Soil systems act as the earth’s largest terrestrial pool of carbon (C), and microbial respiration is the primary means of loss from soils in most forests. Mycorrhizal fungi form crucial symbiotic relationships with most plants and may affect microbial respiration through ecological interactions with soil heterotrophs. Recent evidence suggests that arbuscular mycorrhizal soils have higher rates of respiration than ectomycorrhizal soils, but the mechanism for this remains unclear. In this microcosm experiment, respiration rates were taken repeatedly from identical soil chambers containing either ecto- or arbuscular mycorrhizal hyphae. Live roots were excluded from the chambers but mycorrhizal hyphae were allowed in, so that differences between treatments could be attributed solely to the microbial communities. Isotopically labeled (13C) fungal necromass was decomposed inside the chambers and mass loss was measured.
Results/Conclusions
Variation in fine root biomass across the pots was unrelated to respiration rates (P = 0.67) suggesting that we were successful in isolating hyphosphere respiration from root respiration. Our results suggest that the presence of different mycorrhizal fungi led to variations in microbial respiration, with higher respiration rates in the arbuscular mycorrhizal hyphospheres during the first two weeks of necromass decomposition (P < 0.01). Measurements of 13CO2 content during this time suggested that between 17-46% of the total soil respiration originated from the decomposing substrate. After 35 days of decomposition, more mass had been lost from the substrate decomposing in the ectomycorrhizal hyphosphere than the arbuscular mycorrhizal hyphosphere (P = 0.02), despite the higher rates of respiration in the arbuscular mycorrhizal chambers, suggesting different carbon use efficiencies between these two microbial communities.These findings provide new insight into commonly observed patterns in forest soil respiration.