The Roots of Sustainability: Engaging the Soil Carbon Solution
Introduction: Storing Carbon in Soils
Earth’s atmosphere contains too much carbon. The massive burning of fossil fuels has dumped carbon – as carbon dioxide (CO2) – into the atmosphere faster than nature can remove it. As a result, scientists warn of longterm impacts due to changing temperature and moisture levels, including (but not limited to): more frequent and severe droughts and extreme weather events; loss of freshwater supplies; rising ocean levels; and increasing threats to natural ecosystems and agriculture. Current volumes of atmospheric carbon pose a big problem.
Soils, on the other hand, contain too little carbon. Soil organic matter (SOM) is derived from the secretions and remains of plants and animals and is composed of about 57 per cent carbon. SOM has been decreasing since the beginning of agriculture more than 10,000 years ago. The advent of modern agricultural and soil management methods has exacerbated this problem – the world’s farmlands are estimated to have lost between 30 and 75 per cent of their original carbon. Conventional tilling releases carbon to the atmosphere; using inorganic fertilizers (rather than manure or compost) reduces the rate at which carbon is replaced; and leaving soils exposed to wind and rain increases erosion. Soils need organic matter in order to remain fertile, and people need fertile soils to produce food. Too little SOM is a big problem.
These two big problems, however, may share a common solution. Many soil scientists maintain that it is feasible to remove much of the extra carbon from the atmosphere and put it back into the soil, where it does more good than harm. To shed some light on this intriguing debate, the ECO looked at two basic components of the issue:
- the overall benefits of increasing soil organic matter; and
- the options potentially appropriate for use in Ontario.
Benefits of Increasing Soil Organic Matter
Sequestering carbon in soil removes it from the atmosphere and helps mitigate climate change. This is the most obvious benefit. Building soil organic matter, however, is worthwhile regardless of the climate change implications, since there are many other benefits that result from the subsequent improvements in soil health.
All healthy soils teem with life – over one-quarter of Earth’s living species make soils their home. At the base of this soil food web are bacteria and fungi. These organisms are so numerous and diverse that within a single teaspoon of good soil there can be found thousands of bacterial species, including millions of individuals, and more than a hundred metres of microscopic fungal threads known as hyphae. This web of life – which in a typical farmer’s field or woodland includes many other larger organisms, such as mites, beetles, earthworms and moles – performs many beneficial soil functions upon which all above-ground life depends.
Soil organic matter supports soil life by providing microbes with both habitat and nourishment. As SOM increases in soil, so do the number and variety of organisms in the food web. With this increased variety comes increased benefits, including:
- Clean water: Healthy, biologically diverse soils filter contaminants out of water, often reducing them to harmless by-products. In addition, healthy soils retain more nutrients, reducing the run-off of fertilizer and raw manure that pollutes rivers and lakes.
- Reduced flooding: SOM helps soils hold more water. This can reduce seasonal flooding because the soils “suck up” the water, rather than letting it run over the surface to flood inland areas. For instance, a 2 per cent increase in the SOM of the cultivated cropland around the Red River could provide 23 per cent of the water storage required to prevent flooding.
- Increased agricultural productivity: High-SOM soils require less irrigation and resist drought better than low-SOM soils. The higher numbers and diversity of beneficial microbes also help crops to resist diseases and pests, reducing the need for pesticides. Moreover, higher SOM levels increase yields – adding one tonne of carbon per hectare to a degraded soil increases wheat yield by 20 to 40 kilograms per hectare.
- Enhancement of biodiversity: SOM protects wildlife in at least two important ways. First, the reduced chemical run-off from farms results in less toxicity in the natural environment. Second, increased soil life
provides a greater quantity and variety of food for the life above ground. This enhanced “food chain” can support more overall biodiversity.
Unfortunately, these soil functions are generally invisible to the casual observer and their benefits are taken for granted. For instance, when thinking of biodiversity loss, one tends to think of a decline in the number of larger, more visible species, such as birds and mammals, rarely considering the potentially negative impacts that human activities have on the myriad of microscopic soil species. Without this vital foundation, however, the complex food chain that is necessary to support high levels of diversity among plants and animals could not exist.
Methods of Increasing Soil Organic Matter
Both soil type and climate affect the efficacy of methods for increasing SOM, so it is not a “one-solution-fits-all” issue. Nevertheless, several recurring themes appear extremely relevant to Ontario.
Sustainable Agricultural Practices Generally Build Soil Organic Matter
One of the proven ways to build SOM is to farm organically. Although the reasons for this are not yet fully understood, it is likely because organic farming practices encourage soil biodiversity. For example, organic fertilizers function differently than inorganic or chemical fertilizers. Many of these substances (e.g., manure, seaweed, compost, etc.) add organic matter, but that is only part of the story. Plants can only take in nutrients that are dissolved in water. For the nutrients in organic fertilizers to be made available to plants, they must be first consumed by microbes. In turn, the microbes gradually release the nutrients in plant-available form, primarily in the root zone. Thus organic fertilizers strengthen the natural nutrient cycling process and increase microbial diversity. Commercial inorganic fertilizers, on the other hand, are already water-soluble. This means that they provide nutrients directly to the plants, by-passing much of the microbial participation. Moreover, recent evidence suggests that the readily available nitrogen in inorganic fertilizers encourages the growth of a more limited range of microbes (decomposer bacteria specifically), so that more of the remaining organic matter in the soil is converted to CO2 and lost to the atmosphere.
The key is diversity. The higher levels of microbial diversity in organically managed soils build SOM through a variety of mechanisms. First, not all organic material added to soil degrades immediately; some is converted into a stable substance known as humus, which can remain in the soil for decades. This work is done more slowly by a variety of organisms, not simply the bacterial decomposers. Second, bacteria and fungi work together to build what are called soil aggregates, or tiny clumps of soil, which not only improve soil structure, but also protect any carbon trapped inside the aggregates from further degradation. The role of fungi appears to be particularly important. Studies have shown a direct correlation between the degree of soil aggregation and fungal abundance, as well as between both of these and the level of soil carbon. Finally, well-aggregated soil is less prone to compaction, which allows plant roots to grow deeper, thus sequestering more carbon at lower levels of the soil.
In the U.S., the Rodale Institute has been running a side-by-side comparison of organic and conventional agriculture for almost three decades and has demonstrated clearly that organic farming builds SOM. Over 27 years, a 30 per cent increase was recorded in organic plots compared to no significant increase over the same period in conventional plots.
Conventional farmers can also build SOM, without completely converting to organic methods. Options that have worked well in areas with soils and climate similar to Ontario include the use of cover crops, particularly green manures (i.e., the cover crop is incorporated into the soil while still green or shortly after flowering), and the use of well-designed crop rotations. The latter can be particularly effective if perennial grasses are included, a practice known as ley farming (see below). No-till or reduced tillage practices, on the other hand, do not seem to be effective for carbon sequestration in Ontario, possibly because of our relatively wet temperate climate. In general, however, the most effective method for building SOM in conventional farming is replacing some or all of the inorganic fertilizers with organic materials, such as manure or compost.
Energy from Biomass, if Done Right, Can Build Soil Organic Matter
The growing demand for green energy is creating a growing demand for organic materials (such as municipal food and yard wastes and agricultural residues) for use as fuels. However, as more organic residues are collected and converted into energy, Ontario’s soils will lose a major source of organic matter. Fortunately, alternatives exist that can both produce energy and build SOM. Examples include anaerobic digestion, where residuals (such as manure and food wastes) are used to generate methane gas for power and the solid by-products can be applied directly to agricultural land or composted. Another example is called pyrolysis, where the waste is converted into bio-oils, gas and biochar.
Potential for building soil organic matter also exists with purpose-grown energy crops. Perennial grasses, such as switchgrass, can sequester carbon in soil even when the above-ground portion is regularly harvested. Their deep root systems are part of the story. The other part is that they encourage the growth of mycorrhizae, a class of fungi that co-exists in a symbiotic relationship with plant roots. Plants deliver up to 30 per cent of the carbon produced through photosynthesis to mycorrhizal fungi via root secretions. In return, the fungi provide plants with moisture and nutrients found in areas of the soil the larger diameter plant roots cannot penetrate. Much of the carbon stays in the soil, raising SOM levels and increasing soil fertility. Prior to the widespread adoption of inorganic fertilizers, farmers often included perennial grass pastures (leys) of several years duration in their crop rotations, in order to restore SOM and increase fertility. Perhaps the rise in demand for biomass energy could see that practice revived.
Compost and Biochar, Working Together, May be the Fastest Way to Build Soil Organic Matter
The ECO reported on biochar (charcoal made from biomass) in our 2009/2010 Annual Report and our 2011 Annual Greenhouse Gas Progress Report. Many people are excited about biochar’s ability to sequester large amounts of carbon very quickly. It is comprised of a very high percentage of carbon (between 25 and 95 per cent, depending on original feedstocks) that is extremely resistant to degradation. This form can persist in the soil for centuries or longer.
Soil organic matter delivers many of its benefits by providing food for soil life, with the SOM gradually being consumed by microbes. Accordingly, management changes (e.g., additions of manure, cover cropping, etc.) have to be maintained indefinitely in order to sustain a higher SOM level. With biochar, however, the benefits to the microbes do not come from its consumption; rather, they come from its function as an ideal microbe habitat. The biochar is not consumed, which means that increases in SOM produced by one-time additions of biochar can last for extended periods. Moreover, because biochar provides such good living and working conditions for microbes, the microbe-mediated SOM-building processes described above are also enhanced.
Research on biochar and its value to agriculture is increasing rapidly all over the world, including Ontario. While still in the early stage, some research suggests that biochar works best when combined with well-made compost. This is probably because compost provides the nutrients and microbial inoculant necessary to charge the biochar, which has little nutrient content and no microbial life of its own. Biochar can also be used with inorganic fertilizers. Early research indicates that fertilizer requirements in conventional agriculture could be greatly reduced through the application of biochar, at the same time that SOM levels are increased.
Finally, applying compost by itself also increases SOM. For every dry-weight tonne of compost applied to land, approximately one-half tonne of CO2 is removed from the atmosphere. This has positive implications for municipal waste management, as well as for the ecological management of gardens, grass and turf, where compost could routinely replace inorganic fertilizers. The ECO discussed many other benefits of compost in our 2009/2010 Annual Report.
Soil issues consistently fly beneath society’s radar. Yet soil-related concerns, not the least of which is food security, are of fundamental importance. Ontario has a few programs designed to promote and financially support agricultural best management practices, such as cover cropping, composting, no-till and enhanced pasture management. In addition, the province recently embarked on a small project to investigate ways to accurately measure soil health, using a variety of physical, chemical and biological parameters. These measures constitute a modest beginning; however, they fall far short of the full engagement that is necessary. In particular, the government does not seem to be interested in developing the protocols necessary to allow Ontario farmers to be compensated for building the SOM that benefits society at large.
The ECO recognizes the technical, political and logistical challenges inherent in sequestering carbon in Ontario’s soils. Some significant unresolved issues remain, particularly in the areas of measurement and permanence. Nevertheless, given the numerous environmental benefits that would accrue, and given the potential economic value of these increases to both Ontario farmers and to the public, the ECO strongly encourages the Ministry of Agriculture, Food and Rural Affairs to expeditiously institute a program to evaluate and then develop protocols for the most promising methods for increasing and enhancing organic matter in Ontario soils. This program would have the added advantage of providing the Ontario government with the framework and resources necessary to act on the recommendation made in the ECO’s 2011 Annual Greenhouse Gas Progress Report to investigate and publicly report on the potential for soil carbon sequestration as a greenhouse gas mitigation strategy.
For ministry comments, please see Appendix C.
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|This is an article from the 2010/11 Annual Report to the Legislature from the Environmental Commissioner of Ontario.|
Citing This Article:
Environmental Commissioner of Ontario. 2011. "The Roots of Sustainability: Engaging the Soil Carbon Solution." Engaging Solutions, ECO Annual Report, 2010/11. Toronto: The Queen's Printer for Ontario. 108-112.