Influences of water availability on soil carbon emissions based on flux balance analysis

Soil is the largest carbon reservoir in terrestrial ecosystems, where carbon cycling is primarily driven by microorganisms. However, the effects of soil moisture on carbon emissions remain poorly understood. In this study, flux balance analysis (FBA) was employed to investigate changes in microbial carbon use efficiency (CUE) and CO 2 emission rates in Escherichia coli ( E. coli ) and Salmonella under varying O 2 and water availability. The CO 2 emission rate of E. coli increased from 0.0075 to 0.16 as available water content declined from 0.05 mol L −1 to 0.001 mol L −1 , due to water-facilitated CO 2 assimilation catalysed by phosphoenolpyruvate carboxylase. Under water availability fluctuation, water limitation increased CO 2 emissions and temporarily reduced CUE. During the first hour of water deprivation, deep anoxic soils released 8 μmol L −1 of CO 2 , surpassing the 5 μmol L −1 emitted via aerobic respiration in surface soils. Both CUE and CO 2 emissions were further modulated by microbial metabolic interactions: water-producing microbes reduced CO 2 emissions by 50.1% through metabolic water exchange, while the metabolic water stimulated microbial proliferation, altering their relative abundance and increasing CUE from 0.13 to 0.21. Overall, these findings suggest that water scarcity exacerbates CO 2 emissions from deep anoxic soils, while metabolic water exchange among microorganisms may serve as a mitigating mechanism against soil carbon loss. • Lack of water led to a decrease in microbial CUE under anaerobic conditions. • Water availability stimulated CO2 assimilation and reduced carbon emission rate. • Metabolic water exchange among microbes alleviated the increase in carbon emission rate.