Compost, Soil Health And Climate Change

The "soil food web" is a term scientists use to describe the sum total of the organisms living in any given soil, and the relationships between them. Soil is in fact a living ecosystem and it is the health of that ecosystem (i.e., number or organisms, diversity of types of organisms, their access to energy, air, water, etc.) that dictates the health of the soil itself.

- Although compost can reduce the need for synthetic fertilizer, it should not be viewed as an equivalent replacement, even when soil testing is used to calculate the amount of nutrients supplied by the compost. This is not only because compost can provide additional benefits, such as better soil structure. It is also due to the fact that compost releases its nutrients slowly, over decades. Compost has many nutrients beyond what the soil tests show (they simply show what is available to the plant). These nutrients are tied up in various forms of organic matter. How much of that extra nutrition will be made available will depend on many factors, including the health of the soil. - When compost is added, the soil food web is stimulated. This can enhance the natural nutrient cycling taking place in the soil. One farmer in the United States, who has been practicing soil health for decades, says that he can stop adding fertilizer, with no yield penalty, when his fields reach a soil organic matter (SOM) level of 8 per cent. Most fields are well below that. Compost can help to bring up levels of SOM, but it takes many years if the starting level is low. - Similarly, compost cannot completely eliminate nutrient run-off, and can even contribute to it. However, a healthy soil, with a high SOM level, can greatly reduce nutrient loss. This is because the higher number of soil micobes in healthy soil will ensure that the soil has good structure (aggregation) and therefore lots of storage capacity (SOM also has a very high cation exchange capacity (CEC), which means that the soil will hold on to nutrients better). The nutrients that end up in solution in the soil water don't leave the field because they are held in the pore spaces created by the soil food web. - Finally, compost has been shown to have the potential for major impacts on the adaptability and resilience of soils. Again, this is due to the stimulation of the soil food web and the beneficial soil structure that is the inevitable result of healthy soil best management practices. We should note here that tillage, in particular, can have detrimental effects on soil structure. Compost can perhaps ameliorate this damage, but the best results come from the application of compost to land managed with no or minimum tillage.

The beneficial soil functions (on the left) are the result of activities or conditions created by the soil food web organisms, on the right. - The organisms build soil structure with glues and filaments, which creates pore spaces that allow for good water storage. - Bacteria and fungi extract nutrients from organic matter and minerals; when they are consumed by predators (protozoa, nematodes), these nutrients are released to the plants. - Beneficial microbes suppress disease in many ways, but simple competition for food resources and space is one of the main methods. - Carbon is sequestered when organic matter is trapped in aggregates created by the soil food web organisms. - These same organisms are experts at breaking down toxic molecules, such as those found in pesticides, rendering them harmless and thus cleaning our water supply. - Compost helps with all of these things by providing the soil food web with food energy, habitat, and increased diversity,.

Compost can speed up or enhance the formation of stable soil aggregates, largely by providing food and habitat for the microbes that secrete the glues and grow the filaments that hold aggregates together. Some of the organic matter in compost becomes enclosed within the aggregate. This slows down the rate at which the organic matter decomposes, because the conditions inside the aggregate are less conducive to the the activity of decomposing microbes. As a result, the rate of carbon loss from soil is reduced and carbon levels rise (one of the main mechanisms of soil carbon sequestration).

Compost is comprised largely of organic matter, in the form of substances that are more stable chemically than you would find in uncomposted organics, manure, etc. Microbes are able to use many of these substances as food sources. They also attach themselves to clumps of organic matter, helping build soil aggregates. These aggregates give soil its good structure, which in turn provides the microbes with beneficial habitat, or shelter. Another important benefit of compost is its impact on diversity. Compost made from food wastes, for instance, is chock full of thousands of species of beneficial microbes, many of which may be new to the soil where the compost is added. Finally, compost does not directly provide any protection from predators, but that is a good thing, because predation is the basis for soil fertility -- larger microbes eat the smaller ones, then "poop" out the nutrients in plant-available form.

Although compost is great for soil health, too much added at one time can result in several different problems. For example, it can create a nutrient overload, with subsequent run-off causing pollution of surface waters; also, some types of compost (e.g., C:N ratio below 11) can have detrimental effects on the diversity of some later-successional ecosystems (e.g., grasslands), as the available nutrient levels are too high, encouraging invasion by earlier successional species. Compost has the potential to be a major tool in both the soil-health and climate-change endeavours, but it should always be used within the proper nutrient and ecological context.

By having the soil surface covered with green plants as much as possible, the amount of solar energy captured and converted into biochemical energy is maximized. Up to half of this energy is transferred to the creatures of the soil food web, fueling their beneficial activities. Some of this energy is transferred when plants and animal die, or eliminate waste. However, much is directly transferred by plants to the soil via root exudates -- energy-rich substances secreted through the roots to feed microbes (in return for the nutrients microbes bring to plants). This is the original carbon trading system.

- True. Methane emissions are the result of organic wastes decomposing in anaerobic (low oxygen) conditions; methane (CH4) is 25 times as potent a GHG as CO2 - True. Compost adds organic matter to soil, and it also stimulates the soil food web, which encourages more plant growth and thus even more organic matter added to the soil (as organic wastes and plant root secretions, or exudates); to find out more about how this works, check out the work of the Carbon Cycle Institute in California. Better yet, come to the CCC Annual Conference in Montreal in September and hear the Institute'sTorri Estrada and Dr. Phil Creque talk about how important compost can be in fighting climate change. - False. Adding compost can actually increase the rate at which CO2 is released from soil, because of the stimulatory effects of compost on the soil microbiota. However, as discussed in the previous point, these soils still retain more carbon after compost addition, simply because of the greatly increased amounts of carbon added to the soil. Retained carbon = carbon in - carbon out. In the case of compost, the carbon coming in increases by more than the carbon going out, resulting in a net gain. - True. Because compost stimulates the soil food web, soils with added carbon end up with better structure (more stable soil aggregates). This means more pore space, which in turn means better rainfall infiltration rates and better water holding capacity. This capacity to allow more infiltration reduces flooding during heavy rainfalls; the higher water holding capacity increases drought resistance. - False. The composting process releases a number of greenhouse gases. However, if the process is done properly, the majority of these gases are CO2. Although CO2 is a GHG, its release in composting is not considered to be an addition to the GHGs in the atmosphere, because it is just part of the natural carbon cycle. On the other hand, if the composting process is not managed well, it can release methane and nitrous oxide as well as CO2. Overall, however, most scientists consider the composting process to be fairly close to neutral in terms of net emissions (however, this does not count equipment use, transportation, etc.). - True. Nitrogen fertilizers can create nitrous oxide emissions if not taken up by the crop. By using compost along with fertilizer, the risk of these emissions can be reduced.

- Carbon is the basis for life because it is present in every organic molecule, from simple sugars, to more complex proteins and enzymes, to the most complex humic molecules. Because photosynthesis by plants can convert carbon dioxide (carbon's gaseous form) and water into sugars, the sun's energy is converted into a biochemical form and life is possible. - Nitrogen is an essential plant nutrient (e.g., used for proteins, amino acids) that is not originally derived from the minerals in soil (as are all the others, including potassium, phosphorous, boron, etc.). Instead, it comes from nitrogen gas in the atmosphere. It is "fixed" in biochemical form by certain types of soil microbes. (Of course, once fixed, it becomes available in soil through the breakdown of organic matter). - Carbon dioxide is the most common greenhouse gas (GHG), but not the most powerful. Methane is about 25 times more potent and nuitrous oxide ia bout 295 times more powerful. Fortunately, these gases are not released in nearly as large quantities as carbon dioxide. - Nitrous oxide, or N2O, is often released by soils in small quantities under certain conditions, and can be minimized by careful use of N fertilizers. - Methane (CH4) is released under conditions where oxygen is limited and organic matter breaks down anaerobically. It is 25 times as potent a GHG as CO2. - Soil organic matter (SOM) is about 58 per cent carbon, on average. It consists primarily of the remains of living creatures, their waste products, and carbon-rich exudates from plant roots. These materials are used and re-used by the organisms of the soil food web for their metabolism, to provide energy, and to build soil structure. Most of it is converted back to CO2 and released from the soil into the atmosphere, as part of the carbon cycle. However, some is converted into recalcitrant (difficult to degrade) forms, and some is protected from degradation, either chemically or physically (inside soil aggegates). These recalcitrant and protected forms of SOM can stay in soil for decades or longer and are the basis for soil carbon sequestration. If you would like to learn more about how the soil food web works to build soils, consider attending the Council's Annual Conference in Montreal. Odette Menard, from the Quebec Ministry of Agriculture, will be presenting a plenary talk on healthy soils for a healthy planet.

1 True. Come to the Council's Annual Conference in Montreal and hear how California has made compost application an important part of their fight against climate change, including carbon-offset payments for application of compost to croplands and rangelands. 2 True. As compost matures, its nutrients get tied up in more complex molecules. This is why mature compost is more stable. Fungi are better able to feed on these complex humic molecules than are bacteria, so the microbial composition changes to become fungal dominated over time. This has implications for how the compost should be used. At the Montreal Conference, Vivian Kaloxylos of DocTerre labs will explain the importance of the ratio of fungi to bacteria in compost and in soils. 3 False. Immature compost has more plant-available nutrients, because it has not yet been fully stabilized. Many farmers prefer immature compost for the quick nutrient hit it provides. However, more mature compost has many extra benefits, including long-term slow release of nutrients and higher levels of carbon sequestration (this is due to the higher fungal populations, which are vital to good soil aggregation). 4 False. Studies have shown that adding compost sequesters carbon in soils at a faster rate than does adding raw manure. 5 True. Although these numbers can vary, depending on the type and age of the compost. These figures would be representative of a mature compost with a C:N ratio higher than 11. Composts with a lower C:N ratio than 11 would release N at about double that rate, on average. 6 True. Standard soil lab tests only tell you what is either plant available, (that is, in solution), or in an "exchangeable" state (that is, easily released into solution). Many nutrients are more permanently tied up in organic molecules or in minerals. An active soil food web can release these nutrients over time. 7 False. Up to a point, the opposite is true. As carbon levels rise in soil, the soil food web increases in number and diversity. Since it is the food web that creates the ideal conditions for carbon to be sequestered, higher populations of microbes usually results in greater sequestration. However, that does not mean that this increased ability to absorb carbon will last forever. There is some evidence that it gets harder to sequester carbon after the level in soil reaches 4 per cent. In addition, at some point a soil will reach an equilibrium point, where C is respired as fast as it is sequestered, and no further increase in C will occur. This level will be different for differnt soil types, with clay soils having the highest capacity to sequester carbon, and sandy soils the lowest capacity. However, most agricultural soils worldwide have lost so much carbon since the introduction of the plough, some 10,000 years ago, that they could gain carbon at a substantial rate for many decades before reaching their equilibrium point.