L.A. Dawson, S.J. Grayston and E. Paterson

The soil microbiota in grassland relies to a large extent on the carbon from plant roots and litter for its maintenance and activity. Consequently, plant derived C influences microbially mediated nutrient cycling and the availability of nutrients for plant growth and also the balance of competition between co-existing species. The effect of above-ground herbivory on the timing, quantity and quality of below-ground inputs, and subsequent impacts on the microbial biomass and nutrient cycling remains unresolved. In addition, the ability of plants to cope with below-ground herbivory are even less well defined.

In general, grass root systems respond to above-ground herbivory by a reduction in biomass, diameter, branching, AM fungal colonisation, decomposition rate, a more superficial distribution and increased exudation. Nevertheless, responses to herbivory can be species and even genotype specific and are dependent on soil nutrition and the timing, intensity and frequency of defoliation.

The quantity and quality of organic compounds released from plant roots are also species specific, as are the effects of herbivory on their release. The increase in exudation following defoliation can result in a change in the microbial community structure from one more dominated by fungi when undefoliated to one predominantly of bacteria when defoliated. When roots of pasture species have been grazed by invertebrate larva, an increase in the number of pseudomonads in the soil was found. Linked to this was a greater utilisation of certain carbon compounds in the soils where the larva had been grazing. It is essential that more information on the interactions between plant specific carbon sources and herbivory, both above and below-ground, with plant-associated soil organisms be obtained. Further work on these interactions should help provide a mechanistic understanding of how grazing can affect ecosystem functioning.