Soils

This compendium is, if nothing else, a testament to the key role soils must play if we are to preserve life on earth through the anthropocene. Soils, the engine of every terrestrial ecosystem, are themselves wildly diverse subterranean ecosystems providing habitat to countless trillions of micro- and macro-organisms. These organisms themselves create the soil and its functionality by ingesting dead organic matter, thereby breaking it down so that nutrients become available to plants which in turn feed everything else up the food chain.  In the everyday processes of foraging, reproducing, exchanging, growing, and dying, the biology upon and within the soil regulates the climate by consuming, transforming, and burying molecules of carbon. This buried and embodied carbon would otherwise be atmospheric carbon dioxide. Thus, it’s not the soil as an apparently homogenous substrate that concerns us. It is the living, biodiverse soil ecosystem that matters for maintaining a global climate system hospitable to human life. In this section, we discuss the contents, processes and functions of the soil and the threats to it, with an eye to highlighting the absolutely critical role of soil biology.

Overview

A noteworthy perspective is that a teaspoon of healthy soil holds more microorganisms than there are people on earth. The biodiversity in that teaspoon rivals that of the Amazon rainforest. In fact, it’s these very microorganisms (along with their larger counterparts, like earthworms and mammals) that themselves create the content and structure of soil by breaking down organic material, secreting sticky biomolecules, and burrowing, while also depending on the soil for habitat and food. Because of the interrelatedness of diverse and vital roles played by soil organisms (roles such as fixing nitrogen, suppressing disease, creating channels for water and root penetration, and forming soil aggregates, for example), the soil as a whole could be viewed as a macro-organism, made up of myriad “organs” working together and becoming sick or healthy as if it were a living body (within a field, for instance) [Magdoff 2009].

Furthermore, the particular community composition of microorganisms in a given soil can affect the overall functioning of the soil. It is generally understood that high concentrations of fungi are associated with healthy soil, while soils regularly disturbed by tillage tend to be bacteria-dominant [Magdoff 2009]. Fungal populations are also associated with increased carbon sequestration.

Kallenbach et al [2016] demonstrate that distinct microbial communities, especially those with high fungal concentrations, are a better predictor than clay mineral content of Soil Organic Carbon (SOC) production. Morrien et al. [2016: 1] show “that during nature restoration the efficiency of nutrient cycling and carbon uptake can increase by a shift in fungal composition and/or fungal activity.” Specifically, the proportion of arbuscular mycorrhizal fungi^[5] increases over time leading to greater carbon uptake by fungi, “without an increase in fungal biomass or shift in bacterial-to-fungal ratio.” Recent research by David Johnson of New Mexico State University shows that high fungal to bacterial ratios are correlated with strikingly high carbon sequestration and high plant productivity [Johnson n.d.; Johnson 2017] (see also Croplands section).

Due to the exceptional scope and complexity of its biological activity, soil is a major player in the self-regulating system of Earth’s environment by taking up and releasing large quantities of carbon dioxide, oxygen, methane and other gases [Brady 2008]. Indeed, it is the living organisms on and in the soil that breathe these gases in and out. Under favorable conditions balance is maintained due in part to soil organisms holding carbon in place, both within their own biomass and through ongoing carbon-rich soil creation.

Soils store more carbon than is found in the atmosphere and terrestrial vegetation combined [FAO 2017a]. The capacity for soils to store carbon depends on various factors, such as temperature, moisture level, soil type, and topography. Carbon-rich peatland and organic soil^[6], which occupy 3% of land but contain an estimated 30% of the world’s soil carbon, are commonly located in cold environments or rainy, humid environments, where productivity is high and decomposition slow. Another third of the world’s soil carbon is in drylands (areas where potential evaporation/transpiration exceeds average rainfall) which cover approximately 40% of Earth’s land area, but tend toward lower productivity, slower carbon accumulation, and susceptibility to erosion when mismanaged [FAO 2017b].

Ecological processes have maintained a balanced carbon cycle over time, keeping the Earth at a relatively stable temperature for hundreds of thousand of years. Although there have been temperature fluctuations before, it is virtually certain that human activity (mainly burning fossil fuels, desertifying and clearing land) is upsetting the carbon cycle, causing atmospheric concentrations of heat-trapping carbon dioxide to have risen from about 280 parts per million (ppm) prior to the 19th century to more than 400 ppm today [NASA 2017].  This, in turn, is changing the temperature at a faster rate than at any time in at least 1,000 years [Smith 2015]. Since the start of industrial era, carbon emissions from the soil due to deforestation and land conversion is estimated to be 136 Gt (Gigatons, or one billion tons), a little less than half the amount of carbon emissions from fossil fuel combustion over the same time period [Lal 2016]. Estimates of carbon lost to the atmosphere from land use since the beginning of agriculture range as high as 537 Gt [Buringh 1984: 91].

It is important to note that soil that has been greatly depleted of carbon can become a carbon sink when managed according to ecological/restorative principles. A carbon sink is anything that absorbs more carbon than it releases as CO2. “Soils that are depleted of SOC have the greatest potential to gain carbon, but also the least propensity to do so.” [FAO 2017b: 7] This is because, while depleted soils can theoretically regain the entire massive amount of carbon they’ve lost, they first need to be biologically re-activated through restorative management practices. It is vital to note that degraded lands (and all lands) have this capacity for renewal as the global community seeks both to reduce CO2 emissions and to draw down excess atmospheric CO2 as quickly as possible. Conversely, organic soils, forests, and other intact ecosystems with large existing stores of carbon in the soil and vegetation have the potential to become new sources of emissions, and must therefore be properly managed and preserved.

According to the Rodale Institute, the fastest, least expensive and most efficient way to rebuild agricultural soils is through “regenerative organic agriculture,” designed to bring carbon and biodiversity back into the soils.  

Recent data from farming systems and pasture trials around the globe show that we could sequester more than 100% of current annual CO2 emissions with a switch to widely available and inexpensive organic management practices, which we term “regenerative organic agriculture.” These practices work to maximize carbon fixation while minimizing the loss of that carbon once returned to the soil, reversing the greenhouse effect.  [Rodale Institute 2014: 2] (See more in Croplands section.)

Management practices for building carbon in soils involve increasing input and reducing losses of soil organic matter (SOM). SOM is made up of fungi, bacteria, countless microorganisms, decaying plant matter, decomposing animals, and products formed from their decomposition. SOM also contains humus, an organic material believed to contain stable forms of organic carbon. However, the inherent resistance of humus and other compounds to decomposition and release of carbon dioxide into the atmosphere has recently come into question, as the stability of soil compounds may be an ecosystem property and not an intrinsic property of the compound itself [Schmidt 2011].  Thus the understanding of soils is transformed from a consideration of properties of isolated variables to properties of the system as a whole, with its exceedingly complex interactions, a transformation considered throughout this Compendium.

In sum, SOM provides food for soil organisms and plants, enhances microbial biodiversity, creates pore space, increases cation-exchange capacity (CEC)^[7], and increases buffering capacity (ability to resist change in acidity or pH). All of these factors affect the soil’s ability to hold carbon [Fenton 2008].

Soil organic carbon (SOC) levels are directly related to the amount of SOM. The key factors in SOC levels are photosynthesis, respiration and decomposition. Photosynthesis is the process used by plants to harness energy from sunlight, combined with the CO2 they take from the air and water from the ground, and convert it into energy storage (in the form of sugars) for their own growth and survival. Respiration is the measure of CO2 released from soil microbes and plant roots into the surrounding soil.

Soil C results both directly from growth and death of plant roots and above-ground biomass, as well as indirectly from transfer of carbon-rich compounds from roots to soil microbes. The roots of almost all plants form symbiotic associations with arbuscular mycorrhizal fungi – the roots provide the fungi with energy in the form of carbon while the fungi provide the plant with nutrients. While decomposition of biomass by soil microbes results in carbon loss as CO2 from the soil due to respiration, a proportion of the original carbon is retained in the soil through the formation of numerous stable soil carbon molecules. Carbon is also stored in soil aggregates, which are formed in collaboration with microorganisms.

The multiple soil organisms providing vital ecosystem functions are sensitive to various practices and changing conditions, even those occurring above the soil surface. Applying synthetic nitrogen fertilizer, for example, can affect soil pH, which in turn can negatively affect soil organisms. Organic amendments such as manure and compost, on the other hand, provide direct carbon sources for soil organisms and an indirect carbon source via increased plant growth without negative side effects.

Temperature also affects soil microbes. For example, the scottnema lindsayae nematode cannot survive in its native habitat in Antarctica if temperatures become too warm or too cold, and research suggests that a 65% decline in activity of these nematodes over 12 years could lead to a 30% reduction of carbon cycling in soils [Wall 2014]. Similarly, earthworms, described by Darwin [1881] as “nature’s plow,” are damaged both by cultivation and by the use of toxic herbicides and other agrichemicals often used in “conventional” no-till.

A healthy biosphere has the power to draw down carbon and store water and through this support biodiverse life forms, including humans. Most importantly, in partnership with nature, humanity can restore ecosystems, including agro-ecosystems^[8], to create a functional global biosphere once again. This begins with the soil, and requires at once protecting intact organic soils and their invaluable concentrations of stored carbon, and restoring carbon-depleted mineral soils so as to activate their capacity as massive carbon sinks. Remarkably, taking action to protect and rebuild soil is almost universally available, low-cost, safe, and has the power to mitigate and reverse climate change in a relatively short period of time.  

Eco-restoration has numerous co-benefits such as restoring abundant food production, eliminating floods and droughts, restoring water supplies, building strong local economies and providing shade, beauty, and recreation.

Soil Article Summaries

Crowther et al. 2016. When this paper first appeared in Nature, it seemed to raise serious objections to the hypothesis that soils as carbon sinks could have a significant impact on climate. On further examination, however, it may be more about the perils of isolating variables than about the limitations of soils.

There is growing confidence that warming generally enhances fluxes to and from the soil, but the net global balance between these responses remains uncertain [Crowther 2016: 104].

The authors analyze data on the soil’s response to warming from 49 experimental sites in North America, Europe and Asia, across six biomes. They find that the level of carbon loss from the soil is contingent upon the size of the soil’s original carbon stock, and on the duration and extent of the warming. Extrapolating to a global scale, they estimate that an additional 1oC temperature rise will release anywhere from 30 ± 30 to 203 ± 161 of carbon Gt from the soil.

The authors themselves highlight several limitations in their analysis due to lack of data, including from experimental sites in the tropics, from soil at depths greater than 10cm, and on the effects from biotic responses to warming. It is noteworthy that, as the authors state:

Our current understanding of global feedbacks is dominated by the physical sciences, but changes in the physiology and community compositions of organisms have been shown to have strong effects on the strength of this feedback [107].

In other words, for a more precise prediction of how global warming will affect the soil’s net release of carbon, we need to understand better how plants, animals and microbes may interact and respond to that same warming with respect to their effect on the land-carbon climate feedback.

Like Amundson et al., these authors make projections about the soil’s response to climate change without consideration of the wide spectrum of possible land management practices, ranging from clearcutting and urbanization to reforestation and Holistic Management. Presumably, a more accurate picture of the soil’s response to future warming must incorporate measurements of the soil’s resilience capability, based on its level of exposure to oxidative or erosive elements vs. embeddedness in intact ecosystems.

Grindrod 2017.  A brief review for non-scientists of soil microbiology and the growing understanding that soil health is, in many ways, all about microbes and their complex interactions with mineral, plants and animals.  It likely that micro-organisms are the key to soil fertility as a result of the universe of microbial chemistry that affects, among many things, availability of nutrients and plant health, rendering other forms of dangerous inputs such as biocides unnecessary.  See also David Johnson [2017].

Hart 2015. In a year of severe heat and drought, cover crops and no-till proved their value for North Carolina Farmer Russell Hedrick, reports Southeast Farm Press. In a county where average SOM is 1.5%, Hedrick’s farm has 4.8% SOM after just four years of no-till and cover crops, which have increased the water infiltration in the soil while also cooling the soil surface as much as 20 degrees F. Hedrick also introduced cattle into his system to graze on the cover crops, a strategy his NRCS^[9] advisor told him he “couldn’t afford not to do,” due to the ruminants’ stimulating effects on the life of the soil. During this drought year, while Hedrick’s neighbors lost their crop, he was still able to harvest 110 bushels per acre of corn although it is twice that much in a normal year. Furthermore, Hedrick controls weeds with less herbicide due to the thick mat and allelopathic^[10] effects of the five-plant cover crop mix, and he no longer uses any insecticides. He’s also started to see earthworms on his land for the first time, helping to break down organic matter and carry carbon deeper into the soil.

Kallenbach 2016.

Soil organic matter (SOM) and the carbon and nutrients therein drive fundamental submicron- to global-scale biogeochemical processes and influence carbon-climate feedbacks. Consensus is emerging that microbial materials are an important constituent of stable SOM, and new conceptual and quantitative SOM models are rapidly incorporating this view. However, direct evidence demonstrating that microbial residues account for the chemistry, stability and abundance of SOM is still lacking. Further, emerging models emphasize the stabilization of microbial-derived SOM by abiotic mechanisms, while the effects of microbial physiology on microbial residue production remain unclear. Here we provide the first direct evidence that soil microbes produce chemically diverse, stable SOM. We show that SOM accumulation is driven by distinct microbial communities more so than clay mineralogy, where microbial- derived SOM accumulation is greatest in soils with higher fungal abundances and more efficient microbial biomass production. [Kallenbach 2016: Abstract]

Schmidt 2011.  Complex interactions, not intrinsic chemical properties, may determine the persistence of soil organic carbon molecules and their climate effects:

Most soil carbon derives from below-ground inputs and is transformed, through oxidation by microorganisms, into the substances found in the soil. By moving on from the concept of recalcitrance and making better use of the breadth of relevant research, the emerging conceptual model of soil organic carbon cycling will help to unravel the mysteries surrounding the fate of plant- and fire-derived inputs and how their dynamics vary between sites and soil depths, and to understand feedbacks to climate change. We argue that the persistence of organic matter in soil is largely due to complex interactions between organic matter and its environment, such as the interdependence of compound chemistry, reactive mineral surfaces, climate, water availability, soil acidity, soil redox state and the presence of potential degraders in the immediate microenvironment. This does not mean that compound chemistry is not important for decomposition rates, just that its influence depends on environmental factors. Rather than describing organic matter by decay rate, pool, stability or level of ‘recalcitrance’ – as if these were properties of the compounds themselves – organic matter should be described by quantifiable environmental characteristics governing stabilization, such as solubility, molecular size and functionalization. . . .  Because many, if not most, organic molecules in soils are of microbial origin, experiments are needed that identify the long-term drivers of microbial-cell and microbial-product decomposition, rather than focusing on the immediate fate of fresh plant material. (emphasis added) [Schmidt 2011: 52-3]

This study opens many possibilities for new approaches to soil carbon, including assessment from a systemic as opposed to a reductionist perspective:

More generally, though, the major advances in our understanding of soils will come from research grounded in the theory of many disciplines and in the practice of many approaches. The future research agenda for soils will integrate many different fields and have broader goals than it might have had in the past, with longer time horizons, wider spatial coverage, and an imperative to connect carbon, water and nutrient cycles, so as to understand the soil-plant system as a crucial part of our biosphere. [Schmidt 2011: 55]

Do We Have More Soil for Carbon Storage than We Thought?

This is a discussion of a hypothesis by oligochaetologist^[11] Rob Blakemore, Ph.D. He suggests that accounting for varying planetary topography may dramatically increase the projections for soil-carbon storage potential.  This has significant implications for the importance of eco-restoration efforts to address climate, and the speed with which we can draw carbon from the atmosphere.

We believe that this hypothesis is both reasonable and logical, but support is largely inferential at the current time.  It is our hope that this discussion will stimulate further research into the potential of photosynthetic carbon capture and sequestration in soils, and that this development will motivate an intensification of eco-restoration efforts.

Introduction

Area measurements of the earth’s terrestrial surface are conveniently two-dimensional (2-D), yet the true surface has a certain amount of three-dimensional undulation (3-D).  Therefore the actual surface and volume of soils on Earth may be underestimated by an unknown factor using current measurements and models.  

This leads to the proposition that global soils may have the capacity to store greater quantities of atmospheric carbon than previously assumed.  The role of eco-restoration, with soils as the primary storage medium, with above-ground and below-ground biodiversity as the process for incorporating carbon into soils, is therefore a key to any successful climate strategy.

Why “Flat-Earth” soil estimates may be incorrect

The surface of the earth is conventionally calculated based on an assumption of two-dimensional areas within a set of coordinates.  Yet, despite decades of satellite imagery, accurate information on the actual surface area of the earth is surprisingly elusive: the land has undulating terrain adding to its extent. The importance of this distinction is that the total volume of global soils, as well as primary productivity, i.e., photosynthesis, relate to actual soil surface area, not to a two-dimensional model projection thereof. 

Actual three-dimensional surface area is most important for calculations of our total topsoil resource or, as Darwin [1881: 49] describes in his ground-breaking book on earthworms: “The vegetable mould . . . covers, as with a mantle, the surface of the land . . .”

It is not unreasonable to estimate a flat projection of photosynthesizing terrestrial surface area of 12 billion hectares (gigahectares or Gha).^[12]  Some multiple will account for surface irregularities that increase that surface area, and for the sake of discussion we will use a conservative factor of 2.

A practical example of an applicable mathematical estimate of irregular surface areas is from a paint manufacturer [Resene, n.d.], who estimates that compared to a flat surface, a 200 m2 corrugated sheet has 10.5% larger surface area (= 221 m2), and that Anaglypta or Stucco textures, i.e., bumpy like Earth, have surface area 40-100% greater than that of the base area.  

One can also use geometry and knowledge of fractals in order to estimate a reasonable multiplier of the available flat topography for illustrative purposes.  As a paradoxical (i.e., counter-intuitive) fractal, the actual true land surface area may be infinitely expanded at increasingly finer scales of observation as in a 3-D version of the 2-D “Coastline Paradox” that, in practice, increased the linear distance estimate of Britain’s coastal outline more than six fold^[13].

Therefore, this revised estimate of a true land surface takes into account the area exposed to the Sun’s irradiation, and includes all topsoil that supports the plants upon which we depend. If that surface area is doubled by irregularities in the surface of the land, so proportionately is capture of the Sun’s energy and resultant soil activity, including carbon and water storage.

These are very rough approximations: we can say with certainty only that current “Flat-Earth” surface area, and therefore soil volume calculations, are under-representations, and likely significant ones.  We look forward to more detailed studies from researchers with the resources to pursue them.

Leaf Area Index

The primary productivity providing for most life on Earth operates at the biological scale of a leaf.^[14] Average leaf sizes reportedly range from 0.011 to about 39.5 cm2 but no data are readily available for the topographical surface area and volume of underlying topsoil that supports these plants.  An alternative estimate of effective terrestrial surface area is possible if we apply a Leaf-Area-Index (LAI).

LAI is a dimensionless quantity that characterizes plant canopies defined as the one-sided green leaf area over the flat unit ground surface area (LAI = leaf area / ground area).  In other words, LAI is a factor that derives the effective ground area for which the plant is productive based on how much photosynthesis is actually turning atmospheric carbon dioxide molecules into above- and below-ground biomass.  For example, if the surface area on the ground under a tree occupies 10 m2 and the total leaf surface area is 45 m2, the tree is accomplishing 45 m2 worth of photosynthesis which would be underestimated by only accounting for the tree’s ground-level two-dimensional footprint, as is conventionally reported.

LAI’s range from 0 (bare ground) to ~18 (dense forests) and a global average is 4.5. The authors of this source state that “LAI is a key variable for regional and global models of biosphere-atmosphere exchanges of energy, carbon dioxide, water vapour, and other materials.”  [Asner et al. 2003: 195]

For our purposes, we apply LAI to the recalculated undulating and rough-surface topography. If we therefore take our hypothetical but reasonable estimate of a flat 12 Gha of photosynthesizing land and multiply it by 2, we have 24 Gha of non-flat photosynthesizing land surface area.  If we include the LAI multiplier of 4.5 to those 24 Gha, we arrive at the equivalent of 108 Gha of photosynthesis, or 9 times more carbohydrate production by green plants than would be estimated from flat-surface-area measurements.

Why does it matter?

We are rapidly losing soils, with global topsoil erosion rates reportedly greater than 2,000 tonnes per second^[15] [Pimentel 2013: 447].  Soil is further depleted by agri-chemical pollution and urbanization [Blakemore 2017a], that is, land degradation is due in no small measure to loss of natural soil fertility and excess synthetic nitrogen [Rockström 2009: 472, Fig. 1]. Therefore it is clearly in our best interests, and in the interests of the remaining living organisms on planet Earth, to get accurate information about the 3-D topography of the land, which will alter calculations about surface area and volumes of soils. This in turn will increase the potential for sequestration of carbon and for water storage in soils, vital knowledge that could allow more rapid and effective restoration efforts.

How Much Soil Is There on Earth?

In addition to terrain considerations, we now consider Soil Organic Carbon (SOC) weight in Gigatons, as opposed to previously considered surface area in Gigahectares.  Blakemore [2016a: 11] noted that:

Soil carbon values require allowance for intractable glomalin adding a further 5-27% to almost all SOC tallies (Comis, 2002).  Plus data from deep soils may increase budgets: e.g., Harper & Tibbett (2013) found C up to five times greater in Australian soils at depth >1 m and down to 35 m in some cases.  The Walkley-Black method itself underestimates total C by about 20% with a correction factor of ca. 1.3 often required, whereas latest techniques using mid-infrared (MIR) spectroscopy give more accurate readings.  These three factors combined would surely increase SOC totals.

Glomalin, only discovered in 1996, is a stable fungal molecule tightly-bound to soil particles as a major component of soil organic matter.  Glomalin adds up to 27% of total SOC [Khursheed 2016], and is stable for 7 to 42 years, depending on conditions [Comis 2002: 4].

Thus an answer to “How much soil is there on Earth?” is still elusive. How much soil can be built on Earth through the activity of healthy biological systems and how quickly these transformations can take place remains an intriguing question.  Allowing for glomalin, deep soil data and carbon in living or dead roots [Jackson 1997: T1], soil carbon quantities are likely considerably higher than conventional estimates, as is the potential for future carbon sequestration in soils.

Conclusion

More than two millennia ago Aristotle told us the Earth was not flat and he also concurred with Plato in recognizing that soil erosion and loss of humus and earthworms is catastrophic to civilization [Montgomery 2008: 51].  Leonardo da Vinci’s observation 500 years ago that “We know more about the movement of celestial bodies than about the soil underfoot” seemingly still rings true.  An essential feasible and achievable solution is to apply what Sir Albert Howard termed Nature’s Law of Return, i.e., to vermi-compost all organic ‘wastes” to restock the topsoil [Howard 1945].

Most crucially vital, we must determine the amount of living topsoil remaining and its potential restoration through organic regenerative land management worldwide applying principles and practices of Permaculture (Mollison 1988; Blakemore 2017).  The challenge now is for professional geographers, astronomers and others to provide actual topographic values for land and topsoil contribution to global photosynthesis and the carbon cycle.