Nutrients strengthen density dependence of per-capita growth and mortality rates across all major phyla of soil bacteria

Background/Question/Methods Density dependence, a primary signal of intraspecific competition, has been observed in many macro-organismal ecosystems and is thought to maintain biodiversity. However, the occurrence of density-dependence in microbial ecosystems has not been well studied, partially due to difficulties in measuring rates of growth and mortality of individual taxa in situ. Here, we analyze data from an experiment using quantitative stable isotope probing (qSIP) to measure the per-capita gross growth (i.e., cell division) and mortality rates of bacteria in soils from several ecosystems along an elevation gradient. These soils were subject to nutrient addition of either carbon alone (glucose; C) or carbon with nitrogen (ammonium sulfate; C + N). We used hierarchical linear mixed models to determine the relationship between bacterial population densities and per-capita growth and mortality rates, accounting for variance between and within ecosystems and due to bacterial taxonomic identity.

Results/Conclusions Across all ecosystems, we found that higher population densities had lower per-capita growth rates. This relationship was exacerbated by nutrient amendment such that bacteria in C and C + N amended soils experienced stronger negative density dependence in growth rates than those in soils without labile nutrient addition. Similarly, C + N amended soils had significantly higher mortality rates with increasing population size. Further, we saw no relationship between growth rates, variation in growth, or population density – indicating that larger populations experience lower growth rates but not as a consequence of higher variability in growth. With respect to the effect of density-dependence on diversity, however, we found no indication that soils with higher overall negative density dependence promoted soil bacterial diversity as hypothesized. Instead, and despite stronger negative density dependence, bacterial diversity was lower in C and C + N amended soils. These results represent one of the first examinations of in situ population dynamics of individual microbial taxa and indicate complex relationships between diversity and density dependence in microbial communities.