CANCELLED - Differential nutrient limitation of soil microbial biomass and metabolic quotients (qCO2): Is there a Biological Stoichiometry of soil microbes?

Background/Question/Methods

Variation in microbial metabolism poses one of the greatest current uncertainties in models of global carbon cycling, and the metabolism of soil microbes is particularly poorly understood.  Biological Stoichiometry theory describes biochemical mechanisms linking metabolic rates with the elemental composition of cells, and these relationships have been widely observed in animals, plants, and plankton.  However, this theory has seldom been tested in microbes, which are considered to have fixed ratios of major elements in soil.  To test whether Biological Stoichiometry describes variation in the metabolism of soil microbes, we compiled all existing published data on soil microbial biomass carbon (C), nitrogen (N), and phosphorus (P) pools in soils spanning habitats and land use types across the major terrestrial biomes.  We compared element ratios in these microbial biomass pools to the metabolic quotient qCO₂ (respiration per unit biomass), where soil C mineralization was simultaneously measured in controlled incubations.  

Results/Conclusions

We found significant variation in the C:N:P ratios of soil microbial biomass across land use and habitat types, with some limited evidence suggesting size-dependence in the N:P ratios of microbes, as in animals and plants.  Although we found the direct relationships between microbial stoichiometry and metabolism were weak, microbial stoichiometry was a persistent factor in multiple regression models of microbial metabolism (qCO₂).  While microbial biomass pools in soils appeared to be limited by soil N availability, we found rates of microbial metabolism per unit biomass (qCO₂) were associated with inorganic P availability.  These findings are consistent with the model of cellular metabolism described by Biological Stoichiometry theory, where biomass is limited N availability due to the high N requirements of proteins, but rates of protein synthesis are limited by P availability due to the high P demands of ribosomes.  Incorporation of these physiological processes may improve models of carbon cycling and understanding of the effects of nutrient availability on the turnover of soil C across terrestrial habitats.