Ecosystem properties and microbial community changes in primary successionon a glacier forefront

We studied microbial community composition in a primary successional chrono sequence on the forefront of Lyman Glacier, Washington, United States. We s ampled microbial communities in soil from nonvegetated areas and under the canopies of mycorrhizal and nonmycorrhizal plants from 20- to 80-year-old z ones along the successional gradient, Three independent measures of microbi al biomass were used: substrate-induced respiration (SIR), phospholipid fat ty acid (PLFA) analysis, and direct microscopic counts. All methods indicat ed that biomass increased over successional time in the nonvegetated soil. PLFA analysis indicated that the microbial biomass was greater under the pl ant canopies than in the nonvegetated soils; the microbial community compos ition was clearly different between these two types of soils. Over the succ essional gradient, the microbial community shifted from bacterial-dominated to fungal-dominated. Microbial respiration increased while specific activi ty (respiration per unit biomass) decreased in nonvegetated soils over the successional gradient. We proposed and evaluated new parameters for estimat ing the C use efficiency of the soil microbial community: "Max" indicates t he maximal respiration rate and "Acc" the total C released from the sample after a standard amount of substrate is added. These, as well as the corres ponding specific activities (calculated as Max and Acc per unit biomass), d ecreased sharply over the successional gradient. Our study suggests that du ring the early stages of succession the microbial community cannot incorpor ate all the added substrate into its biomass, but rapidly increases its res piration. The later-stage microbial community cannot reach as high a rate o f respiration per unit biomass but remains in an "energy-saving state," acc umulating C to its biomass.