I have mentioned that we exist in cyclical systems rather than linear ones. In an extremely simplified form, we are involved in a cycle of …birth – growth – death – decomposition – birth… An over-emphasis on birth/production and growth has caused a breakdown in the links of this cycle. For sustainability, this breakdown must be healed. I suggest that increased attention to the less-appreciated half of the cycle – the death and decomposition — may allow us to move toward increasing cycles in a sustainable way.
After life has departed from earthly materials, we are left with residues which have both material and energy aspects. The material includes carbon and all of the other nutrients which were incorporated during life. The energy is the residue of what was originally incorporated by photosynthesis and not used up to support plant and animal bodily processes.
There are a range of options for management of these organic residues. One may note that all of the thermo-chemical options make use of the embodied solar energy. However, while they may provide some ash or char which is of some nutrient and amendment value to soils, all nitrogen and other volatile nutrients are lost in the gases.
There has been much interest recently in the carbon sequestration potential of biochar produced through pyrolysis and incorporated into soils. I suspect that this may well be highly appropriate for residues with high lignin content (e.g. woody residues). I suggest that the biological pathways are more appropriate for most other organic residues. The embodied energy can be accessed/utilized, especially with anaerobic digestion; the humus (most of the carbon) is more biologically active; and, nutrients are conserved, again especially with anaerobic digestion. See Anaerobic Digestion.
As to the biological pathways, the biochemical production of alcohol does make use of much of the energy in the feed stock. However, there remains a mass of material which must then be further managed — through feeding to livestock or entering upon one of the two remaining pathways — aerobic or anaerobic decomposition (which might be better understood as re-composition-for-regrowth).
Both of these pathways result from the integrated life-cycle activities of huge populations of various micro-organisms — as well as insects and other macrophages in the case of aerobic compost piles. Speaking ecosystemically, their job is to break down the complex organic molecules formed during growth into simple molecules, which are then available for regrowth.
Attention to these biological pathways offers a huge carbon sequestration potential. Many agricultural soils in the USA, which contained 8-10% high-carbon organics before the start of farming, are down to 1-2% today. A focus on returning uncontaminated organic material to the soil provides a natural, highly significant, and multi-dimensionally beneficial means for ameliorating overly high atmospheric carbon levels. (See Soils.)
The pathways appear quite similar in many respects. Both result in a stabilized humus composed of longer-lasting organic materials with varying levels and molecular forms of nutrients. When incorporated into the soil, this material adds significantly to long-term tilth and productivity.
There are, however, significant differences. While the aerobic organisms require large amounts of easily available oxygen, most of the crucial anaerobic organisms are destroyed by free oxygen, even in solution. While the solar energy embodied in decomposing residues is largely released as heat during aerobic processes, it is largely contained in the methane gas (CH4, the principal component of natural gas) produced during the anaerobic process. Also, the heat generated in the aerobic process tends to drive off much of the nitrogen and other volatile nutrients, while nearly all nutrients in the original residues are maintained in the anaerobic end products.
Aerobic decomposition is most widely known as composting. It occurs naturally on forest floors and grasslands, in the upper layers of well-aerated soils and bodies of water, and in earlier organs of digestion of larger animals. Composting is increasingly being utilized from back-yard through farm and commercial scales as the importance of returning these materials to our soils has become increasingly recognized. Aerobic processes are also widely used in sewage treatment systems where air is pumped through the liquids to maintain adequate oxygen levels.
I would direct interested parties to the web for more information and discussion of aerobic decomposition and composting.