Microbial EPS as a relevant pathway for non-growth C investment: a study in two agricultural soils

Microorganisms drive soil C cycling, yet microbial metabolism is commonly conceptualized as a balance between growth (usually increase in biomass) and respiration. This simplified view neglects substantial microbial investments into non-growth pathways, such as the production of extracellular polymeric substances (EPS), which may strongly influence soil biogeochemical processes. EPS contribute to soil aggregation, resource acquisition, and microbial stress tolerance, but their role in microbial C allocation and soil C cycling remains poorly quantified. In this study, agricultural soils with different fertilization histories were incubated for 70 days with ¹³C-1-glucose and ¹⁵N-U-urea to trace substrate allocation among microbial biomass (MB), EPS, and CO₂ efflux. Our main hypothesis was that even though most substrate C and N would be allocated to MB, a significant portion would be incorporated into EPS. As results, we found that most added substrates were allocated to MB. However, 2 ~ 15% of added C and 10 ~ 15% of added N were recovered in EPS, corroborating our hypothesis that this non-growth pathway can account for a meaningful portion of microbial resource use. Further, we also observed that soil intrinsic characteristics, rather than their fertilization history, had the most significant effects over C and N partitioning in the studied sites. Microorganisms residing in clay-rich soils allocated more substrate to EPS than those in sandy soils. Finally, we also found that the incorporation of labelled C and N correlated positively in both MB and EPS. This supports the hypothesis of a coupled microbial C–N metabolism, in which EPS production accompanies growth rather than occurring independently of it. A larger set of soils is needed to incorporate non-growth C allocation pathways (other than EPS) into conceptual and quantitative models of soil biogeochemistry, in order to improve our understanding of microbial resource allocation for soil C and N stabilization.