LINKING ABOVEGROUND AND BELOW-GROUND INTERACTIONS - HOW PLANT-RESPONSES TO FOLIAR HERBIVORY INFLUENCE SOIL ORGANISMS

Studies of the effects of above-ground herbivory on soil organisms and decomposer food webs, as well as the processes that they regulate, ha ve largely concentrated on the effects of non-living inputs into the s oil, such as dung, urine, body parts and litter. However, there is an increasing body of information which points to the importance of plant physiological responses to herbivory in regulating soil organisms and therefore, implicitly, key soil processes such as decomposition and n utrient mineralisation. In this review we identify the mechanisms by w hich foliar herbivory may indirectly affect the soil biota and associa ted below-ground processes through affecting plants, so as to better u nderstand the nature of interactions which exist between above-ground and below-ground biota. We consider two broad pathways by which above- ground foliar herbivory may affect soil biotic communities. The first of these occurs through herbivore effects on patterns of root exudatio n and carbon allocation. These effects manifest themselves either as s hort-term changes in plant C allocation and root exudation or as long- term changes in root biomass and morphology. Evidence suggests that th ese mechanisms positively influence the size and activity of the soil biotic community and may alter the supply of nutrients in the rhizosph ere for plant uptake and regrowth. The second of these involves herbiv ores influencing soil organisms through altering the quality of input of plant litter. Possible mechanisms by which this occurs are through herbivory enhancing nitrogen contents of root litter, through herbivor y affecting production of secondary metabolites and concentrations of nutrients in foliage and thus in leaf litter and through selective fol iar feeding causing shifts in plant community structure and thus the n ature of litter input to the soil. While the effects of herbivory on s oil organisms via plant responses maybe extremely important, the direc tions of these effects are often unpredictable because several mechani sms are often involved and because of the inherently complex nature of soil food-web interactions; this creates obvious difficulties in deve loping general principles about how herbivory affects soil food-webs. Finally, it is apparent that very little is understood on how response s of soil organisms to herbivory affect those ecosystem-level processe s regulated by the soil food-web (e.g. decomposition, nutrient mineral isation) and that such information is essential in developing a balanc ed understanding about how herbivory affects ecosystem function. (C) 1 998 Elsevier Science Ltd. All rights reserved.