Michelsen et al. 1999

Michelsen, A., Graglia, E., Schmidt, I.K., Jonasson, S., Sleep, D., and Quarmby, C. 1999. Differential responses of grass and a dwarf shrub to long-term changes in soil microbial biomass C, N and P following factorial addition of NPK fertilizer, fungicide and labile carbon to a heath. New Phytologist 143(3): 523-538.

These authors measured the responses of two tundra plants, one graminoid and one shrub, to additions of fertilizer, labile carbon, and a fungicide over four years in a heath in subarctic Sweden. The underlying hypothesis is that differences in growth strategy between these two groups of plants would be reflected in their responses to changes in microbial biomass, also measured by these authors.

Molecules inaccessible to plants by virtue of being incorporated into microbial cells in soils dominate their respective pools. For example, in this study, microbial N concentrations were up to 50 times higher than free, inorganic N such as ammonium. Similar ratios were found for C and P. Rapid incorporation of these nutrients into microbes therefore constitutes competition between microbes and plants. Consistent with the variety of ecological and growth strategies that plants have evolved interacting with each other and with the abiotic features, plant strategies impact their interactions with microbes and how they react to such competition.

As predicted, the graminoid Festuca ovina responded rapidly to changes in nutrient levels. Added N from fertilizers led to increased ground cover by this grass, while added labile C, in the form of sugar, led to decreased ground cover, presumably due to a rapid increase in microbial biomass and associated increased microbial uptake of soil N and P. In contrast, the shrub Vaccinium uliginosum did not appreciably react to changes in nutrient levels, consistent with a long-view growth strategy and successful escape from severe competition with soil microbes. This may have been mediated by the mycorrhizal associations the shrub has but the graminoid lacks.

This paper, interestingly, does not cite any of the work by Wardle or Nilsson, the authors who severely criticized the earlier work by this research group (Michelsen et al. 1995).