Belowground effects of organic and conventional farming on aboveground plant–herbivore and plant–pathogen interactions

Soil organisms may significantly affect the aboveground system. However, the influence of farming practices in modifying the effects of soil organisms on aboveground systems is poorly understood. The aim of our study was to investigate: (1) How important are soil organisms for plant growth and the development of herbivores and pathogens above the ground? (2) How do agricultural management practices affect interactions between soil organisms, plants and their aboveground herbivores and pathogens? To answer these questions we investigated the effect of experimental defaunation of soils from organic versus conventional farms on growth of wheat, abundance of aphids and infection of wheat by Septoria fungi. Plant biomass in soil from conventional farms exceeded that of soils from organic farms, presumably due to the higher nutrient input in the conventional farming system. Soil defaunation likely mobilized nutrients that increased plant growth. Aphid abundance and Septoria infection was reduced by defaunation but only in organic soils. This suggests that soil organisms in organic farming systems are more important for aphid performance and the infection rate by Septoria than in conventional systems. Hence, changes in the soil animal food web caused by farming practice feed back on aboveground organisms, and this appears to be more pronounced in organic farming systems. Further, the results indicate that soil organisms may modify higher trophic levels (aphid and pathogen infection) without significantly affecting lower trophic levels (plant growth).