Fate of bacterial biomass derived fatty acids in soil and their contribution to soil organic matter

Soil organic matter (SOM) is a major pool of the global C cycle and determines soil fertility. The stability of SOM strongly depends on the molecular precursors and structures. Plant residues have been regarded as the dominant precursors, but recent results showed a major contribution of microbial biomass. The fate of microbial biomass constituents has not yet been explored; therefore, we investigated the fate of fatty acids (FA) from ¹³C labeled Gram-negative bacteria (Escherichia coli) in a model soil study [Kindler, R., Miltner, A., Richnow, H.H., Kästner, M., 2006. Fate of gram negative bacterial biomass in soil—mineralization and contribution to SOM. Soil Biology & Biochemistry 38, 2860–2870]. After 224 days of incubation, the label in the total fatty acids (t-FA) in the soil decreased to 24% and in the phospholipid fatty acids (PLFA) of living microbes to 11% of the initially added amount. Since the bulk C decreased only to 44% in this period, the turnover of FA is clearly higher indicating that other compounds must have a lower turnover. The ¹³C label in the t-FA reached a stable level after 50 days but the label of the PLFA of the living microbial biomass declined until the end of the experiment. The isotopic enrichment of individual PLFA shows that the biomass derived C was spread across the microbial food web. Modelling of the C fluxes in this experiment indicated that microbial biomass is continuously mineralized after cell death and recycled by other organisms down to the 10% level, whereas the majority of biomass derived residual bulk C (～33%) was stabilized in the non-living SOM pool.