Lee, Z.M. and T.M. Schmidt
Presented at the All Scientist Meeting (2011-04-15 to 2011-04-15 )
Bacteria in soil are responsible for converting labile root exudates into microbial biomass that is less readily oxidized. This transformation of soil organic carbon (SOC) is important for stabilizing soil organic matter and maintaining soil fertility. Bacterial partitioning of SOC into biosynthesis of new biomass or mineralization to carbon dioxide is defined as Bacterial Growth Efficiency (BGE). BGE is an integral component of models that simulate carbon dynamics in soil. However, BGE variation in terrestrial ecosystem is not well understood, nor are environmental factors that influence BGE. BGE is calculated from bacterial production (BP) and respiration (BR) using the formula BP/(BP+BR). In this study, BP and BR were measured for soils collected from deciduous forest and three croplands that are differently managed at the Kellogg Biological Station Long Term Ecological Research site. We measured BP as 3H-leucine incorporation rate into protein and BR as oxygen consumption rate, both without exogenously added substrates. We found that BGE was not constant, but varied from 0.23 to 0.63. Bacterial communities in soils from soybean monoculture cropland tended to have a higher BGE than those in deciduous forests or rotation cropland soils. BGE in cropland soils exhibited a large seasonal variation not observed in forest soils. Inclusion of measured BGE into DAYCENT, a model simulating plant-soil nutrient cycling in terrestrial ecosystems, revealed the sensitivity of equilibrium soil carbon values to changes in BGE. When simulating carbon dynamics in a forest ecosystem, decreasing the default BGE of 0.45 by 10% to 0.35 reduced the active carbon fraction by 22%. This led to a 5% reduction in the predicted total soil carbon at equilibrium. We conclude that site-specific BGE is important for improving the predictive capacity of SOC models, especially when investigating the effects of changes in land management on labile SOC transformation.
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