Low amounts of root herbivory positively influences the rhizosphere microbial community of a temperate grassland soil.

Denton, Crystal S. and Bardgett, Richard D. and Cook, Roger and Hobbs, Phil J. (1999) Low amounts of root herbivory positively influences the rhizosphere microbial community of a temperate grassland soil. Soil Biology and Biochemistry, 31 (1). pp. 155-165. ISSN 0038-0717

Abstract

It has been suggested that an important factor relating the dynamics of microbial communities and nutrient flux in grassland soils is a change in nondetrital inputs, such as root exudates, associated with below-ground herbivory. Our objective was to quantify the effects of different amounts of root infestation, and hence herbivory, by clover cyst nematodes (Heterodera trifolii) on the biomass and structure of rhizosphere microbial communities of white clover (Trifolium repens) plants grown in a grassland soil of low nutrient status. An additional aim was to test the hypothesis that these biotic interactions are influenced by soil texture, in particular a prevalence of soil pores of less than 30 μm dia. We found that at day 89, low amounts of root herbivory by H. trifolii, below the damage threshold for white clover, resulted in significant increases in total microbial biomass, measured by phospholipid fatty acid analysis (PLFA) and the abundance of gram-positive and gram-negative specific PLFAs in the rhizosphere soil. At this time there were also significant increases in the abundance of fatty acids synthesized by fungi and actinomycetes in the rhizosphere of infested clover plants. However, an increase of root herbivory above the damage threshold for clover, had no, or a negative effect on soil microbial biomass and individual PLFAs. The positive effects of root herbivory on rhizosphere microorganisms at day 89 were thought to correspond with an infestation by a second generation of nematodes. The results suggest that changes in the biomass of microorganisms in the rhizosphere of lightly-infested plants were not accompanied by changes in the structure of the microbial community as the fungal-to-bacterial biomass ratio was unaffected by the herbivory treatment. Soil texture had no effect on the ability of clover cyst nematodes to infest clover roots. There was also no soil type x nematode interaction, suggesting that the effects of root herbivory on the rhizosphere microbial community were similar in both soils. However, microbial biomass and the abundance of several individual PLFAs were higher in the clay loam than the sandy loam soil. The significance of these results in terms of nutrient flows in sustainable grassland systems is discussed.