Susan J Grayston, Amy M Treonis, Philip J Murray, Lorna A Dawson, Jasmine Ross, Shona Pratt, Eileen J Reid & Ruth MacDougall

The increasing emphasis on low input extensive agriculture means that achieving sustainable plant growth will depend on devising strategies to maximise the use of soil nutrient resources. In such systems, which include upland grasslands, the soil microbial community plays an important role in nutrient acquisition for plants. The availability of carbon is the limiting factor to microbial growth in soil and the quality and quantity of carbon present, coupled with N and P availability determine whether microbial biomass immobilises or mineralises N and P. Therefore, it is probable that changes in carbon and nutrient flow, due to root herbivory by insect larvae, will have a major impact on microbial communities and subsequently nutrient cycling. However, despite the importance of microbial processes in these systems, few studies have linked them to root herbivory. One aim of the NERC Soil Biodiversity Programme is to increase our understanding of biological diversity and functioning in soil. As part of this programme the impact of root herbivory by insect larvae on plant biomass and soil microbial communities is being investigated.

In this study perennial ryegrass (Lolium perenne) and clover (Trifolium repens) were grown singly and as mixtures in pots containing soil from the NERC field site at the Macaulays' Sourhope Research Station, located in the Borders of Scotland. Leatherjacket larvae (Tipula paludosa), a dominant insect root herbivore at this site, were added at field density to half the pots and the impacts of their feeding on plant shoot and root biomass and soil microbial communities was determined after 10 days. Larval herbivory had a significant negative effect on shoot growth of both grass and clover and root biomass of grass. In mixed swards larvae preferentially fed on clover. Soil microbial community structure was altered in the presence of larvae with populations of pseudomonads being significantly increased. The implications of these findings for the understanding of the effect of root herbivory on microbial communities and the consequences for nutrient cycling and vegetation dynamics are discussed.