Do growth yield efficiencies differ between soil microbial communities differing in fungal: bacterial ratios? Reality check and methodological issues

Soil communities dominated by fungi such as those of no-tillage (NT) agroecosystems are often associated with greater soil organic matter (SOM) storage. This has been attributed in part to fungi having a higher growth yield efficiency (GYE) compared to bacteria. That is, for each unit of substrate C utilized, fungi invest a greater proportion into biomass and metabolite production than do bacteria. The assumption of higher fungal efficiency may be unfounded because results from studies in which fungal and bacterial efficiencies have been characterized are equivocal and because few studies have measured microbial GYE directly. In this study, we measured microbial GYE in agricultural soils by following C-13-labeled glucose loss, total CO2-C, and (CO2)-C-13-C evolution at 2 h intervals for 20 h in two experiments (differing in N amendment levels) in which the fungal: bacterial biomass ratios (F:B) were manipulated. No differences in efficiency were observed for communities with high versus low F:B in soils with or without added inorganic N. When calculated using (CO2)-C-13-C (in contrast to total CO2-C) evolution, growth yield efficiencies of soils having high and low F:B were 0.69 +/- 0.01 and 0.70 +/- 0.01, respectively. When soils were amended with N, soils with high and low F:B had growth yield efficiencies of 0.78 +/- 0.01 and 0.76 +/- 0.01, respectively. Our experiments do not support the widely held assumption that soil fungi have greater growth efficiency than soil bacteria. Thus, claims of greater fungal efficiency may be unsubstantiated and should be evoked cautiously when explaining the mechanisms underlying greater C storage and slower C turnover in fungal-dominated soils. (c) 2005 Elsevier Ltd. All rights reserved.