Scale and density dependent relationships among roots, mycorrhizal fungi and collembola in grassland and forest

Belowground foodwebs, which dominate energy flow in temperate systems, may yield insights into general foodweb function: do consumers drive resource levels or simply respond to them? Controversy exists about the relationships among fine roots, mycorrhizal fungi and invertebrate grazers because of the difficulty of studying belowground processes. We used contrasting temporal scales to examine relationships among roots, fungi and collembola in the top 90 cm of soil in native grassland and forest over three years, with data collected from minirhizotrons. Root production was generally similar between habitats, but fungal production and collembola density were mostly significantly greater in forest than grassland. All variables decreased significantly with increasing depth, except for forest mycorrhizal fungi. This exception suggests that forest mycorrhizae are especially important for nutrient uptake at greater soil depths. Relationships among roots, mycorrhizal fungi and collembola varied with the temporal scale considered. Over the entire growing season, collembola were significantly positively correlated with root production in forest, and with both fungal and root production in grassland, reflecting the broad positive associations between these groups. In contrast, over the shortest sampling interval (i.e. two weeks), strong negative relationships suggested that collembola grazing decreased fungal and root production. Data from all dates and depths considered together revealed that both fungal and root production were significantly higher at intermediate collembola density than at low or very high density. In summary, belowground consumers appear to drive resource levels at short time scales, but respond to broad variation in resource availability at annual time scales.