Dynamics of bacterial growth, and life-history tradeoffs, explain differences in soil carbon cycling due to land-use.

Abstract

Soil contains a considerable fraction of Earth's organic carbon. Bacterial growth and mortality drive the microbial carbon pump, influencing carbon use efficiency and necromass production, key determinants for organic carbon persistence in soils. However, bacterial growth dynamics in soil are poorly characterized. We used an internal standard approach to normalize 16S ribosomal RNA gene sequencing data allowing us to quantify growth dynamics for 30 days following plant litter input to soil. We show that clustering taxa into three groups optimized variation of bacterial growth parameters in situ. These three clusters differed significantly with respect to their lag time, growth rate, growth duration, and change in abundance due to growth (ΔN_g) and mortality (ΔN_d), matching predictions of Grime's CSR life-history framework. In addition, we show a striking relationship between ΔN_g and ΔN_d, which reveals that growth in soil is tightly coupled to death. This result suggests a fitness paradox whereby some bacteria can optimize fitness in soil by minimizing mortality rather than maximizing growth. We hypothesized that land-use constrains microbial growth dynamics by favoring different life-history strategies and that these constraints control carbon mineralization. We show that life-history groups vary in prevalence with respect to land-use, and that bacterial growth dynamics correlated with carbon mineralization rate and net growth efficiency. Meadow soil supported more bacterial growth, greater mortality, and higher growth efficiency than agricultural soils, pointing toward more efficient conversion of plant litter into microbial necromass, which should promote long-term C stabilization.