Croplands

Cultivated land covers 1.6 billion hectares globally [FAO 2011]. About 62% of cropland produces food directly for human consumption, while 35% is dedicated to producing animal feed, and 3% to biofuel feedstock, seed and other industrial products [Foley 2011: 338]. Agriculture is a major source of emissions of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), contributing 10-12% (including crop and livestock production) of total greenhouse gas emissions [Smith 2007]. Agricultural emissions are driven by the globally dominant industrial model, which favors monocultures and fossil-fuel intensive inputs, and results in soil organic carbon loss and overall soil degradation. However, rather than being a source of carbon emissions globally, agriculture can become a powerful carbon sink. This section looks at the carbon sequestration outcomes of farming practices, such as cover cropping, agroforestry and no-till, which are designed to minimize erosion and boost soil biodiversity, thus restoring soil ecosystems to health and resilience. While more research is needed on holistic approaches that combine multiple soil-building practices, such as permaculture and agroecology, recent research suggests that restorative agriculture could sequester “more than 40% of annual emissions (an estimated 21 Gt CO2 each year [5.7 Gt C/year])” [Rodale 2014], and likely far more, as indicated below.

Cultivation thus began an ongoing slow ignition of

Earth’s largest surficial reservoir of carbon ^[16]

Overview

The purpose of this compendium, once again, is to emphasize possibilities, the “positive deviants” which lead us to expand our conceptual limits.  Only when we can conceive of exceptional and inspiring outcomes may we find the motivation to overcome obstacles to attain them.  Fortunately the evidence that supports regenerative land management is rapidly growing, and there are indications that it may outpace climate disruption and provide us with the time and opportunity to address the many difficult circumstances resulting from widespread eco-destruction, including the poster child, global warming.  In this section we address the challenges of croplands and their ability to capture atmospheric carbon and recover quickly from millennia of mistreatment.

Under careful human management it is possible for soil organic carbon to reach amounts greater even than under natural, pre-agricultural conditions. A classic example is the Terra Preta soils of the Amazon, “where intensive management and high levels of organic matter additions were practiced over many years, resulting in greatly enhanced soil C” [Paustian 1997: 231].

In spite of a long history of soil carbon loss and a body of scientific literature that views carbon-poor soils as “normal,” many examples of building high levels of soil carbon exist among today’s ecologically minded land managers. California Farmers Paul and Elizabeth Kaiser, for instance, use 5-10 times more compost than average, never till, rotate fields with an extremely diverse mix of vegetable varieties, surround their crops with native trees, shrubs and flowers and have thus built up a thick topsoil containing 10% SOM [Oppenheimer 2015; Kaiser 2017].

In most scientific studies, carbon sequestration rates for croplands measure below 1t C/ha/yr (0.4t C/ac/yr), despite some exceptions as highlighted below. Leading soil scientist Rattan Lal [2016] estimates the global sequestration potential for cropland soils to be 0.8 to 1.2 t C/ha/yr, or “as much as 62 t/ha over the next 50 to 75 years … with a total C sink capacity of ~88 Gt on 1,400 Mha” [Lal 2016: 20A]. That amounts to an average annual global sequestration rate between 1 and 2 Gt C/year, compared to annual carbon emissions from fuel combustion and land use conversion of more than 10Gt C/yr [Lal 2016]. Similarly, Smith [2008] estimates that, under improved management, agriculture could offset 20% of global emissions. Both authors note that conservation-oriented agricultural is a small, albeit crucial, piece of the whole climate mitigation puzzle.

Yet, for a couple of important reasons, these estimates likely greatly underestimate the potential of global croplands to absorb carbon. First, samples are commonly taken to a depth of 30cm or less [Torres-Sallan 2017; Minasny 2017]. This is the default sampling depth recommended in the 2006 IPCC Guidelines for National Greenhouse Gas Inventories, despite acknowledgement in these same guidelines that land use and management is likely to have a major impact on deeper soil layers [FAO 2017b].

Indeed, significant amounts of carbon sequestration occurs in deeper soil profiles – even beyond a 1 m (3 ft) depth [Follett 2012, Liebig 2008, Schmidt 2011: 51]. Harper et al. found that half to three-quarters of total SOC to bedrock was in the surface 5 m with the remainder below that depth. The authors speculate that deep carbon may have been deposited directly by deep-rooting plants. “Where deep soils coincide with deep rooting the biological deposition of carbon from roots (and their associated biota) is inevitable at depths at which SOC has rarely been measured” [Harper 2013: 642].

Second, many studies measure sequestration rates for just one or two soil-building techniques, isolating them from additional, potentially synergistic, practices. In fact, intact ecosystems are based on countless synergistic relationships among organisms and their environment.  In other words, many studies measure minor tweaks to conventional, industrial cropping systems.  

For example, Minasny et al. [2017] compiled sequestration rates from around the world to assess the viability of the France-led “4 per 1000” initiative (seeking to halt the annual increase in atmospheric CO2 by increasing soil carbon by 0.4% per year). The authors estimate “that an annual rate of 0.2-0.5t C/ha/yr is possible after adoption of best management practices such as reduced tillage in combination with leguminous cover crops.” However, most of some 40 studies of best management practices on arable land assessed only one or two of many – often minimally improved – practices, such as “reduced use of summer fallow,” “rice-rice with NPK,” “inorganic fertilizer,” and “pasture.”

Similarly, an often-cited study by West and Post [2002], compiling 276 paired treatments from 67 long-term experiments, analyzes the sequestration rates for either increased rotation complexity (0.2+/-0.12t C/ha/yr) or a change from conventional tillage to no-till (0.57+/-0.14t C/ha/yr). While both practices were used at some sites, the data were not analyzed according to whether a single practice or combined practices were used. However, the authors suggest that using both practices together can be additive:

Data used in this analysis was stratified separately with regard to a change in tillage or a change in crop rotation. In practice, these changes could occur simultaneously. It can be inferred from our results that if of a decrease in tillage and an enhancement in rotation complexity occur simultaneously, the short-term (15–20yr) increase in SOC will primarily be caused by the change in tillage and subsequent decrease in the rate of SOC decomposition, while the long-term (40–60yr) increase in SOC will be primarily caused by the rotation enhancement and residue input and composition [West & Post 2002: 1943].

If moving to a combination of two restorative practices can increase carbon sequestration somewhat above the use of a single improved practice, then what is possible when many restorative practices are stacked one upon another within an agroecosystem? Permaculture, a design framework with “two broad conceptual criteria: ecosystem mimicry and system optimization,” where multiple restorative practices are indeed combined, represents a counterexample to industrial farming. Yet, sequestration rates from such a system have rarely, if ever, been measured. In fact, very little scientific study of any kind has been conducted in permaculture systems, despite the concept having been developed by scientist Bill Mollison, and adopted to favorable results globally for over 30 years [Ferguson and Lovell 2013].

In light of the centrality of agriculture’s role in ecosystem restoration due to the vast surface area it occupies, we present some literature representing agriculture’s maximum potential contribution to climate change mitigation, and argue that the focus of future research do the same.

Specifically, future studies should consider the effects of greater ecological intensity, diversity and potentially additive and synergistic interactions that can exist among multiple soil-building practices, rather than continuing to pursue measurement of their individual effects, which yield minimal outcomes. Future research must also measure SOC changes to greater depths in the soil horizon in order to capture the full benefit of any given practices. Such changes would likely present both a more accurate and more promising real-world potential for the climate mitigation potential of agriculture.

For a relevant and effective comprehensive assessment of regenerative management practices, one that supports the profound shift necessary in conventional 21st century agriculture, a scientific paradigm shift is necessary so that economics and policy will follow.

For 10,000 years, humans have been clearing patches of forest and grassland to plant crops. While clearing land by burning it visibly turns organic carbon into smoky CO2, plowing and tilling releases soil organic carbon by breaking up soil aggregates that protect carbon. Exposed soil organic carbon is consumed by microbes, and converted to CO2 through respiration.  Tilling soil also subjects it to erosion. “Since tillage-based farming began, most agricultural soils have lost 30% to 75% of their soil organic carbon (SOC), with industrial agriculture accelerating these.” [Teague 2016: 157]  

Agriculture is a source not only of CO2 emissions, but also of greenhouse gas emissions of methane (CH4) and nitrous oxide (N2O). In cropland soils, CH4 is produced by anaerobic decomposition of organic matter, usually in waterlogged soils like rice paddies. However, soils can also be a methane sink due to the presence of methanotrophic bacteria, which oxidize methane. N2O is produced by microorganisms, which transform excess ammonia fertilizer into nitrate and then N2O. “Upon conversion to NO2– or NO3–, excessive fertilizer N becomes subject to denitrification and thereby contributes to terrestrial emissions of N2O, which have been found to increase with the rate of N fertilization” [Mulvaney 2009: 2296].

Industrial agriculture compensates for soil carbon loss by abandoning degraded land or using chemical inputs for the nutrients and pest resistance that an otherwise carbon-rich, biologically active soil provides. However, the farming methods that rebuild topsoil without relying on synthetic inputs, while also ameliorating the worst effects of drought, are the same ones that can make agriculture a major sink for atmospheric CO2. Such methods, which can be used together as a complementary suite of practices include: no-till; cover-cropping; agro-forestry; diverse crop rotations, including integrating livestock grazing; use of compost, manure, and biochar; and use of deeper-rooting plants and perennials.

Cropland Article Summaries

Cover crops

Cover crops protect the soil during a time of year when no cash crops are growing and the soil would otherwise be bare. “Cover crops, also named inter-crops or catch crops, are crops that replace bare fallow during winter period and are ploughed under as green manure before sowing of the next main crop.” [Poeplau 2015: 34] Cover crops can also be rolled and crimped or mowed, instead of plowed, in preparation for the main crop.

Using cover crops reduces erosion, nutrient leaching, and drought stress, and add carbon through continued plant cover and growth as well as increase biodiversity. Leguminous cover crops also fix nitrogen. Furthermore, “in contrast to other organic amendments, a large part of the C input from cover crop is added as roots, which was found to contribute more effectively to the relatively stable carbon pool than aboveground C-input” [Poeplau 2015: 38].

Vick 2016. This Montana study demonstrates that leaving farmland fallow “depletes carbon stocks and thereby soil quality” [Vick 2016: 129], thus illustrating the importance of keeping land continuously covered with living vegetation. “Fallow” is the stage of crop rotation where no crop is grown.  

In this study, a CO2 emissions rate of 1.35 tC/ha/yr (0.54 tC/ac/yr) was measured from land left fallow during the 2014 summer growing season; an adjacent field planted in winter wheat (summer 2013) and spring wheat (summer 2014) was a net carbon sink, measuring carbon input from the atmosphere into the soil at ~2 tC/ha/yr (0.8 tC/ac/yr) and ~1 tC/ha/yr (0.4 tC/ac/yr), respectively. Other parts of this study show a dramatic effect on area cooling as well as increased moisture and rainfall.  These results occur only from ending the practice of fallowing.

The researchers observed that a widespread decline of land left fallow in agricultural areas of the Canadian Prairie Provinces coincided with a summertime cooling trend since the 1970s. They noted that extreme temperature events now occur less frequently than in the recent past, maximum summer temperatures have decreased by ca. 2° C (3.6° F), relative humidity has increased by some 7% and summer precipitation has increased by an average of 10 mm/ decade across parts of the Canadian Prairie Provinces. A remarkable 6 W/m2 summer cooling has been observed compared to a ca. 2.5 W/m2 warming globally since the dawn of the Industrial Era.

Even in degraded croplands, relatively small changes can lead to significant differences in rainfall, soil carbon sequestration, and ambient temperature. A 2016 study in Montana demonstrated the effects of reversing the practice of fallowing of wheat fields in the upper midwest.  Fallowing is “the practice of keeping a field out of production during the growing season” (Vick 2016:129):

Fallow is a common management practice in the dryland wheat-growing regions of the northern North American Great Plains to conserve water for subsequent crops (Lubowski et al., 2006). Fallow however also increases erosion (Wischmeier, 1959) and soil carbon loss (Cihacek and Ulmer, 1995), and fallow-small grain management strategies are not considered sustainable from the soil conservation perspective (Merrill et al., 1999). [Vick 2016:130].

As a result of farmers’ experience, fallowing has progressively decreased across many areas of the northern midwestern plains since the 1970s, providing an environment suitable for comparison study:

The area of fallow in the Prairie Provinces of Canada has decreased from over 15 Mha in the 1970s to under 2 million ha at the present (Fig. 1) as producers have realized that the water-savings benefit of fallow is outweighed by the economic losses of not planting (Dhuyvetter et al., 1996). The area under fallow in the United States has likewise decreased from 16 Mha to 6 Mha across the same time frame (Lubowski et al., 2006), largely in the northern Great Plains and other areas of the semiarid West . . . Despite the decreasing trend in fallow area across the North American northern Great Plains, fallow remains common in many regions including major land resource area

(MLRA) 52 in north-central Montana – the largest wheat-growing region in the state – where some 40% of agricultural lands may remain in fallow in any given year. In contrast, fallow has been reduced in northeastern Montana (MLRA 53) by hundreds of kha over the past decade (Long et al., 2014, 2013) as producers have adopted continuous cropping or alternate cropping practices.  [Vick 2016:130].

The effects of this relatively simple change of practice led to some remarkable results:

The widespread decline of fallow in agricultural areas of the Canadian Prairie Provinces (Fig. 1) has coincided with a summertime cooling trend since the 1970s (Betts et al., 2013a, 2013b; Gameda et al., 2007; Mahmood et al., 2014). Extreme temperature events now occur less frequently than in the recent past, maximum summer temperatures have decreased by ca. 2০C, relative humidity has increased by some 7% (Betts et al., 2013b), and summer precipitation has increased by an average of 10 mm/decade across parts of the Canadian Prairie Provinces (Gameda et al., 2007). A remarkable 6 W/m2 summer cooling has been observed (Betts et al., 2013a); for reference, anthropogenic greenhouse gasses are responsible for a ca. 2.5 W/m2  warming globally since the dawn of the Industrial Era (IPCC, 2007). These climate benefits have only occurred during the growing season; fall, winter, and early spring temperatures have followed global trends (Betts et al., 2013b) . . . In other words, the observed regional climate cooling is broadly consistent with the effects of fallow avoidance on climate processes.  [Vick 2016:130-131]

As dramatic as some of these changes are with only reduced fallowing, there are other land-management practices with significant impacts on water cycles, soil carbon, biodiversity and productivity.  Such practices hold additional potential, and include cover-cropping and green mulches, pasture cropping, elimination of synthetic inputs which encourage renewed activity of important soil biota, especially worms – and perhaps most importantly on grasslands that co-evolved with animals, the reintroduction of animals themselves.

Finally, it is worth noting that there may be a significant underestimation of surface area and volume of soils in grasslands, as well as in other ecosystems, since natural topographies are not uniformly flat. Topographical variations would add volumes of soil carbon, water, etc. to prior estimates of areas that are typically calculated on the basis of two-dimensional map projections [Blakemore 2016: Fig. 5]. The implications are that there may be considerably greater volumes of soil amenable to regenerative management, carbon capture and water storage than is conventionally assumed. Such adjustments to soil volume calculations would positively affect carbon drawdown estimates in considering the potentials of eco-restoration in climate (see section, Do We Have More Soil for Carbon Storage than We Thought?).

Pimentel 2011. Arguing for cover crops as an effective way to reduce erosion and conserve nutrients in soil, Pimentel notes that “Growing cover crops on land before and after a primary crop nearly doubles the quantity of solar energy harvested in the agricultural system per hectare per year. This increased solar energy capture provides additional organic matter, which improves soil quality and productivity.” [Pimentel 2011: 41]

Crop rotation

Crop rotation diversification can enhance pest resistance, nitrogen input (when leguminous crops are added), soil penetration for better water infiltration (when deeper rooting plants are added), and residue input (when crops that produce more biomass are added). The effects on carbon sequestration from increases in crop rotation diversity vary depending on what crops are included. “Crop species can vary significantly in growth patterns, biomass production, water requirements, and decomposition rates, all of which affect net GHG emissions. Therefore, many rotations could be adapted with alternative species or varieties of annual crops to promote soil C sequestration—increasing root and residue biomass, increasing root exudates, or slowing decomposition—or otherwise reduce emissions” [Eagle 2012: 13].

Clearly, crop rotation is something of an umbrella term, describing a variety of practices, and even leaving space for practices that would not seem to offer much in the way of soil restoration. For example, as West and Post [2002] state,

. . . enhancement of rotation complexity refers to (i) a change from monoculture to continuous rotation cropping, (ii) a change from crop–fallow systems to continuous monoculture or rotation cropping, and (iii) an increase in the number of crops used in a rotation cropping system. In this analysis, continuous cropping is a cropping system without a fallow season, monoculture is a system with only one crop grown, and rotation cropping indicates two or more crops rotated over time on the same unit of land. [West & Post 2002: 1931]

Thus, even “continuous monoculture” can be considered as a crop rotation meant to increase carbon sequestration capacity. On the other hand, crop rotation can also involve great diversity, such as at Paul and Elizabeth Kaiser’s farm, where 3-7 crops/year rotate through vegetable beds, sometimes intercropped two crops at a time [Kaiser 2017].

Teague 2016. This study argues for greater use of no-till, cover crops, and crop rotation, including integrating livestock rotation into cropping systems.

Crop production can be managed to maintain permanent ground cover through the rotation of forage and row crop mixes, including cover crops, and legumes to increase soil fertility by fixing N. Grazing livestock can accelerate nutrient cycling through the consumption and decomposition of residual aboveground biomass.” [Teague 2016: 159]

The authors present a set of testable hypothetical scenarios suggesting the adoption of conservation cropping and adaptive management grazing (including grass-finishing cattle).

No-till

No-till (NT) allows farmers to plant without disturbing the soil, thus protecting it from water and wind erosion, leaving soil aggregates intact, and preventing a flush of oxygen from activating microbial breakdown of organic matter and releasing CO2. No-till can contribute to climate mitigation both by reducing emissions from the turnover of soil organic matter caused by tillage, and by sequestering carbon, especially in the surface layer [Mangalassery 2015].

Brown 2016. North Dakota Farmer Gabe Brown began practicing no-till in 1994. Since then, he has added cover crops (a diverse mixture of 70 species), complex crop rotations, orchards, livestock grazing (including cattle, sheep, pork and chicken), vegetable production, and bees. Through a long-term commitment to building the soil through no-till, keeping the ground always covered, and favoring as much biodiversity as possible (including a wide diversity of cash crops), Brown reports SOM has increased from 1.7% in 1993 to 11% in 2013. Furthermore, water infiltration has increased from ½ inch to more than 14 inches over the same time span.

Follett 2012. Measured to a depth of 150 cm (~5 ft), no-till continuous maize grown in eastern Nebraska, fertilized with 120 kg/ha of nitrogen and stover left on the field after grain harvest, sequestered 2.6 tC/ha/yr (1 tC/ac/yr). Notably, more than 50% of sequestered carbon was found below 30 cm (1 ft), illustrating that studies failing to sample below this depth (a common practice) risk greatly underestimating sequestration rates.

Organic vs. synthetic inputs

Organic farming uses “cultural, biological, and mechanical practices that support the cycling of on-farm resources, promote ecological balance, and conserve biodiversity” according to the USDA, which prohibits the use of most synthetic pesticides and fertilizers on certified organic farms. Organic farmers must find alternatives to synthetic inputs for managing pests and fertility. For example, vermi-composting is commonly used in organic farming. It is a natural and proven enhancement of the humification process that uses specific earthworms (e.g. Eudrilus eugeniae [Blakemore 2015]) to rapidly convert all organic “wastes.” Returning this vermicompost to soil renders synthetic fertilizers and pesticides unnecessary, as vermicompost often confers natural resistance to pests [Howard 1945, Balfour 1975] and it enhances resident earthworms [Blakemore 2000, 2016a; see also Earthworms section].  

While the organic law provides a baseline for organic practices, the term “organic” encompasses a wide range of approaches to farming. For instance, some organic farmers may do little more than substitute naturally occurring inputs into an otherwise conventional, industrial operation, likely leaving the soil similarly depleted. Other organic farmers put into practice several of the methods mentioned in this section, aiming to truly build the functionality of the soil to resist pests and provide fertility. The studies included below highlight benefits from organic inputs and problems that come with using synthetic fertilizers with respect to soil carbon and biodiversity.

Johnson 2017. Using fungal-dominant compost in a 4.5-year trial at Leyendecker Field Research Site in New Mexico, researchers recorded an annual carbon sequestration rate of 10.7t C/ha/yr (4.8t C/ac/yr). Based on the observed trajectory of increasing productivity, they estimate a potential rate of 19.2t C/ha/yr (7.67t C/ac/yr). Chief investigator David Johnson found that increased plant growth is correlated most closely with the fungal to bacterial ratio. At a fungi:bacteria ratio of 0.04, only 3% of carbon flow went into plant biomass production, with the remainder of the carbon going into other functions, including nitrogen fixation, the soil, and respiration. At a fungi:bacteria ratio of 3.68, plant growth was more efficient with 56% of carbon flow going to biomass production.

Rodale 2014. Compiling data collected from around the world, Rodale Institute concluded that if all cropland were converted to their regenerative model^[17], it would sequester 40% of annual CO2 emissions. Adding pastures to that model would add another 71%, effectively exceeding the world’s yearly carbon dioxide emissions.  

On-farm soil carbon sequestration can potentially sequester all of our current annual global greenhouse gas emissions of roughly 52 gigatonnes of carbon dioxide equivalent (GtCO2e). Indeed, if sequestration rates attained by exemplar cases were achieved on crop and pastureland across the globe, regenerative agriculture could sequester more than our current annual carbon dioxide (CO2) emissions. Even if modest assumptions about soil’s carbon sequestration potential are made, regenerative agriculture can easily keep annual emissions within the desirable lower end of the 41-47 GtCO2e range by 2020, which is identified as necessary if we are to have a good chance of limiting warming to 1.5°C. [p.5]

Ryals and Silver 2013. This study examined the effects on plant growth and respiration from compost application on annual grassland in both coastal and valley sites in California. They found that a single application of compost during the three-year study resulted in a carbon sequestration rate of 1.45t C/ha/yr (0.58t C/ac/yr) and 0.54t C/ha/yr (0.22t C/ac/yr) at the valley grassland and coastal grassland, respectively. This enhanced net primary productivity was partially offset by CO2 emissions from increased respiration, but the compost did not affect CH4 or N2O fluxes. The authors conclude that:

Our results have important implications for rangeland management in the context of climate change mitigation. Urban and agricultural green waste is often an important source of greenhouse gas emissions (IPCC 2001). Here we show that an alternative fate for that material can significantly increase NPP and slow rates of ecosystem C losses at the field scale. This approach provides important co-benefits to landowners, such as the sustained increase in forage production measured here [Ryals & Silver 2013: 56].

While these results are low compared to some of the other studies noted here, this study illustrates positive use for green waste, and a potential tool that may contribute to climate-positive management.

Khan 2007. This five-decade study of nitrogen fertilization effects on SOC in Illinois shows that, despite progressively greater corn crop residue input during the second half of the 20th Century (increasing from 20,000 or 30,000 to 69,000 plants/ha since 1955), partly due to synthetic fertilizer use, SOC content did not increase, and in most cases declined. SOC declines were most pronounced in subsurface (16-46cm) of the soil horizon, compared to the surface layer (0-15cm). These results are despite crop residue being incorporated, rather than removed, in most plots since 1955, and in all plots since 1967.

These findings implicate fertilizer N in promoting the decomposition of crop residues and soil organic matter and are consistent with data from numerous cropping experiments involving synthetic N fertilization in the USA Corn Belt and elsewhere, although not with the interpretation usually provided. [Khan 2007: 1821]

Perennial systems, agroforestry, and permaculture

Unlike annual plants, perennials live for many years – thousands of years in some cases. Because of their deep (>2m, or 6 ft) and extensive root system, and longer growing seasons, perennials are likely to sequester carbon better than annual cropping systems [Glover 2007].

Agroforestry is the practice of integrating trees (a type of perennial) into a cropping system, including alley cropping, windbreaks, riparian buffers, silvopasture, and forest farming [Eagle 2012; Nair 2009]. Agronomic practices are notable for adding significant amounts of carbon to aboveground biomass, which is often measured separately from soil organic carbon sequestration [Nair 2009]. One of the strengths of agroforestry is its enhancement of an agroecosystem’s functional diversity:

The utilization of the environment by species includes three main components: space, resources, and time. Any species utilizing the same exact combination of these resources as another will be in direct competition which could lead to a reduction in C sequestration. However, if one species differs in utilization of even one of the components, for example light saturation of C3 vs. C4 plants,^[18] C sequestration will be enhanced.” [Udawatta 2011: 19]

Toensmeier 2017. Compiling carbon sequestration rates from individual studies, reviews, and expert estimates, and organizing them into groups of annual versus perennial systems, woody versus herbaceous crops, and polyculture versus monoculture, Toensmeier observes that “the general trend is that systems that incorporate trees sequester more carbon.” The highest sequestration rate listed, 18 tC/ha/yr (7.2 tC/ac/yr) falls into the perennial woody polyculture group, and more than half of all sequestration rates listed under perennials are more than 6 tC/ha/year (2.4 tC/ac/yr), while most rates for annual cropping systems are less than 1t C/ha/yr (0.4 tC/ac/yr).  

Lawton 2016. On 10 acres of the Arabian Desert in Wadi Rum, Jordan, Permaculture Designer Geoff Lawton built an organic, multi-species food forest on what had previously been bare desert ground. Using wastewater from a nearby irrigated farm to get started, he designed a microclimate that would prevent evaporation in every way possible. Key elements included: date palm trees for wind protection and shade; smaller fruit tree and trellised grapevines for additional shade; a succulent ground cover, which also catches nutrient-rich desert dust; a shaded swale for irrigation; and cut legume trees for mulch.

From https://www.facebook.com/greeningthedesert2/.

Lawton sought to “build organic matter within the system as quickly as possible with any living elements that will achieve those ends.” Once the soil came alive, it became productive. Lawton explains that strategic arrangement of the space is especially important in the desert. That’s why crops were grown in two rows in between three slightly wider rows of mixed fruit trees for protection. After four years, this orchard/farm was producing an abundance of fruits and vegetables, showing that it is possible to work with nature and avoid industrial inputs to achieve a productive landscape even in the harshest environment.

DuPont 2010. A Land Institute study measured the effect on soil properties and biota from perennial polyculture systems as compared to annual grain crop systems. Since the latter are typically intensively managed, “the effects of tillage and plant community composition are often confounded” [DuPont 2010: 25]. To control for management effects, this study compared the soil carbon and root biomass outcomes from no-tilled annual crops (rotation of soybean, sorghum and wheat) versus a perennial polyculture. Total root biomass in no-till annual plot measured at only 43% of that in a perennial grass plot in the top 1m of soil. Also, the authors found significantly higher levels of readily oxidizable carbon (ROC) and microbial biomass in the perennial plots compared to the annual crop plots. ROC measures soil carbon that is more available to soil microbes.

Small changes in ROC and other labile fractions of SOC may provide an early indication of soil degradation or improvement in response to management practices. Changes in active carbon pools can be two to four times greater than changes in total C after the initiation of new management practices and they are more highly correlated with other soil quality indicators including microbial respiration, aggregate stability and plant productivity [DuPont 2010: 28].

The authors conclude that “even in the absence of tillage and under best management practices, annual cropping can reduce soil carbon and impact soil biota and food webs important in nutrient cycling after just three years” [DuPont 2010: 25].

Soto-Pinto et al. 2009. In this southern Mexico study of land-use change in various agroforestry systems, the authors show that converting “traditional fallow” (secondary growth woods following cropping, averaging 23.4 years in age) to maize (with beans, squash and pepper) production results in 94% loss of living biomass carbon. However, transitioning to (a) “taungya” (maize, beans, squash and peppers intercropped between rows of timber and multipurpose trees), (b) shaded coffee systems, or (c) “improved fallow” (adding timber trees to traditional fallow plots) preserves living biomass carbon. This study points to the mounting relevance of agroforestry systems that can provide economic benefits to small-scale farmers, while avoiding carbon emissions from land use change from forest to agriculture and livestock production, which accounts for 35% of total emissions in Mexico, according to the authors.

Association for Temperate Agroforestry 2004:

Agroforestry practices are intentional combinations of trees with crops and/or livestock which involve intensive management of the interactions between the components as an integrated agroecosystem.

Intentional: Combinations of trees, crops and/or animals are intentionally designed and managed as a whole unit, rather than as individual elements which may occur in close proximity but are controlled separately.

Intensive: Agroforestry practices are intensively managed to maintain their productive and protective functions, and often involve annual operations such as cultivation, fertilization and irrigation.

Interactive: Agroforestry management seeks to actively manipulate the biological and physical interactions between the tree, crop and animal components. The goal is to enhance the production of more than o­ne harvestable component at a time, while also providing conservation benefits such as non-point source water pollution control or wildlife habitat.

Integrated: The tree, crop and/or animal components are structurally and functionally combined into a single, integrated management unit. Integration may be horizontal or vertical, and above- or below-ground. Such integration utilizes more of the productive capacity of the land and helps to balance economic production with resource  conservation.

Liebig 2008. Measured to a depth of 120 cm (~4 ft), switchgrass grown for bioenergy at 10 farms across the Great Plains in the United States sequestered 2.9 tC/ha/yr (1.16 tC/ac/yr). Of that, only 1.1 tC/ha/yr (0.44 tC/ac/yr) was found in the first 30 cm (1 ft) depth, with the remainder measured below 30 cm. The authors explain what makes switchgrass effective in carbon sequestration:

Increases in SOC [soil organic carbon] under switchgrass were likely caused by belowground C input from root biomass and rhizodeposition and decreased soil organic matter losses by erosion. Research conducted by ecologist John Weaver and his graduate students over 60 years ago provide ancillary support for increased SOC under switchgrass. Their detailed surveys of prairie grass roots indicated switchgrass to have the deepest root system of all grasses examined, with roots extending to a soil depth of 3m (~10 ft). This finding, coupled with observations that prairie grass roots regenerate by replacing dying roots with new, live roots indicates the potential for significant C input to the soil under switchgrass.

Montagnini & Nair 2004. Agroforestry systems are multifunctional with respect to carbon capture. Agroforestry can: increase the soil carbon content and fertility of cropland, while allowing for continued food production; create greater sequestration efficiency through diversity of vegetation; and allow for harvest of forest products, potentially keeping carbon sequestered in wood products for many years, and thereby also decreasing pressure on natural forests. And because of the mixed use of agroforestry systems:

[T]he amount of biomass and therefore carbon that is harvested and ‘exported’ from the system is relatively low in relation to the total productivity of the tree (as in the case of shaded perennial systems). Therefore, unlike in tree plantations and other monocultural systems, agroforestry seems to have a unique advantage in terms of C sequestration [Montanigni & Nair 2004: 285].

A few sequestration rates highlighted in this article include: A Costa Rica study of cacao grown under two different species of shade trees Erythrina (a leguminous tree) and Cordia (a timber tree), measured C sequestration in perennial plant biomass at an average of 4.28t C/ha/yr (1.7t C/ac/yr) for the cacao-Cordia system, and 3.08t C/ha/yr (1.2 tC/ac/yr) in the cacao-Erythrina system . In another study, tropical smallholder agroforestry was projected to sequester 1.5-3.5t C/ha/yr (0.6-1.4 tC/ac/yr).

Onim 1990. Tropical agroforestry was observed to increase SOC (soil organic carbon), at the 0-30 cm depth, to a maximum of 8.34 tC/ha/yr (3.38 tC/ac/yr)  and minimum of 0.73 tC/ha/yr (0.30 tC/ac/yr).

Biochar

Biochar is organic matter that has been decomposed through pyrolysis (burning) under controlled, low-oxygen conditions, where it emits relatively little CO2.  Biochar is then added to the soil for long-term carbon storage and/or enhancing availability of soil nutrients, oxygen and water to plants and microbes. Because charred biomass has been observed to persist in the soil for centuries or millennia, biochar is seen as a stable or recalcitrant form of carbon that that may prove to be a useful tool for reversing climate change. Not only is the biochar itself a stable form of carbon that can remain in soils long-term, but also it helps build healthy soil structure which increases plant growth and therefore photosynthetic capacity, resulting in carbon being removed from the atmosphere and stored in biomass or soils. [McLaughlin 2017; Taylor 2010; Paustian 2016; Weng 2017; Remediation Magazine 2017]

It is worth noting that depending on the pyrolysis technique, the resulting biochar may range in quality from poor to excellent. One hopes that as the industry matures, the understanding of the importance of biochar quality in assessing results will grow as well.

McLaughlin 2017.  Hugh McLaughlin, Ph.D., P.E. is an expert on the properties and production of chars created by pyrolyzing biomass, and the subsequent conversion to activated carbons. He has published extensively on biochar and biomass-derived heat production. In this video he gives a short but comprehensive review of the qualities and use of biochar.

Paustian 2016. Biochar application to soils is considered in this article among several activities (such as compost application, cover cropping, residue retention, no-till, and others, as previously mentioned in this compendium) designed to increase soil C stocks by increasing organic matter inputs or reducing decomposition rates. Biochar acts as a soil amendment stimulating plant growth, thereby allowing for greater C storage through greater biomass production, while also embodying a generally stable form of buried carbon.

Biochar mineralizes 10–100 times more slowly than uncharred biomass. Thus a large fraction of added C …  can be retained in the soil over several decades or longer, although residence times vary depending on the amendment type, nutrient content and soil conditions (such as moisture, temperature and texture).

 

However, because the organic matter originates from outside the ecosystem ‘boundary’, a broader life-cycle assessment approach is needed, that considers the GHG impacts of: (1) offsite biomass removal, transport, and processing; (2) alternative end uses of the biomass; (3) interactions with other soil GHG-producing processes; and (4) synergies between these soil amendments and the fixation and retention of in situ plant-derived C. In many cases, net life-cycle emissions will largely depend on whether the biomass used as a soil amendment would have otherwise been burnt (either for fuel, thereby offsetting fossil fuel use, or as waste disposal), added to a landfill, or left in place as living biomass or detritus [Paustian 2016: 50].

Remediation Magazine 2017. A popular report on Weng 2017, quoting the authors:

The project’s leader, DPI [Department of Primary Industries] researcher and SCU [Southern Cross University] adjunct professor Lukas Van Zwieten said the research threw up some unexpected results. “We immediately saw an increase in soil carbon from the biochar, as expected, but what we didn’t expect was that soil carbon content continued to increase. This research demonstrates the ongoing benefits of biochar in farming systems to improve pastures and grasslands and increase farmers’ production and profitability . . .  the researchers found that biochar enhanced the below-ground recovery of new root-derived carbon by 20% – that is, more of the carbon photosynthesised by plants was retained in the biochar-amended soil. Biochar accelerated the formation of soil microaggregates via interactions between organic matter and soil minerals, thus stabilising the root-derived carbon. . . . The increased microbial activity and improved physical structure of the soil would also ultimately improve the effectiveness of fertiliser use, making the application of biochar particularly beneficial for high-end, intensive crop production”

“[T]he improved structure of the soil protected the naturally occurring carbon, as well as the carbon added”, said Southern Cross University’s associate professor Terry Rose, a co-author of the study. “Importantly, the biochar also slowed down the natural breakdown of native soil organic carbon by more than 5%.

Taylor 2010.  An anthology of articles written by biochar pioneers.  Covers biochar history, testing, production, challenges and uses.  Suitable reading for general audiences as well as land management and industry professionals.

Weng 2017.  Biochar can increase the stable C content of soil. However, studies on the longer-term role of plant–soil–biochar interactions and the consequent changes to native soil organic carbon (SOC) are lacking. . . . We found that biochar accelerates the formation of microaggregates via organo-mineral interactions, resulting in the stabilization and accumulation of SOC in a rhodic ferralsol (s.a. Remediation Magazine 2017).