Recycling Biodegradable Wastes? | Take Your Cue from Mother Natureby Terry Green on 12/22/13
Do you believe composting is Nature’s way of recycling organic waste? That composting minimizes pollution of the environment? That compost is safer to add back to soil after the core temperature of the decomposing waste rises high enough to kill potential pathogens? That compost is a rich source of nutrients that increases soil fertility? Or that burying organic waste in soil before it is completely composted should be avoided? These common axioms are often cited in gardening and recycling circles. There are earth friendly alternatives in recycling organic waste which more closely mimic the way waste gets recycled in Nature (see, for example, Black Soldier Fly (BSF) Processing of Organic Wastes, fermentation and its integration with BSF in facilitating year round processing of waste materials in Recycle Food Scrap at Home and Save Big, and Is Composting an Earth Friendly Method of Recycling Plant and Food Scrap Waste?). This blog reviews the biological processes by which organic waste gets degraded and returned to soil, and discusses why composting as an earth friendly way of recycling organic waste is not all that it is cracked up to be.
Unlike what occurs in Nature, composting bypasses anaerobic and micro fauna synergistic processing of organic waste. Instead by design it exploits the oxidizing activity of aerobic microbes as the principle means of degrading waste. To compost waste it needs to be turned and laid in piles built to optimize air flow, all aimed at maximizing its oxidative degradation. It must be wetted to keep it from drying out. It off loads noxious byproducts into the atmosphere in varying chemical forms including organic amines, sulfur dioxide, ammonia and, in particular, two well-known greenhouse gases, carbon dioxide and nitrous oxide. To be done properly, it takes attention to detail, requires the input of work and energy, pollutes the atmosphere with greenhouse gases, and wastes copious amounts of precious water reserves better used for other purposes (irrigation, potable drinking water, washing and bathing, etc.).
In Nature, organic debris stacks up and layers over the ground as it falls onto the ground. Degradation occurs passively with no input of work, minimal generation of heat, and correspondingly far less off gassing of greenhouse gases into the atmosphere. No one has to aerate or water down the organic matter recycling each year in forests and fields undisturbed by humans.
Nutrients recycle directly into the soil as the waste mineralizes. Aerobic activity occurs principally near the surface of the layered pile. Facultative anaerobic microorganisms beneath the upper layer, and deeper still, strict anaerobic microorganisms, breakdown the decaying organic matter by different metabolic pathways relative to the way to those used by aerobic microorganisms. Micro fauna, including insects and worms, furthermore partake in the recycling of the waste, feeding on it, assimilating some of its nutrients, and leaving behind worm castings and insect frass which further enriches the decaying waste.
This more natural degradation process constitutes a heterogeneous, synergistic, multipronged biological process that ensures more efficient recycling and incorporation of carbon, nitrogen, phosphate, potassium and trace minerals, all of the essential nutrients, back into the soil. Leachate, released as organic debris decays carries with it water soluble nutrients and hydrolytic enzymes (denatured and inactivated by heat generated in the core of a compost pile) further drive the decomposition process forward as the leachate percolates through the layered pile and back into the soil.
Since most carbon in organic waste comes from plants, the fate of glucose, a proxy for the fate of carbon is instructive and worth contemplating regarding the relative differences between carbon matter degraded in Nature relative to what occurs in composting processing of the waste. Composting glucose with aerobic microorganisms uses up six equivalents of oxygen in oxidizing the 6-carbons of glucose, yielding 6 equivalents of carbon dioxide and 6 water equivalents. This simple calculation underscores the combustive nature of composting. All of the carbon stored in the form of glucose, having decayed through forced aerobic activity, blows off into the atmosphere as carbon dioxide. Aside from polluting the atmosphere, the loss of carbon to the atmosphere accompanying composting diminishes the quality and viability of soil stripped of essential soil organic carbon had the carbon been retained and returned to the soil.
The importance of returning carbon to soil cannot be underemphasized. There is much evidence that soil organic carbon plays an essential role in maintaining the fertility of soils, in general, and that the soil organic carbon content in many parts of the developed world has fallen dramatically with deforestation and implementation of modern agricultural practices that have greatly increased losses of carbon from soils (see, for example, Soil Carbon Sequestration— Fundamentals, Organic matter decomposition and the soil food web, and Soil carbon sequestration to mitigate climate change).
In this regard, even though a field plowed and left with organic matter exposed in furrows may look beautiful (Fig. 1), left in this state it is at its most vulnerable to losses of valuable nutrients washed and leached from the soil and oxidation leading to the venting of nutrients into the atmosphere, especially carbon and nitrogen (see, for example, Greenhouse gas emissions during cattle feedlot manure composting and Carbon Sequestration in Arable Soils is Likely to Increase Nitrous Oxide Emissions, Offsetting Reductions in Climate Radiative Forcing).
Fig. 1. A view of arable farmland left plowed with organic matter left to oxidize at the surface in plowed furrows can lose significant amounts of greenhouse gases into the atmosphere, particularly carbon dioxide and nitrous oxide, reducing the soil’s organic carbon and nutrient content and long term viability. The soil-air interface and high surface area provides for efficient aerobic oxidation and loss of greenhouse gases into the atmosphere. Cover crops and incorporation of organic matter left unturned over the top soil interface can substantially reduce off gassing, preserve more of the soil organic carbon and reduce overall losses of nutrients from the soil.
In considering what occurs in Nature regarding the turnover of organic waste, the fate of carbon and nitrogen, and other water soluble nutrients, is very different. In the glucose example cited above, in this latter situation glucose can no longer get broken down through normal aerobic metabolic pathways used by aerobic microorganisms. Far more of the carbon ends up in the form of intermediate organic acids (pyruvic, lactic, acetic, propanoic, butyric, valeric, caproic, etc.) (see Oxygen Effects in Anaerobic Digestion – A Review) which get carried into soil where they support the growth of soil micro fauna and beneficial soil microbes.
With regard to the fate of glucose, only 2 equivalents of carbon dioxide end up given up to the atmosphere through this process for every 6-carbon glucose molecule processed. Acetate, 2-carbon fragments derived metabolically as glucose breaks down, is readily upon entering the soil is readily taken up and metabolized by soil microbes which facilitate the build-up of organic biomass and fertility of soil. Thus 4 of every 6 carbons in glucose are conserved in the pile of degrading organic matter processed by mechanisms used in Nature relative to a loss of all carbons in glucose in waste aerobically processed and combusted into carbon dioxide.
Nitrogen, on the other hand, gets converted to ammonia in the reducing environment established beneath the upper layers of the pile where oxygen tension is lowest (see Enhanced Ammonia Content in Compost Leachate Processed by Black Soldier Fly Larvae). There is less loss of nitrogen given off as nitrous oxide to the atmosphere. Overall, anaerobic degradation occurring beneath the top layers of waste ensures better retention and recycling of both carbon and nitrogen back to soil where both are most needed.
Humic substances, and certain lignocelluloses complexes resistant to further microbial degradation, left behind in composted residue, can bind minerals, and therefore reduce to some extent the washout of nutrients after getting added back to soil. They can increase the capacity of soil to retain water while helping build up soil structure upon addition of compost to soil. But it can hardly be claimed that composting is Nature’s way of recycling organic waste. In this regard, when it comes to recycling organic matter, keep in mind that Nature has devised sophisticated methods of recycling its own waste through millions of years of evolutionary history. We can benefit by learning more about and following its cues when it comes to recycling our own organic wastes if we are only willing to look more closely in learning the lessons it can teach us!
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