Manure decomposition in a natural setting was, in days gone by, primarily an aerobic process. Today, we stockpile animal waste in such depth and quantity that it can be used only by anaerobic bacteria. The hitch? The chemical and biological output of anaerobic decomposition is entirely different from aerobic decomposition.

Because of the nature of the compounds produced by the two processes, it could be said aerobic is generally good, while anaerobic decomposition is mostly bad. It's a standard that is especially apropos when we consider how these two methods of animal waste handling affect crops and soils.

Nature's way

There are actually three basic types of bacteria at work in animal waste, soil and other organic compounds undergoing decomposition. These are anaerobic, facultative and aerobic.

Anaerobic bacteria function in the absence of free oxygen. Aerobic bacteria live and work where adequate oxygen is available. Facultative bacteria can function in either situation, but are more efficient at capturing nutrients through the aerobic process, as are aerobic bacteria.

Anaerobic and facultative bacteria played a major role in moving a young Earth toward its current state by releasing oxygen during metabolism. But most life on Earth today is dependent on aerobic conditions and aerobic decomposition.

Anaerobic decomposition is much less common in nature today, with the exception of swamps and peat bogs, both of which humans have never favored as living sites. That's because they stink, are structurally unstable, full of toxic gases and largely devoid of the things that support higher life forms such as mammals.

The case against no air

Anaerobic bacteria produce a completely different set of enzymes and by-products than aerobic bacteria, explains Elaine Ingham. She is president of Soil Foodweb in Corvallis, OR, and New South Wales, Australia, and research director at Soil Foodweb in New York. Throughout the 1990s, she was a leading university researcher in soil biology and its relationship to soil chemistry and plant nutrition.

Ingham says that anaerobic decomposition has two primary problems: The majority of plants can't tolerate the by-products of anaerobic decomposition, and too many nutrients are lost as gases.

Compared with aerobic decomposition, decomposition under anaerobic conditions is essentially “incomplete” because it doesn't finish breaking down hydrocarbon chains. Aerobic decomposition does, leaving primarily water and CO2.

Anaerobic decomposition leaves behind many carbon-based organic compounds, Ingham says. Several of those compounds are alcohols — carbon chains with variations of oxygen-hydrogen bonds attached — known as carboxls. They're among the world's most toxic compounds to all plants except those that thrive in aquatic environments, she adds.

Anaerobic bacteria convert a great portion of the nitrogen in organic compounds, such as manure, to ammonia (NH3), a nitrogen form that is unavailable to plants until converted by bacterial action to nitrate (NO3) or ammonium (NH4). Further, huge amounts of ammonia, nitrous oxide (N2O) or nitrogen gas escape into the atmosphere after anaerobic decomposition.

Most organic compounds, such as plants and cows, and manure and milk, have an approximate nitrogen-to-phosphorus ratio of 6:1. But anaerobically handled manure products usually have a nitrogen-phosphorus ratio of 1:1. This explains scientific measurements that show 75% to 80% of nitrogen in most hog and dairy waste-storage facilities is lost, mostly as ammonia.

Micronutrients are lost in similar anaerobic reactions. One example is hydrogen sulfide (H2S), which is the rotten-egg smell common around confined animal facilities. In contrast, aerobic decomposition produces plant-available and more-stable sulfate (SO4).

In fact, many of the compounds produced by anaerobic decomposition have strong odors. “If it stinks, there's anaerobic metabolism occurring,” Ingham says.

A small amount of the phosphorus undergoing anaerobic decomposition is lost as phosphene gas (PO4). This is the same light-colored but heavy fluorescent gas that's seen hanging at knee or waist level in swamps at night, Ingham says.

Anaerobic decomposition also produces organic compounds known as phenols, she adds, similar to some toxic compounds found in petroleum (an anaerobic product).

Ingham says some very toxic organic acids such as acetic, proprionic, butyric, valeric, and many other low-pH organic acids are produced in anaerobic conditions.

“Once you've created anaerobic material, it's very toxic and low in nutrients,” she says.

The case for air

In contrast, aerobic decomposition produces water-soluble, relatively stable, plant-available compounds, such as nitrates, ammoniums, sulfates and phosphates.

When the primary bacterial action is aerobic, many of the nutrients, and much of the organic matter — carbon-based materials — are stored in the billions of living organisms suspended in the water, waste material or soil. In an aerobically driven cycle, life feeds on life and spirals upward to higher numbers of life.

The primary byproducts of aerobic decomposition — the “waste products” of the process — are carbon dioxide and water. Both are immediately usable by plant life.

In healthy soils or aerobically decomposed animal waste, there exists an entire food chain of bacteria feeding on other bacteria, and larger organisms feeding on the smaller ones, to ultimately excrete materials available to plants. The food chains under the soil are very much a miniature version of the food chains we observe on the Earth's surface and in her oceans, but with many times more organisms involved.

The constant cycling of microbial life, together with their interaction with plant and animal life, mines the nutrients from the soil and air and puts them into the life cycles at a high rate. This is why well-managed, healthy prairie and forest was, from the earliest times, such an amazing perpetual motion machine.

In most of the world's ecosystems, aerobic bacterial action is nature's favored model. There's still much work to be done by anaerobic bacteria, and many jobs are done by a combination of anaerobic and aerobic action.

Once we understand nature's preferences and the biologic principles, it's only logical to look for ways to introduce higher levels of aerobic decomposition back into our modern manure handling systems.

Alan Newport is a freelance ag writer based in Carnegie, OK.