, in which plants with a growth strategy favouring nutrient storage rather than immediate use are less harmed. Such traits appear to be widespread among the dominant plants of the Arctic tundra, including Cassiope tetragonal and probably other prostrate shrubs. Many of these plants may form associations with mycorrhizal fungi, which provide some protection against microbial competition.

This paper is directly relevant to the discussion section of my current-high-priority Pits & Probes manuscript. The patterns of soil respiration and microbial GHG activity under some of the lowland communities are consistent with successful competition against soil microbes by Cassiope tetragonal and possibly Salix arctica plant roots.

This paper was critiqued quite harshly by Wardle and Nilsson (1997).