Microbial physiology plays a fundamental role in soil organic carbon sequestration. The amount of carbon stabilized in soils depends on how heterotrophic microorganisms convert carbon into biomass relative to the amount of CO2 respired back to the atmosphere (i.e. Carbon Use Efficiency; CUE). This mechanism is affected by the availability of nitrogen relative to carbon, as abundant nitrogen can enhance CUE. Consequently, CUE is predicted to differ between soils dominated by trees with arbuscular mycorrhizal (AM) associations, which are more nitrogen rich than soils dominated by ectomycorrhizal (ECM) trees. Similarly, chronic atmospheric N deposition should increase CUE, but the extent of these changes in AM vs. ECM soils is unknown. In this study we incubated soils with 13C and 15N labelled aspartic acid and measured the fraction of this substrate released as CO2 or inorganic nitrogen vs. what was incorporated into microbial biomass. This dual labeling, along with quantitative stable isotope probing, allowed us to compare substrate use efficiency between soil types, identify microbial taxa driving those patterns, as well as testing the impacts of chronic atmospheric N deposition.
Results/Conclusions
Across mycorrhizal associations, N deposition decreased total soil CO2 respiration, but did not affect the amount of 13C from aspartic acid respired or incorporated into microbial biomass. Consequently, microbial CUE was not affected by N deposition in the AM or ECM associated soils. In non-fertilized soils, microbial communities from AM stands showed greater 13C respiration than communities from ECM stands, as well as a trend towards greater 13C incorporated into microbial biomass. Despite these differences in the amount of substrate use, the balance of assimilation and mineralization was unaffected, producing no differences in CUE between mycorrhizal associations. Nitrogen fertilization reduced NUE, as we observed greater isotopic signature in the inorganic N pool under this treatment, and no differences in the amount of 15N incorporated into microbial biomass. The composition of microbial communities using the labeled substrate differed between N-fertilization treatments and mycorrhizal association, but we did not find interactive effects between factors. Overall, our results suggest that N deposition altered the composition of microbial communities degrading organic N, but it did not change the efficiency at which this substrate was used. This implies a low impact of N deposition on microbial CUE measured via organic N use, suggesting that this mechanism does not play an important role at explaining why more organic C is stored in soils receiving chronic N deposition.