Fig. 2: Formation of bioNERs during the biodegradation of a heavy C- or H-labeled substrate (here: 2-13C- and 2-C–D2-glyphosate).
Please note that the same processes apply to 14C and T as well, but are for simplicity only shown for 13C/D. The C/H-cycle starts with the breakdown of C–D bonds of the C–D-substrate by degrading microorganisms in multiple oxidation steps (catabolism) to free both energy and C(12C/13C)-building blocks needed for biomass synthesis (anabolism). The C-building blocks mined during the central C-metabolism (e.g., acetyl-groups generated from the pyruvate produced by the glycolysis pathway48) are integrated into C-monomers during anabolism (1st level). Contrastingly, after C–D bonds breakage, most of the substrate-D is first lost via the coenzyme nicotinamide adenine dinucleotide (NAD(D)) into ambient water, where it is diluted with unlabeled H. Then, the H/D is transferred by the coenzyme nicotinamide adenine dinucleotide phosphate (NADP(D)) to C-building blocks during de novo C–H bonds formation of C-monomers in anabolism49. When biomass decays, the necromass is either assimilated by other living microorganisms (primary consumers) or stabilized in the soil matrix forming bioNERs. A direct assimilation of C-monomers (here glycine) into microbial macromolecules is also possible after a partial breakdown of C-substrates (like glyphosate) or C-necromass. 13CO2 is slowly released from the soil matrix during microbial degradation at each trophic level, whereas D2O is lost more rapidly and hardly taken up again due to an estimated 100,000 times dilution with unlabeled H2O. Hence the retention of an easily biodegradable H-substrate in soil after one year is about sixfold lower (5%) than for C (30%); see small inserted Fig. adapted from Paul et al.38 according to a CC BY 3.0 license. The N-cycle is similar to the C-cycle except that N-substrates are mineralized to gaseous N2O or N2 and the substrate-derived N (e.g., a NH2-group) is transferred to C-monomers (e.g., amino acids) in anabolism; therefore, it is not shown here.
