Towards Ecological Relevance — Progress and Pitfalls in the Path Towards an Understanding of Mycorrhizal Functions in Nature. In: van der Heijden MGA, Sanders IR, editors. Mycorrhizal Ecology. Berlin: Springer Berlin Heidelberg; 2003. pp. 3-29. [DOI: 10.1007/978-3-540-38364-2_1]

Plant community composition, not diversity, regulates soil respiration in grasslands

Soil respiration is responsible for recycling considerable quantities of carbon from terrestrial ecosystems to the atmosphere. There is a growing body of evidence that suggests that the richness of plants in a community can have significant impacts on ecosystem functioning, but the specific influences of plant species richness (SR), plant functional-type richness and plant community composition on soil respiration rates are unknown. Here we use 10-year-old model plant communities, comprising mature plants transplanted into natural non-sterile soil, to determine how the diversity and composition of plant communities influence soil respiration rates. Our analysis revealed that soil respiration was driven by plant community composition and that there was no significant effect of biodiversity at the three levels tested (SR, functional group and species per functional group). Above-ground plant biomass and root density were included in the analysis as covariates and found to have no effect on soil respiration. This finding is important, because it suggests that loss of particular species will have the greatest impact on soil respiration, rather than changes in biodiversity per se.

Tree species and mycorrhizal associations influence the magnitude of rhizosphere effects

Previous research on the effects of tree species on soil processes has focused primarily on the role of leaf litter inputs. We quantified the extent to which arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) tree species influence soil microbial activity and nutrient availability through rhizosphere effects. Rhizosphere soil, bulk soil, and fine roots were collected from 12 monospecifc plots (six AM and six ECM tree species) planted on a common soil at the Turkey Hill Plantations in Dryden, New York. Rhizosphere effects were estimated by the percentage difference between rhizosphere and bulk soil samples for several assays. Rhizosphere effects on soil microbes and their activities were significant for ECM species but in only a few cases for AM species. In AM tree species, microbial biomass, net N mineralization, and phosphatase enzyme activity in the rhizosphere were 10-12% greater than in bulk soil. In ECM tree species, rhizosphere effects for microbial biomass, C mineralization rates, net N mineralization, and phosphatase activity were 25-30% greater than bulk soil, and significantly greater than AM rhizosphere effects. The magnitude of rhizosphere effects was negatively correlated with the degree of mycorrhizal colonization in AM tree species (r = -0.83) and with fine root biomass (r = -0.88) in ECM tree species, suggesting that different factors influence rhizosphere effects in tree species forming different mycorrhizal associations. Rhizosphere effects on net N mineralization and phosphatase activity were also much greater in soils with pH < 4.3 for both AM and ECM tree species, suggesting that soil pH and its relation to nutrient availability may also influence the magnitude of rhizosphere effects. Our results support the idea that tree roots stimulate nutrient availability in the rhizosphere, and that systematic differences between AM and ECM may result in distinctive rhizosphere effects for C, N, and P cycling between AM and ECM tree species.

The effect of two arbuscular mycorrhizal inocula of contrasting richness and the same mycorrhizal potential on the growth and survival of wild plant species from La Gran Sabana, Venezuela

Diversity of arbuscular mycorrhizal fungi (AMF) is potentially important for the productivity and functioning of plant communities. However, from results published to date, it is unclear whether the increase in productivity and plant diversity obtained with increased AMF richness is a consequence of higher richness in itself or whether it is caused simply by an uncontrolled, higher mycorrhizal potential in the more diverse AMF inocula used. The aim of the present work was to evaluate the effect of AMF richness on the biomass and survival of several wild plant species from La Gran Sabana, Venezuela, comparing AMF inocula of varying richness but with similar mycorrhizal potential. We studied the effect of two inocula (containing two and five AMF species respectively, each with 600 propagules/100 g of soil) on growth and survival in a community composed of three woody species indigenous to La Gran Sabana. No differences were found in dry mass production or plant survival when the two inocula of contrasting richness were compared. Reasons for such results include the life history strategies of the AMF used in the present work.Key words: arbuscular mycorrhizal fungi, arbuscular mycorrhizal fungal diversity, microcosm, species richness, biodiversity, tropical species.