Shifts in microbial CUE as a function of available organic C resources and electron acceptors under changing soil redox conditions

Soil microbial carbon use efficiency (CUE) represents an important driver of soil organic C formation and turnover. The balance between anabolic and catabolic processes are known to regulate SOC formation through microbial growth and stabilization of microbial residues, and SOC mineralization, respectively. CUE is largely affected by environmental factors but also regulated by the availability of organic substrates and electron acceptors. This is particularly the case in soils that are temporarily subjected to shifts in redox conditions, as those that occur when rice paddy soils are flooded or drained. In such soils, microbial CUE may be affected by electron donor availability (e.g. presence or absence of labile crop residues) as well as electron acceptor availability as O₂ becomes limiting and other oxidized species like nitrate and Fe^III minerals are reduced. These changes in metabolic activities may also be accompanied by a change in microbial communities which are more adapted to changes in redox conditions and use their resources more efficiently affecting the overall community CUE.

The aim of this work is to explore the effects of short-term changes in soil redox conditions (i.e. from aerobic to anaerobic) on the microbial physiology of a rice paddy soil, by unravelling the effects of management-related differences in electron donors and acceptors on microbial growth, respiration and CUE, as well as their dependence on changes in microbial community composition. For this we set up a microcosm experiment where a paddy soil was incubated for 17 d with a factorial combination of (i) redox conditions (oxic vs. anoxic), (ii) with or without rice straw, and (iii) with or without added nitrate. Soils were destructively sampled after 4, and were analysed for DOC, dissolved nitrate and Fe^II , and microbial biomass C (MBC). Soil aliquots were incubated with D₂O for 48 h to measure rates of microbial respiration (CO₂ and CH₄) and growth by tracing isotope incorporation into phospholipid fatty acid (PLFA) biomarkers, to calculate CUE. 

Our preliminary results showed that under anoxic conditions nitrate was rapidly consumed within 4 d while Fe(III) was reduced at a later stage particularly where easily degradable rice straw was added. This could mean that in the first few days the microorganisms are not facing C deficiencies, however, as the anoxic conditions persist the C input enhances microbial activity leading to an increase in Fe(II) in solution. This was also seen in the MBC, as the presence of rice straw enhanced MBC, irrespective of the other treatments (i.e. redox conditions and NO₃^- addition). Redox-driven changes in community level microbial physiology (growth, respiration and CUE) as well as changes in the active microbial community (PLFA-based) will provide further insights on the role of changing redox conditions and management on key parameters related to soil carbon accrual.