PNNL

Abstract

Density dependence in an ecological community has been observed in many macro-organismal ecosystems and is hypothesized to maintain biodiversity but is poorly understood in microbial ecosystems. Here, we analyze data from an experiment using quantitative stable isotope probing (qSIP) to estimate per-capita growth and mortality rates of bacterial populations in soils from several ecosystems along an elevation gradient that were subject to nutrient addition of either carbon alone (glucose; C) or carbon with nitrogen (glucose + ammonium sulfate; C+N). Across all ecosystems, we found that higher population densities, quantified by the abundance of 16S gene copies per gram of soil, had lower per-capita growth rates in C and C+N amended soils as compared to unamended control soils. Bacterial mortality rates increased with population densities in all treatments although the slope of density dependent mortality was steepest in C+N amended soils. Despite statistical significance, population densities explained a small proportion of the variance in per-capita rates. In contrast to the hypothesis that density dependence would promote or maintain diversity, soils with stronger negative density dependence (lower growth rates and higher mortality rates for larger populations) had reduced bacterial diversity. Here, soil nutrients were the key determinant in diversity and exerted small but significant control over density dependence trends in soil bacterial communities.