From energy to (soil organic) matter

In this concept paper, we propose a new view of soil organic matter (SOM) formation: microorganisms use most of the organics entering the soil as energy rather than as a source of carbon (C), while SOM accumulates as a residual by-product because the microbial energy investment in its decomposition exceeds the energy gain. During the initial stages of decomposition, the nominal oxidation state of C (NOSC) in litter increases and the energy content decreases. This reflects the rapid mineralization of available compounds with low NOSC. Subsequently, however, the NOSC of remaining compounds drops to -0.3 units and the oxidation rate decreases due to the relative accumulation of aromatic and aliphatic compounds and entombment of the necromass. Ultimately, incompletely decomposed plant residues will have 1-2.5% more energy per C unit than the initial litter. The linear decrease in energy density by 106 kJ mol-1 C per NOSC unit upon oxidation is supported by experimental data on litter degradation. Preferential recycling of energy-rich reduced (lipids, aromatics, certain amino acids, amino sugars) and the microbial degradation of oxidized compounds (carboxylic acids) also energetically enrich SOM. Despite the high energy content, the availability of energy stored in SOM is lower than in litter. This explains why SOM is not fully mineralized, especially in the absence of plant C to provide new energy (e.g., in bare soil). Energy from litter activates decomposers to mine nutrients stored in SOM because the nutrient content of SOM is 2-5 times higher than that of litter. This results in only 0.4-5% y-1 of litter-derived C being sequestered in SOM, whereas SOM stores 1-10% y-1 of the total litter-derived energy. Thus, energy captured by photosynthesis is the main reason why microorganisms utilize organic matter, whereby SOM is merely a residual by-product of nutrient storage and a mediator of energy fluxes.