Richard Freeman, Designed Ecosystems
Presentation #3 of the Eco Garden Series
Hi, my name is Richard Freeman. The below text is the substantive basis for a live presentation I give to public audiences. During these live presentations, I summarize much of the text, limiting discussion of key principles to two or three sentences, unless the situation calls for more detail. During subsequent Question-and-Answer sessions, I often go into a topic further when it seems pertinent.
My passion is designing and teaching about ecological horticulture. I design and teach about systems that produce abundantly, conserve resources, streamline workflow, and use common, affordable technology. I rely on ecological and permaculture principles to guide my design and teaching.
In this brief presentation, I introduce some easy-to-adapt gardening and farming practices that will conserve and increase (sequester) soil carbon in the short-term and long term.
The presentation will following this sequence:
1. I’ll begin by discussing some key principles.
2. Then I’ll discuss management objectives that reflect these principles.
3. Next, I’ll outline some easy-to-do practices for implementing these objectives.
4. I’ll conclude by tying this presentation into other presentations I will be offering to the public.
Soil Carbon and Soil Organic Matter.Soil organic matter (SOM), includes the entire range of carbon-based material found in the soil profile, ranging from intact, living organisms to the smallest organic materials resulting from decomposition of tissue.
“Soil organic matter contains more organic carbon than global vegetation and the atmosphere combined” according to soil scientist Johannes Lehmann.
Aside from the benefits of increasing soil carbon in the context of global warming and climate change, soil organic matter is fundamentally vital and necessary for growing nutritious food in the short-term and long-term. It confers many benefits – from water and resource conservation to enhanced nutrient cycling – so sequestering soil organic matter is an important goal.
Some forms of soil organic matter can take millenia to decompose, while others can decompose in one to five years. The term labile denotes rapidly decomposable organic matter. The term recalcitrant denotes slowly decomposable organic matter. As bacteria and fungi decompose soil organic matter, they oxidize it into CO.
Compost, leaf litter and short-lived micro-organisms are examples of labile SOM.
Living organisms, large dead tree roots, and char are examples of recalcitrant SOM. Both forms are important to soil ecology.
Conservation-Sequestration Continuum. Managing soil organic matter involves simultaneously conserving and sequestering soil organic matter. Sequestering organic matter in practical terms means increasing the mass of organic matter over a defined area – a garden for example. Conserving soil organic matter means slowing the decomposition of organic matter over that area. Sequestering AND conserving soil organic matter results in a net increase. Most horticultural practices that conserve soil organic matter also help to sequester it, and vice-versa.
Biodiversity. Another fundamental concept is biodiversity, short for biological diversity. Biodiversity describes the diversity of the organisms in an ecosystem. Ecologists describe three types of biodiversity, including diversity of composition, diversity of structure and diversity of function.
The term composition refers to the species and other taxonomic groups that are present in an ecosystem. Taxa, short for taxonomic groups, include individual species, genera and families, all the way to kingdoms and domains. The term composition also encompasses the population patterns of the taxa that are present. So, compositional diversity refers to the diversity of species, families, etc. in an ecosystem.
The term structure describes the spatial and temporal distribution of organisms and taxa in terms of volume, mass and shape. So, structural diversity refers to diversity of organisms and taxa in physical, spatial terms.
The term function describes the biological processes at work. Functions contribute to net productivity in an ecosystem. Functions are equivalent to the effects that organisms have on each other and how these effects determine productivity. One species can affect another species or it can affect an entire family and so on. Likewise, a family can affect another species or another family, and so on.
Organisms can affect other organisms in three main ways: by influencing behavior, physiology and/or metabolism. In combination, these effects, or functions, determine the net productivity of the entire ecosystem.
So, functional diversity refers to diversity of functions that contribute to productivity in the ecosystem.
Biodiversity is key to effective horticulture, even at a small scale. The most robust, resilient ecosystems exhibit high diversity in function, structure and composition.
Especially with the smallest organisms – from microbes to insects – more species representation is better than less. The vast majority of the smaller organisms are beneficial or neutral to gardens and crops. A biodiverse ecosystem with high species representation will usually deter the small minority of pest species that are present and keep damage within a management comfort zone.
Succession & Disturbance. In ecology and ecological design, Succession and Disturbance are important concepts. Succession is equivalent to the maturation of an ecosystem. In this process, shade tolerant plants gradually supplant sun-loving plants.
During succession, several patterns emerge that are beneficial for horticulture:
1. An increase in long-term soil organic matter.
2. An increase in microbial networking and resource transport and exchange;
3. An increase in biodiversity towards the maximum possible diversity on a site.
Long-term soil organic matter increases due to three factors:
1. Increased perennial root mass;
2. Accumulation of dead root mass, and
3. Increasingly intensive microbial networking.
Live root mass is a major component of soil organic matter. It varies widely between ecosystems. As a percentage of total dry plant matter, it can typically range from 21% (boreal and temperate forests) to 49% (shrublands) and higher (xeric and desert ecotypes).
Dead root mass is also a major component of soil organic matter. As large plants die, their dead root mass increases long-term organic matter and contributes resources to the plants that emerge to replace them.
Microbial networking, which increases soil organic matter, is also its own benefit. Microbial networking links organisms into resource transactions and optimizes resource use. Resource loss from the ecosystem decreases.
Meanwhile, plant diversity increases, driving diversity of other organisms, which depend upon plants.
An ecosystem will proceed through succession until one of two states emerge.
1. In one state, ecosystems tend towards a climax stage. In a climax ecosystem, the most shade-tolerant plants that can grow on the site’s conditions dominate plant composition.
2. In the other state, disturbance reduces vegetation and increases sunlight to the forest floor. Thus, after a major disturbance, pioneer species emerge. Pioneer species require full sun for vigor and they can tolerate soils with relatively less organic material. Examples of disturbances include fire, flood, logging and harvesting.
Rhizosphere. Another important concept is the rhizosphere. The rhizosphere is the soil environment immediately surrounding the root. The rhizosphere’s physical, chemical and biological properties determine the vigor and survival of the plant.
Soil Food Web. The Soil Food Web, another fundamental concept, describes the biology of the rhizosphere.The soil food web is a community of interdependent organisms making up the soil ecology.
In the soil food web, plants transform sunlight and nutrients into plant tissue and fluids (or solutions), which are energy-rich food for a multitude of organisms. Plants exude fluids into the rhizosphere in the form of root exudates. These protein- and sugar-rich solutions provide a major food source for soil organisms. They commonly contain enzymes, hormones, proteins, glyoproteins, sugars, starches, minerals, and other useful food resources.
Among the organisms that feed off root exudates are symbiotic bacteria and fungi. These organisms, in turn, provide the plants with minerals and compounds, for example plant growth-promoting hormones. Often, bacteria and/or fungi live within the plant’s tissue in a symbiotic relationship.
Other organisms that feed off exudates and plant tissues are pathogenic, or disease-causing organisms.
In turn, other microorganisms, the saprophytes, feed off dead plant and animal tissue.
All these micro-organisms, in turn, become food for protozoa, nematodes, other bacteria and fungi and a multitude of other predators. These predator organisms release nutrients as they digest their prey. In turn, these organisms become food for yet other organisms – more nematodes, micro-arthropods, worms, tardigrades and a multitude of others. All these organisms release nutrients as they digest their prey.
The soil food web is key to maintaining robust nutrient-cycling and to averting pest problems. Furthermore, it constitutes a huge portion of the soil organic matter and is responsible for soil aggregation, fundamental to soil health. A biodiverse soil food web builds on itself by establishing the conditions for increased productivity. Productivity is measured in terms of living tissue and/or agricultural crops. Soil food webs become more complex, massive and robust as ecosystems mature.
Goals for Increasing Soil Carbon by Gardening
1. Maximize soil biodiversity. Maximizing soil biodiversity in structure, function and composition is fundamentally important. Emphasize building fungi and fungal mycelia.
2. Increase live root mass. Almost one half a tree’s total dry weight is root mass, and shrubs are about the same. So, growing woody perennials transforms atmospheric CO2 into living organic matter, much of it in the soil. In addition, the live roots increase the soil food microorganisms.
3. Conserve and increase buried wood and char. Sub-surface char and wood creates recalcitrant soil organic matter, which holds water and provides resources and niches for soil micro-organisms.
4. Conserve and increase stores of labile carbon. Soil stores plenty of labile carbon bound to minerals, which is a source of nutrients – for example compost. These carbon molecules are easily oxidized into CO2. But, in the soil, they provide resources and services to the ecosystem.
Horticultural Practices that Maximize SOM. Several of these practices support numerous objectives for increasing SOM:
1. Use live, biological fertilizers, and minimize fertilizer salts. Plants directly uptake the salts, temporarily bypassing the symbiotic organisms. Without food, these microbes then dwindle in population, in turn reducing the mass of organisms that feed on them.
2. Minimize soil disturbance, especially tilling and compacting. Tilling soil disrupts fungal mycelia, consequently favoring a less diverse, bacteria-dominated soil environment. The resulting bacteria-dominated soil rapidly oxidizes labile carbon – small organic particles – into CO2.
3. Maximize plant diversity. Diverse plants attract and encourage diverse soil organisms as well as encouraging above-ground, pest-suppressing arthropods.
4. Eliminate or minimize pesticide use. Pesticides kill soil organisms, most of which are beneficial. Killing beneficial organisms further encourages pests, which often evolve to become resistant to the pesticide.
5. Cultivate semi-wild mushrooms. Fungal mycelia makes excellent SOM and becomes food for a variety of organisms. In addition, mushrooms can be nutritious, high-quality sources of food and medicine for people.
6. Bury wood. A judicious use of underground dead wood in a garden can improve water-holding and build SOM.
7. Bury char. Char confers a multitude of benefits that contribute to soil productivity and health. Whenever tilling, one should introduce char. It can also be introduced in holes dug for transplants and perennials. Adding char to compost will improve its benefits and the compost will last longer. Charring should replace any burning.
Increasing SOM is an important part of any gardening project. It creates the necessary environment for sustaining a healthy, productive, clean and sustainable garden. From this foundation, one can go in many directions ranging from annuals-intensive cropping to annual/perennial mixes to full-fledged forest gardens. Several of my presentations focus on various topics directly pertaining to ecological gardening, including ecological pest management, eco-intensive gardening and perennial polycultures.
Lehmann, Johannes, Markus Kleber. 2015. “The contentious nature of soil organic matter.” Nature. Published on-line.