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Effects of Plant Function Group Removal on the soil Microbial Community Diversity and Composition

Marshall, C. B. 2008. M.Sc. Thesis. University of British Columbia, Vancouver, BC

Abstract

A major objective of biodiversity-ecosystem functioning (BDEF) research is to determine the consequences of species loss, caused both naturally and anthropogenically, on the functioning of ecosystems. The impact of plant species loss on the soil microbial community has not received much attention even though soil microbes influence many important ecosystem functions such as decomposition and nutrient cycling. The objective of this research was to investigate how the functional group composition of the aboveground plant community influenced the belowground microbial community. Plant functional groups (graminoids, legumes and non-leguminous forbs) were removed from a northern grassland system in the Yukon Territory, Canada. One metre square plots had one of the three functional groups removed or left intact as a control and this was crossed with a fertilizer treatment and a fungicide treatment that targeted mycorrhizal fungi. After five seasons (2003-07) of implementing treatments the soil microbial community was analyzed using substrate-induced respiration (SIR, a measure of metabolic diversity) and phospholipid fatty acid analysis (PLFA, a measure of community composition). Plant functional group removal had almost no effect on the soil microbial community. The only response detected was an increase in stress (indicated by the PLFA stress ratio of cy19:0 to 18:17c) which occurred when legumes were removed and fertilizer was not added, indicating that legumes had a positive effect on the nutrient status of microbes. Likewise, soil properties (total carbon, pH, moisture and nutrients) showed limited response to plant removals. Fertilization decreased the metabolic diversity of the soil microbial community. We detected no soil microbial or plant biomass response to the fungicide indicating that mycorrhizae had little influence in this system. Based on the low-productivity of the grassland, and the lack of response in both the soil properties and the microbial community, we hypothesize that the main determinants of the microbial community may be litter input. When litter decomposition rates are slow, such as in this northern system, five growing seasons may not be sufficient to detect the impact of a changing plant community on the soil microbes.