Growth of soil microbes is not limited by the availability of nitrogen and phosphorus in a Mediterranean oak-savanna

The environmental conditions under which the availability of inorganic nutrients such as nitrogen (N) and phosphorus (P) influence soil microbial growth are poorly understood, especially with regards to how fertilization changes specific aspects of microbial growth such as carbon-use efficiency (CUE). Microbial CUE is the fraction of C converted into biomass out of all C taken in and plays a critical role in global C budgets. Using the 18O labeled water method we tested short vs. long-term effects of N and/or P fertilization on microbial growth, CUE, and C, N, and P-acquiring enzyme activities in two soils from an oak-savanna, which differ in their soil organic matter (SOM) content. We hypothesized that soils with more SOM (from under tree canopies) would have higher microbial growth rates than soils with less SOM (from open grassland), and that microbial growth and CUE would increase with fertilization. We further hypothesized that these increases would be associated with a decrease in enzyme activity and a shift towards older SOM substrates in the short-term, in contrast to substrates from recently fixed C resulting from increased plant productivity in the long-term. We found that nutrient additions did not affect microbial growth or CUE in the relatively high SOM habitat on either time scale. In contrast, the low SOM habitat had lower growth and CUE when single nutrients were added, with significantly reduced growth when P alone was added, but was unchanged when N and P were added together. Our results show that short-term, stoichiometric imbalances can reduce microbial growth and that microbial growth at this site is limited not by nutrients but by the amount of C available to soil microbes.