Bacterial growth efficiency varies in soils under different land management practices

• Bacterial metabolism in soil drives CO2 production and carbon biosequestration.
• Ratios of CO2 production to biomass accumulation vary with land management.
• Metabolic efficiency is influenced by substrate availability and stoichiometry.
• Metabolic efficiency in forested soils is more constant than agricultural soils.

Transformations of plant-derived carbon into soil organic carbon (SOC) influences both local and global carbon cycles. Soil bacteria play a major role in SOC transformations, which are important for maintaining soil fertility and for stabilizing carbon in soil. SOC is consumed by bacteria in soil and transformed into biomass or respired to carbon dioxide. This bacterial-driven partitioning of SOC is defined as Bacterial Growth Efficiency (BGE) and it is an integral component of models that simulate carbon dynamics. We tested the variability of BGE in microbial communities from soil by measuring bacterial production (BP) and respiration (BR), the two components of BGE, in slurries of soils collected from deciduous forests and croplands at the Kellogg Biological Station Long Term Ecological Research site. BP was measured as 3H-leucine incorporation into protein and BR as oxygen consumption. The differences in BP and BR in soil under different land management practices revealed that BGE was not static but varied from 0.23 to 0.63, supporting more recent SOC models. Bacterial communities from soils of soybean monoculture cropland tended to have a higher BGE than those from deciduous forests. BGE of cropland soil microbes exhibited a large seasonal variation not observed in forest soils. Nutrient amendments on rotation cropland soil microcosms showed that BGE is sensitive to substrate availability and nutrient stoichiometry. Using a range of growth efficiency expected of terrestrial ecosystem, simulations of carbon dynamics in a forest using the DAYCENT model revealed the sensitivity of equilibrium soil carbon values to changes in growth efficiency. Decreasing the default growth efficiency of 0.45 to a growth efficiency of 0.35 reduced the active carbon fraction by 22%. This sensitivity emphasizes the importance of site-specific BGE measurements for improving the predictive capacity of SOC models, especially when investigating the effects of changes in land management practices on labile SOC transformation. The weak correlation of BP and BR in most soil tested also showed that BGE is a more valuable measurement than the common interpretation of bacterial activity based on BR.