Soil organic matter as a mediator of energy fluxes - a new perspective

The transformation of “energy to (soil organic) matter’’ has long been the focus of scientific attention, but a definitive conceptual framework does not yet exist. Following the classical definition of energy given by Odum and Odum (1977) and the principles and laws of energy, we have developed an experiment-based review of the complex process of microbial conversion of energy and carbon (C) from litter to soil organic matter (SOM). Based on the transformation rate of plant residues, the amount of plant-derived energy persisting in soil (after one year) ranges from 7 to 20 % of total energy input depending on the plant community (for example, spruce and broadleaf forests and grasslands were taken). This represents 0.8-10 % of the energy already stored in SOM but only adds 0.4-5 % C to the existing SOM pool. We have introduced two new parameters - energy quality representing primarily substance, and energy availability representing the ability of microorganisms to utilize that substance (or pool of substances) under actual soil conditions. According to these parameters, we have assigned the main classes of organic substances to one of the three groups that show the availability of energy stored in microorganisms. When the energy availability is >1, microorganisms gain more energy than invest by the decomposition of organic substances; when energy availability is <1, then energy investment is required for the co-mining of nutrients, and some compounds are unsuitable for energy mining due to low efficiency, and in this case, they will be partially decomposed by co-metabolism (no energy gain). We have estimated the energy investment of soil microorganisms for exoenzyme production and concluded that the disadvantage of enzymatic degradation could explain the ‘stability’ of the SOM because the energy input (investment) required for degradation exceeds the energy gain. Following the linear decrease in energy density (by 106 kJ mol^-1 C) of a broad range of organic substances per nominal oxidation state of C (NOSC) unit upon oxidation and experimental data on litter decomposition, we have developed the concept showing changes in the NOSC and the energy content of plant residues during decomposition and formation of SOM. Mineralization, recycling, and accumulation processes control energy and NOSC changes in organic pools. Mineralization processes lead to energy losses and an increase in NOSC, while SOM accumulation increases energy content and decreases NOSC. Recycling can shift both the energy content and NOSC values depending on the environmental conditions of the soil and the quality/quantity of litter input. As a result, the SOM pool is different from the initial litter in the energy content and NOSC. The SOM has a more diverse molecular composition but a narrower range of NOSC values than plant residues, consists of microbial necromass and substances recycled by microorganisms, and contains, on average, substances with a higher energy content than the initial plant residues. Based on the developed concept, we have concluded that plant-derived C and energy that persist in the form of SOM ensure energy fluxes in the soil system.