Turnover of gram-negative bacterial biomass-derived carbon through the microbial food web of an agricultural soil

Soil organic matter (SOM) represents an important terrestrial carbon reservoir in the biosphere, and microorganisms have been recognized as significant material contributors to the formation of SOM. However, the turnover of microbial biomass residues with respect to their detailed microbial food web remains elusive. To elucidate this turnover process, we traced the fate of Gram-negative (Gram−) microbial biomass carbon through the microbial food web over time, using the concentrations and isotopic compositions of biomarker phospholipid fatty acids (PLFA) in a soil incubation with isotopically (13C) labelled model bacterial cells (Escherichia coli). We found that after 120 days 46.5% of the recovered E. coli derived 13C remained in the soil, whereas 53.5% was emitted as 13CO2 (recovery 77.0% of the initially added 13C). The 13C in microbial biomass decreased to 5.3% of the initial value. This indicates that about 41% of the E. coli biomass in our experiment was transformed to SOM. The PLFA patterns over time demonstrate the pathway of 13C transformation associated with different groups of microorganisms along the incubation. The E. coli-derived 13C was utilized by fungi first, then the label was shifted from fungi to Gram-positive (Gram+) bacteria, arbuscular mycorrhizal fungi (AMF), actinomycetes, and to Gram− bacteria other than E. coli. Finally the carbon was transferred from all consumers to the next consumer level; this is reflected in ongoing loss of 13C-PLFA without a shift in the 13C-PLFA pattern. In summary, our study details the turnover process of microbial biomass residues via the microbial food web to necromass and finally to SOM. This supports the soil microbial carbon pump concept, i.e. carbon channelling and the assimilation of easily degradable carbon into microbial biomass and a significant contribution of these residues in SOM.