Increase Harvested Wood Use and Preservation

According to many studies, the main climate benefit of increasing the use of harvested wood products is emissions reductions from the substitution of wood products for materials such as concrete and steel that require more fossil fuels for production (e.g., Hashimoto et al., 2002; Perez-Garcia et al., 2005; Sathre and O'Connor, 2010). However, increasing the preservation of harvested wood by improving preservative treatment methods (Song et al., 2018) or advanced landfilling could be a significant CO₂ removal approach with additional economic benefits. Zeng (2008) proposed harvesting live trees and other biomass from managed forests and burying the logs in trenches or otherwise storing them to prevent the carbon from being released. Improving the preservation of wood products from existing harvest operations and potentially increasing harvest with high levels of product preservation could be viable approaches to increasing carbon removal.

Wood removals from the world's 4 billion ha of forests average around 3 billion m³/y or 0.65 percent of the growing stock, of which about half is for timber products and half for fuel (FAO, 2015b). During harvest, a significant portion of the live biomass is left in the forest as logging slash (not including tree roots), ranging from roughly 30-40 percent of the pre-harvest biomass at the national scale, with much higher variability at more local scales (Oswalt et al., 2014; Winjum et al., 1998). Some of this logging slash is used for fuelwood and charcoal. Much of industrial wood removals comes from approximately one-half of the world's forests designated for timber production or multiple use (FAO, 2015b).

Much of the biomass removed from forests for timber products is emitted during primary processing into products, with losses ranging from roughly 20 percent to 60 percent depending on conversion efficiency (Bergman and Bowe, 2008; Ingerson, 2009; Kline, 2005; Liski et al., 2001). After the end of their useful life, wood products are typically deposited in landfills that are often designed for relatively rapid decomposition, or subject to other fates such as deposit in dumps that emit their stored carbon (Skog, 2008). According to several estimates, about 0.5 to 0.7 Gt/y CO₂ of the harvested carbon is sequestered in use or landfills after accounting for the inputs from current harvests and inherited losses from past harvests (Hashimoto et al., 2002; Miner and Perez-Garcia, 2007; Pan et al., 2011; Winjum et al., 1998).

Conventional Cropland and Grassland Practices

The vast majority of agricultural lands around the world are not managed optimally for increased soil carbon storage. Most annual croplands in temperate climates have bare-fallow conditions outside of the main crop-growing season, and intensive tillage practices are still widespread. Some annual cropland is on marginal lands that are subject to continuing soil degradation. Many pastures and rangelands employ unimproved grazing systems and suboptimal forage management. However, numerous conservation management practices are available that can increase carbon stocks in soils and are successfully practiced by progressive farmers and ranchers. In many cases these practices have been well studied, with long-term field experiments and comparative field observations. Table 3.2 lists several categories of management practices, classified according to their main mode of action in either increasing carbon inputs to soils and/or reducing carbon losses from soils.

TABLE 3.2

Examples of Conventional Agricultural Management Actions That Can Increase Organic Carbon Storage and Promote a Net Removal of CO₂ from the Atmosphere.

Following implementation of improved practices, soil carbon accrual rates can continue over several decades but attenuate over time as soil carbon contents tend toward a new equilibrium state with no further carbon gains unless additional carbon-accruing management practices are adopted. Furthermore, because of the dynamics of mineral-organic matter interactions that largely control the residence time of carbon in soil (Lehmann and Kleber, 2015), soils with initially low carbon concentrations have a greater propensity to gain carbon compared to soils with already high carbon concentrations. Thus there is effectively a “saturation limitation,” which varies as a function of soil texture and mineralogy (Stewart et al., 2007) and hinders further carbon accrual for mineral soils with very high organic matter contents.

Improved Annual Cropping Systems

On annual croplands, farmers may adopt several cropping choices that increase inputs of carbon into soils: replacing winter bare-fallow with seasonal cover crops, planting crops that produce large amounts of residues, promoting more continuous cropping (reduced summer fallow frequency) in semi-arid environments, and increasing the proportion of perennial grass/legume forage crops within crop rotations. Such cropping choices can maximize the time during which live vegetated cover is maintained on the soil and increase the amount of root-derived carbon added to the soil (Rasse et al., 2005). In the past few years, interest has grown in the use of cover crops, which have been strongly promoted by the U.S. Department of Agriculture's (USDA's) Natural Resource Conservation Service. Cover crop adoption is rising; however, adoption rates in the United States remain low (<5 percent of cropland area; USDA, 2014). This reflects unfamiliarity for growers, barriers from additional costs, restrictions related to crop insurance, and possibly immature technology in some areas. Systems to increase cropping frequency and reduce summer-fallow in semi-arid cropland have been successful in increasing productivity as well as soil carbon stocks (Peterson et al., 1998), although large areas dominated by alternate year wheat summer-fallow practices remain. Wider adoption of crop rotations that incorporate 2 to 3 years of grass or legume hay with annual crops (common in mid-20th century Corn Belt agriculture) is limited by the higher prices for main commodity crops (e.g., corn, soybean), which encourages continuous grain mono-cropping. Hence, for a variety of reasons, best cropping practices for sequestering carbon are not widely used on U.S. annual cropland, which means there is ample room to increase adoption rates if soil carbon sequestration becomes a more prominent policy goal.

Farmers use tillage to manage crop residues and prepare a seed bed for crops, and it is the main source of soil disturbance in croplands. Intensive tillage tends to accelerate decomposition rates of SOM (Paustian et al., 2000). Advances in tillage implement technology and agronomic practice have allowed farmers in recent decades to reduce tillage frequency and intensity, sometimes ceasing tillage altogether with a practice known as “no-till.” Reduced tillage systems, particularly no-till, can increase the mean residence time and slow decay of SOM (Six and Paustian, 2014), promoting greater soil carbon storage (Table 3.3). Many field studies show increases in soil OC following adoption of reduced till and no-till, with variations due to soil texture and climate. However, there are instances in which no-tillage does not increase soil carbon relative to conventional tillage, particularly in wet, cool climates where productivity under no-till may be reduced (Ogle et al., 2012) and in soils with already high topsoil carbon contents, where organic matter stabilization efficiency may be lower than for residues mixed deeper into the soil with tillage (Angers and Eriksen-Hamel, 2008; Ogle et al., 2012). No-till and reduced tillage are not widely adopted on most annual cropland in the United States, although there is significant regional variation.

TABLE 3.3

Examples of Published Summaries and Meta-Analyses for Soil Carbon Sequestration Rates with Conservation Practice Adoption on Annual Cropland.

In summary, a combination of an extended vegetated period, increased residue (particularly root-derived) inputs, and minimal tillage systems could be broadly implemented as best practices for increasing soil carbon stocks on annual cropland. Rates vary significantly as a function of climate, soil, and land-use history, but rates of 0.2-0.5 tC/ha/y over time periods of 20-40 years are representative, as observed in numerous meta-analyses of long-term field experiment data both globally and in the United States (Table 3.3).

Organic Matter Amendment

Organic matter additions such as animal manures and composts can increase soil carbon contents, both by virtue of the added carbon in the amendment itself and through improving soil physical attributes and nutrient availability. Cropland soils receiving substantial organic amendments invariably show increases in soil carbon concentrations derived from the amendment itself. However, this does not equate necessarily with a CO₂ removal from the atmosphere, but simply a transfer of carbon from another location (Leifeld et al., 2013). To the extent that the amendments improve soil performance and thus increase in situ plant productivity and residue carbon inputs, then the amendment can in fact stimulate real increased CO₂ removals. Hence a full life cycle assessment (LCA) approach in which the boundaries of the assessment extend outside the farm to include the greenhouse gas (GHG) and net carbon emissions with the production and baseline utilization of the amendment is needed for an accurate accounting of net CO₂ impacts. Because nearly all raw animal manure is land applied (Wu et al., 2013), increasing rates of manure addition at one location would necessitate an equal reduction elsewhere. Hence, counting the manure carbon added as part of the overall carbon balance is problematic. For other waste streams, such as municipally sourced compost, where the alternative use may be landfilling, an LCA approach that accounts for net emissions associated with a landfilling alternative vs production and land application of compost could be evaluated as a CO₂ emission strategy. One study of compost applications to grasslands in California (DeLonge et al., 2013; Ryals and Silver, 2013) estimated a net CO₂ removal and GHG reduction of 23 tCO₂e/ha for the first 3 years following compost application, because of soil carbon increase (discounting added compost carbon) and net avoided GHG emissions from diverting lagoon-stored manure and landfilled green waste to compost. However, similar studies are lacking for other areas in the United States.

Conversion of Annual Cropland to Perennial Vegetation

Probably the most effective means of increasing soil carbon stocks on annual cropland is to convert to perennial vegetation, either for grazing and forage production, afforestation (see previous section), dedicated energy crops (e.g., switchgrass, miscanthus), or as conservation set-aside. Perennial grasses in particular allocate a large fraction of their carbon assimilates to below-ground production, which enter the SOM pool through root exudation, sloughing, and turnover. Over time, in the absence of long-term soil degradation, SOC stocks can approach or equal those in precultivation native grassland systems. Perennial grasses purposed as dedicated energy crops (see Chapter 4) and established on former cropland typically increase soil carbon stocks, which can contribute to the net GHG balance of those systems (Field et al., 2018; Liebig et al., 2008). Conversion to perennial woody vegetation (e.g., forest) increases stocks of woody biomass carbon but may also increase soil carbon stocks. In many cases, accrual rates are similar to those for conversion to grassland vegetation (Guo and Gifford, 2002; Post and Kwon, 2000), although some studies suggest that soil carbon gains occur under deciduous species whereas soil carbon gains under coniferous woody vegetation are minimal (Laganière et al., 2010; Morris et al., 2007). Several meta-analyses have documented soil carbon gains on cropland converted to perennial vegetation (Table 3.4).

TABLE 3.4

Examples of Published Summaries and Meta-Analyses for Soil Carbon Sequestration Rates Following Conversion of Annual Cropland to Perennial Grassland or Forest Vegetation.

Agroforestry

Agroforestry involves the incorporation of trees into agricultural systems, either in combination with annual crops or grazed pastures (the later often referred to as silvopastoral systems). There are many different practices in which trees may be interspersed with crops (e.g., alley cropping) or herbaceous forage, used as border or buffer plantings (e.g., living fences, windbreaks, forest buffers) or used in a time sequence or rotation with annual crops (e.g., improved tree fallows). Regardless of form, the inclusion of perennial woody species in combination with annual crops or pastures typically results in increased soil carbon stocks and woody biomass stocks (Table 3.5).

TABLE 3.5

Examples of Carbon Accrual Rates for Biomass and Soil Carbon from Recent Meta-Analyses and Multisite Assessments of Temperate Zone Agroforestry Practices.

Improved Grazing Land Management

Grasslands contain some of the highest soil carbon stocks of any managed ecosystems. With the exception of some managed pastures, grazing lands are infrequently or never tilled, and the perennial grasses that dominate in most rangelands and pasture allocate a substantial portion of their photosynthetically fixed carbon below ground, thus supporting comparatively large soil carbon stocks.

Grasslands in the United States can be loosely categorized into pastures, which are typically in more mesic (i.e., moister) environments, with higher productivity, often with selected (sown) species and more intensive soil management such as fertilization, liming, and irrigation. Pastures in some cases may be rotated with annual crops and typically have soils that were tilled at some time in the past. In contrast, rangelands are mainly found in semi-arid and arid climates, are usually dominated by native grassland vegetation, often have never been tilled, and usually have no (or minimal) management interventions in terms of fertilizer or irrigation use. Grazing management is the primary management “lever” for rangelands.

Grazing management influences grassland productivity and carbon storage (Table 3.6) in both pastures and rangelands, via the amount, timing, and duration of vegetation removal by grazing animals (Milchunas and Lauenroth, 1993). With excessive grazing pressure (overgrazing), plant productivity and hence carbon uptake is reduced and soil carbon stocks decrease (Dlamini et al., 2016). Therefore, reducing stocking rates and grazing intensity can allow vegetation productivity to recover and carbon stocks to increase. However, other than eliminating overgrazing, there is considerable debate regarding the impact on soil carbon of more intensively managed grazing systems (i.e., rotational grazing, adaptive multipaddock, mob grazing), compared with less intensively managed, continuous grazing systems. It has been suggested that management-intensive grazing—short heavy-grazing periods followed by long grazing-free periods—can increase soil carbon storage (Chaplot et al., 2016; Wang et al., 2015), although many experiments do not show a significant difference between light to moderate continuous grazing vs management-intensive grazing (Briske et al., 2008). However, the interactions that determine vegetation productivity and soil carbon responses for different grazing systems are complex (McSherry and Ritchie, 2013). In general, grazing systems that maintain plant cover and maximize plant vigor and productivity are most conducive for building and maintaining soil carbon stocks (Eyles et al., 2015). Additional field-based research and improved models are needed to determine the best grassland- and region-specific systems for promoting soil carbon sequestration and their capacity for net GHG reductions and CO₂ removal (Conant et al., 2017; Eyles et al., 2015).

TABLE 3.6

Examples of Soil Carbon Sequestration Rates from Recent Meta-Analyses and Field Studies of Improved Grassland Management Practices.

Rewetting of Organic Soils

Organic soils (i.e., peat and muck soils), which develop under wetland vegetation and are saturated with water to the soil surface for a substantial part of the year, have extremely high organic matter content (>12 percent of soil mass as OC, for a minimum 20 cm horizon depth; FAO, 1998) that can extend to several meters in depth. When converted to agricultural use (or forest plantations) following drainage, liming, and fertilization, they can be extremely productive soils, but with rapid rates of organic matter decomposition and carbon loss rates as high as 20 tC ha/y (Armentano and Menges, 1986). Consequently, restoring the wetland hydrology and perennial vegetation can reverse the processes driving soil carbon losses and greatly reduce CO₂ losses compared to drained organic soils and in many cases can reestablish the soil as a net carbon sink, although increased methane (CH₄) emissions following rewetting can decrease the overall net sink. In a global review, Wilson et al. (2016) reported that rewetting managed organic soils led to net annual CO₂ removals for most soil classes (usually <1 tC ha/y), varying as a function of thermal regime, site productivity, water table height, and time since restoration. Most lacking are data for impacts of restoration on tropical organic soils.

Frontier Cropland and Grassland Practices

The management interventions described above are well known and used to varying degrees in the United States and elsewhere and have been the subject of previous (and ongoing) research. In most cases, causal mechanisms and relative magnitude of carbon stock responses are known, although important basic and applied science questions remain. In contrast, the committee characterizes several approaches as “frontier technologies” because they are in an early stage of development; many questions regarding rates and capacities for carbon removals remain unanswered, and there has been little or no deployment of these technologies outside of field research situations. Two of the technologies involve increasing the residence time and stability of OC in soils, and the other two involve developing plants with greatly enhanced capacity to add carbon to the soil.

Biochar Amendment

Pyrolysis (heating in the absence of oxygen) of plant material is an exothermic (i.e., energy-yielding) process that produces volatile compounds and oils (which can be purposed for bioenergy and biomaterials), as well as solid, “charred” organic residues with high aromaticity and reduced O:C and H:C ratios, that are much more resistant to microbial decomposition than the original plant residues (e.g., straw, wood, shells). The longevity of biochar applied to soil can vary substantially depending on the pyrolysis temperature and duration, biomass type, and climate and soil conditions. Highly condensed, low H:C chars derived from wood are highly recalcitrant and can remain in soils over century timescales (Lehman et al., 2015). Naturally occurring pyrogenic carbon (e.g., black carbon, charcoal) comprises a substantial portion of the total soil OC in many fire-prone grassland and forest ecosystems (Skjemstad et al., 2002), and additions of charred biomass by Neolithic farmers have resulted in soils with greatly increased OC stocks (e.g., Terra Preta soils of the Amazon; Glaser and Birk, 2012).

Pyrolysis is a bioenergy pathway (predominantly for the production of liquid biofuels) that could be part of a bioenergy production strategy (see Chapter 4). Pyrolysis BECCS aims to both displace fossil-based fuels as well as produce a biochar coproduct that can be added to soils for long-term carbon storage. Alternatively, purposed biochar production, with the pyrolysis liquids and volatiles combusted for process heat and/or energy generation, can produce biochar as the main product for soil application to sequester carbon and improve agronomic performance. The extent to which biochar production and addition to soil represents a net CO₂ removal depends on a full lifecycle consideration of all GHG emissions associated with the biomass sourcing, its production and use, and fossil fuel offset values (Roberts et al., 2010; Woolf et al., 2010). In addition, indirect impacts of soil application of biochar on other biogenic GHG emissions and plant carbon uptake factor into assessments of its potential to reduce net GHGs. Recent meta-analyses suggest that biochar applications may reduce nitrous oxide (N₂O) emissions from soil on the order of 10 percent (Verhoeven et al., 2017), although the response varies between soils, with some biochar additions increasing N₂O flux. Sánchez-García et al. (2014) suggest that biochar decreases N₂O emissions from the denitrification (anaerobic) pathway but increases emissions from the nitrification (aerobic) pathway, possibly accounting for some of the differences between soil types and soil conditions in biochar effects on N₂O flux. Biochar applications also appear to decrease CH₄ emissions from flooded (e.g., rice) soils and in acid soils (Jeffery et al., 2016), whereas application to nonflooded soils with neutral or alkaline pH may reduce rates of CH₄ oxidation (a CH₄ sink). Finally, biochar application often increases plant productivity and hence plant carbon inputs to soil, particularly in highly weathered and acidic soils, averaging about a 10 percent increase in recent meta-analyses (Jeffery et al., 2011; Verhoeven et al., 2017). However, plant responses vary by soil type, biochar attributes, and management systems, and thus further research is needed to better identify conditions under which biochars can best increase plant carbon uptake, reduce non-CO₂ GHG emissions, and contribute to net CO₂ removals from the atmosphere.

Deep Soil Inversion

Placing organic matter–rich surface soil into subsoil layers, through a one-time deep inversion tillage, may substantially increase the overall SOM residence time. For most soils, microbial activities decrease sharply with depth, because of less aeration, cooler temperatures, and sparser and more dispersed organic matter inputs. In addition, the placement of low organic matter content subsoil at the top of the soil profile where the majority of the root system resides will increase soil carbon sink generated by root system and photosynthesis process. Alcantara et al., 2016 found that deep plowing and burial (to >50 cm depth) of organic matter–rich surface soil horizons, coupled with subsequent enhanced carbon accumulation in surface-exposed and carbon-depleted subsoil horizons, yielded an average carbon accumulation rate of ca. 3.6 tCO₂ ha/y over a 40-year time period at several cropland sites in Germany. Testing of deep inversion tillage for carbon storage has recently started in some other locations (e.g., New Zealand; M. Beare personal communication), but not to the committee's knowledge in the United States. A large portion of U.S. cropland in humid regions, particularly in the Central and North-Central states, might be potential areas for implementation.

High Carbon Input Crop Phenotypes

The most direct way to increase soil carbon storage is to increase the rate of OC addition to soils in the form of plant residues, particularly below ground. Here we discuss two approaches: (1) increasing the relative amount of carbon allocated to roots for the annual crops that dominate current agricultural food production systems and (2) developing new perennial food crop species, which have inherently higher below-ground carbon allocation and could mimic the high soil carbon storage capacity found in perennial pasture grasses.

Breeding for Annual Crops with Increased Carbon Input

A main reason that annual crops are less effective at increasing soil OC stocks compared to perennial grasses is the smaller amount of dry matter allocation below ground to roots and rhizosphere deposition. Most soil OC is derived from roots (as exudates and through root death and turnover; Rasse et al., 2005), and thus an option to increase soil OC stocks would be to develop crops that allocate more dry matter below ground and/or have deeper root systems where the decomposing OC compounds would have a longer mean residence time (Kell, 2012). Increasing carbon allocation to roots without reducing above-ground yields will be a key issue for crop breeders, but is possible for several reasons. Many root characteristics (e.g., architecture, depth distribution, size) are strongly controlled by genetic traits that can be selected for (Hochholdinger et al., 2004; York et al., 2015). Where available nutrients such as phosphorus limit yields (common in tropical soils), enhanced root growth can increase nutrient acquisition and increase total assimilation and yield (Lynch, 1995). Finally, sink size inhibition often limits total plant carbon assimilation and thus plant breeding because an increased root carbon sink does not necessarily reduce above-ground productivity and yields (Jansson et al., 2010). Preliminary estimates suggest that widespread adoption of annual crops with enhanced root-phenotypes could increase carbon stocks in U.S. soils by 500-800 Mt/y CO₂e over several decades (see Table 3.7).

TABLE 3.7

Published Estimates of Global and U.S. Soil Carbon Sequestration Potential on Managed Nonforested Lands.

Conventional Forest Practices

Afforestation/Reforestation

For the United States, comprehensive analyses have shown that the potential carbon removal from afforestation/reforestation ranges from 0.0 to 0.45 Gt CO₂ annually, over a period of approximately 50 to 100 years (Table 3.8). The soil component of this increase in carbon removal may be significant according to a recent study that estimated a range of 0.05 to 0.08 Gt/y CO₂ (Nave et al. 2018).

TABLE 3.8

Total Global and U.S. Estimates of Carbon Removal Potential for Forestry Activities.

At the global scale, the range of published estimates for afforestation/reforestation is large, from 2.7 to 17.9 Gt CO₂ annually based on global analyses using top-down approaches for the high end of the range, and bottom-up approaches for the low end (Table 3.8). The wide range reported in studies represents different assumptions and modeling approaches, and variable prices or incentives for implementing activities.

At the low end of the range, the assumed price of carbon is low and secondary impacts are few—for example, sufficient marginal agricultural land is available and landowners are willing to participate in mitigation. At the high end, the price of carbon would be as high as $100 tCO₂, and tens of millions of hectares would be incentivized to convert from crop or grass production to forest. Food production would be significantly impacted, so that the total land area that could be reforested or afforested is constrained by the higher value of alternative land uses (i.e., food price impacts from cropland conversion). The net CO₂ reduction may be reduced over time by leakage (i.e., timber harvest elsewhere to meet continued economic demand for forest products), and concerns about permanence (subsequent clearing or natural disturbances).

Near-term forest carbon removal potentials

For a recent and practical reference point, summing the forestry activities proposed by countries in their intended nationally determined contributions (INDCs) to the Paris agreement yields a total expected global carbon removal benefit from afforestation/reforestation and improved forest management (excluding avoided deforestation) of approximately 1.0 Gt CO₂. This is comparable to the low end of the estimates reported above, because the INDCs were purposely modest, representing what different governments considered achievable toward a target of limiting climate change to 2°C.

A modest program of afforestation/reforestation near the lower end of published ranges could be 0.15 Gt/y CO₂ for the United States and 1.0 Gt/y CO₂ for the world, taking into consideration availability of land and secondary impacts (described later in this chapter). Considering land constraints, a practical upper limit for carbon removal from afforestation/reforestation would be about 0.4 Gt/y CO₂ for the United States and likely no more than 6.0 Gt/y CO₂ for the world (Box 3.1). Similarly, the near-term carbon removal from improved forest management is likely to be near the lower end of these ranges, about 0.1 Gt/y CO₂ for the United States and 1.5 Gt/y CO₂ for the world. The upper limit of a more aggressive program to improve forest management could achieve a rate of carbon removal of 0.2 Gt/y CO₂ for the United States and 3.0 Gt/y CO₂ for the world.

BOX 3.1

SUMMARY OF TERRESTRIAL CARBON REMOVAL ESTIMATES.

Frontier Forestry Practices

Conventional Cropland and Grassland Practices

During the past 20 years, several estimates of the soil carbon sequestration potential for the United States and the world have been released (Table 3.7). Nearly all represent a technical or “biophysical potential” assuming nearly complete practice adoption. As such, they represent upper-bound estimates of the carbon sequestration potential for the practices considered. However, the estimates do account for limitations relating to land availability, such as the limited land-use conversions to high carbon storage practices (e.g., permanent grassland set-aside that could occur without compromising food and fiber production). Most published estimates use aggregate data stratified by broadly defined land-use and climate categories (in some cases soil type), together with representative per hectare rates for different land-use/management practices determined from long-term field experiments or other measurements (e.g. chronosequence).

Frontier Cropland and Grassland Practices

Detailed analyses of the global sink potential of the frontier practices described earlier (i.e., high carbon input phenotypes, deep soil inversion) have not been published, with the exception of biochar amendment. Recent estimates of the global carbon sink capacity of large-scale deployment of biochar application to soil suggest gigaton per year potential (e.g., 6.6 Gt/y CO₂e by Woolf et al. [2010], 2.6 Gt/y CO₂e from Smith [2016], and 0.5-2.6 Gt/y CO₂e by Fuss et al. [2018]). Because biochar applications mainly target existing current cropland where large addition rates (up to 50 tC/ha; Smith, 2016) are agronomically feasible, there is no land constraint for application, although there is for biomass feedstock sourcing (see Land Requirements section below). Key uncertainties remain, including net life cycle GHG impacts, long-term impacts on crop productivity and economic feasibility of large-scale biochar deployment.

Forest

The impact on climate from reducing CO₂ by afforestation/reforestation may be partially offset by albedo changes (the proportion of radiation reflected by the land surface). Generally, an increase in forest cover reduces surface reflectivity causing more surface warming, whereas a harvest increases surface reflectivity causing a cooling effect, implying that the effect of increasing harvest or forestland clearing would have a less negative impact on climate than indicated by only accounting for GHG changes. Albedo effects are typically larger in the years immediately following harvest, last longer for land use changes, and are most significant in response to changes in conifer coverage above the snow line (Cherubini et al., 2012; Holtsmark, 2015). These effects on climate forcing are usually less influential than the effects of changes in GHGs but may be highly variable (Anderson-Teixeira et al., 2012). In general, afforestation/reforestation in boreal zones will have a warming effect that exceeds the cooling effect of reducing GHGs, and the opposite effects in the tropics. In temperate zones, the effects are highly variable in space, depending on vegetation type, timing of snow cover slope, aspect, and other factors. Recent work has extended the analysis of radiative effects to include nonradiative processes such as air turbulence, which may have larger and locally more variable effects than albedo changes (Bright et al., 2017).

Cropland/Grassland

In designing and implementing CO₂ removal strategies involving agricultural land management, impacts on other GHGs, particularly N₂O and CH₄, are of paramount importance. Agricultural soils are the single largest source of N₂O, driven by the large additions of nitrogen to promote plant growth. Flooded cropland soils (rice) are a major CH₄ source. Soil-based CO₂ removal strategies should avoid requiring additional nitrogen fertilizer, which has large embodied fossil carbon emissions for its manufacture, in addition to impacts on N₂O emissions (van Groenigen et al., 2017). Thus, any additional nitrogen requirements to support increased plant production and carbon inputs should be sourced from more efficient use of nitrogen already applied to agricultural soils as well as from biological nitrogen fixation. Management options that offer synergisms with both increased soil carbon stocks and reduced N₂O (and CH₄) emissions should be prioritized.

Agricultural practices to increase CO₂ removal likely have minimal impact on other (e.g., aerosols, albedo) radiative forcing factors. For example, conversion of winter-fallow bare soils to lighter surface of senescent winter vegetated cover, as well as reduced tillage systems that retain surface residues, increases albedo in most cases, providing a small cooling effect (Davin et al., 2014). Most other agricultural carbon removal practices will have negligible impacts on albedo.

Forestry

Even relatively low amounts of carbon removal and carbon sequestration from afforestation/reforestation (0.15 Gt/y CO₂) in the United States would require converting 3-4 Mha of nonforested land to never-harvested forest. Improved forest management would require changes on 11-19 Mha of existing forest for a modest increase in carbon removal of 0.1 Gt/y CO₂. Global land requirements for afforestation/reforestation to achieve 1.0 Gt/y CO₂, and improved forest management to achieve 1.5 Gt/y CO₂, are 70-90 Mha and more than 1,000 Mha of existing forestland, respectively. These levels of activity should be achievable at low carbon prices (i.e. $10-50/Mg CO₂), but safeguards would be necessary to reduce negative impacts and ensure permanence. In addition, implementation would require a very high participation rate by landowners.

Although higher levels of carbon removal are technically achievable from afforestation/reforestation, attaining 0.4 Gt/y CO₂ in the United States would require much larger areas of land (up to 20 Mha) and a high carbon price (i.e., >$50/Mg CO₂). Globally, the area of land needed to support a 6 Gt/y CO₂ carbon removal from afforestation/reforestation would require up to 500 Mha of land. These higher levels of afforestation/reforestation would have significant negative impacts, including reduced food production and increased food prices.

Higher levels of improved forest management in the United States to achieve 0.2 Gt/y CO₂ would require 22-38 Mha of existing forestland. Worldwide, the levels to achieve 3.0 Gt/y CO₂ would require more than 2,500 Mha of existing forestland. These management changes could result in leakage by shifting some harvesting elsewhere and would require participation by most private landowners, which would be difficult to achieve.

Agriculture

The majority of management practices considered for carbon removal in agricultural soils do not involve changes in land use and hence would largely be operative on the existing land base devoted to agricultural production. To the extent that many practices improve soil health and productivity, carbon removal activities could contribute toward meeting increased food and fiber demands without increasing the area of land under agriculture. The main activity that could contribute to displaced agricultural production and potential land-use conversions elsewhere would be setting aside cropland into perennial grassland (or forest, see above), for conservation purposes. To the extent that degraded or otherwise marginal agricultural lands are targeted for set-asides, leakage effects (i.e., displacement of agricultural production resulting in land-use conversion and soil carbon losses elsewhere) would be minimal. Currently, roughly 9.8 Mha of cropland is in set-asides under the U.S. Conservation Reserve Program (CRP). At its height during the late 1990s, the CRP area exceeded 13 Mha (Mercier, 2011), which resulted in estimated leakage of around 20 percent of the sequestered carbon (Wu, 2000), although potentially larger leakage has been suggested (Murray et al., 2007).

BECCS

As discussed in Chapter, 4 the committee calculated the low-end estimate for this approach assuming that sources of biomass from existing land uses (e.g., wood and other unutilized organic waste) are sufficient and no land-use changes are necessary.

Dedicated land requirements for the high-end estimate for the United States are based on reported economically feasible energy crops that could produce up to 0.65 Gt CO₂e annually (U.S. DOE, 2016), Assuming an average productivity of 18 t CO₂e per ha, this level of production would require 36 Mha of land. Scaling this estimate up to our high end of 1.4 Gt CO₂e indicates a requirement of 78 Mha, equivalent to almost 20 percent of the current agricultural land base (Table 3.9). Smith et al., 2016 estimate the global land area required to deliver 12 Gt/y CO₂e at approximately 380-700 Mha in 2100 for high productivity energy crops, such as willow, poplar short-rotation coppice, and miscanthus. Therefore, the large-scale implementation of BECCS is expected to compete with terrestrial carbon capture and storage initiatives, as well as with food production (e.g., Smith et al., 2010) or the delivery of other ecosystem services (e.g., Bustamante et al., 2014).

Aggregated Carbon Removal Land Requirements and Availability Of “Marginal Land”

The total area of land managed for agriculture and forestry is 791 Mha in the United States and 7,130 Mha globally (Table 3.10). Some of this land may be considered marginal by landowners, which could serve as an indicator of the amount of land that could be shifted to another use without significant effects on production of necessary services, particularly food. There is no generally accepted definition of marginal land (it is an economic decision that varies over time or is based on subsistence needs), but an indicator in the United States is the ca. 10 Mha of farmland currently enrolled in the CRP. A similar global estimate is more difficult to determine, but one recent study estimated that the global pool of marginal land is about 1,300 Mha. This land supports about one-third of the world's population and therefore only a fraction would be available for afforestation/reforestation and BECCS. Another global analysis estimated that 760 Mha of land are available and suitable for afforestation in IPCC non-Annex 1 countries, which are mostly in tropical and subtropical biomes (Zorner et al., 2008).

TABLE 3.10

Managed Land Area by Selected Land-Use Categories in 2015 (Mha).

Because new land dedicated to BECCS would likely be planted with fast-growing species such as miscanthus (on more productive sites), the area required per unit of carbon removal would be much less than that required for afforestation/reforestation. Humpenöder et al. (2014) highlighted the trajectories and relative land requirements for afforestation and BECCS (Figure 3.3) using a land-based modeling approach that simulated a similar level of carbon removal as the high-end calculation in this report. They suggest that the area required for BECCS is much lower than that required for afforestation (Table 3.9) based on strongly increasing yields per hectare for herbaceous biofuel, up to 25-30 t/(ha y) C, compared with afforestation yields that are estimated at a much lower rate for natural regeneration of forests, about 2-6 t/(ha y) C (Table 3.1). However, as noted in the BECCS chapter, the temporal nature of bioenergy production should also be considered.

FIGURE 3.3

Simulated time series of global land use for business as usual (BAU), afforestation (AFF), BECCS, and AFF + BECCS. NOTES: Both increased afforestation and BECCS require additional land that significantly impacts land used for food and pasture. By the (more...)

In summary, at the low end of the committee's estimates for carbon flux and capacity for afforestation/reforestation and BECCS, any competition for land between these two NETs would not be significant (because BECCS biomass could be sourced entirely from existing land uses), nor would there be competition for land currently required for food production. Land requirements for afforestation/reforestation both in the United States and globally could be met by tapping into land considered “marginal” for food production, although globally, some of the required marginal land is used to support population needs for food and fiber. At the high end shown in Table 3.9, the total area required for afforestation/reforestation and BECCS combined would likely exceed the availability of marginal land. Considering the additional impact on land required for food production, this level of carbon removal is not likely to be realized.

High Carbon Input Crop Phenotypes

Research is needed to develop crop varieties with altered root morphology and biomass (e.g., more roots, deeper root distributions, more recalcitrant to decomposition) to add and maintain high root carbon inputs to soil, while maintaining high aboveground yields. Specific research areas include crop breeding and selection for annual crops with larger and deeper roots, perennialization of major grain and oilseed crops, advanced root phenotyping technologies, new crop performance trials, and soil and ecosystem responses to new crop varieties. Such a research program can build on the recently initiated ROOTS (Rhizosphere Observations Optimizing Terrestrial Sequestration) program established by ARPA-E.² The cost for this research is approximately $40-50M/y for an initial 20-year period and could be conducted through USDA, the National Science Foundation (NSF), and/or the Department of Energy (DOE). For comparison, current annual research and development (R&D) funding for conventional crops improvement and genetics in the United States is approximately $1.5 billion from the public sector and $1.8 billion from the private sector (Fuglie and Toole, 2014).

Soil Carbon Dynamics at Depth

Research is still limited on the controls on decomposition and stabilization of SOM in subsoil, that is, below 30 cm. Several promising technologies to increase soil carbon stocks (e.g., prairie restoration, deep soil inversion and carbon burial, enhanced root phenotypes) are based on increasing carbon additions to subsoil layers. Studies are needed to determine how stabilization and residence times of organic residues vary as a function of depth gradients in aeration, microbial community composition, soil physiochemical properties (e.g., soil texture, soil mineralogy), and plant (root) residue composition. The cost for this research is approximately $3-4M/y for 5 years and could be conducted by USDA and NSF.

Harvested Wood Preservation

Research is needed in two areas: The first area is landfill designs for achieving the lowest possible rate of wood decomposition. Reducing wood decomposition has not been a stated goal of landfill designs; rather, they have been designed to contain buried waste, collect contaminated precipitation that percolates through the waste (leachate), and collect and control gas emissions. A large body of landfill design research at several representative sites forms a foundation for focused attention on minimizing wood decomposition (U.S. Forest Service [USFS], U.S. Environmental Protection Agency [EPA], and NSF; Cost: $2.4M/y for 3 projects lasting 3 years). The second area is integrated assessment of net greenhouse balance, costs, and required land, including the implications for worldwide consumption of wood products and their life cycle emissions. Any change in consumption of wood products will have widespread effects on emissions related to other sectors of the economy, for example, construction or material transportation. (For the existing research capacity and cost, see the integrated assessment section in the BECCS chapter.)

Biochar Amendment Studies

Although much is already known about residence time in soil for different types of biochar and how biochar characteristics vary as a function of the feedstock and the pyrolysis process used, additional research is needed to assess secondary impacts of different biochars on crop performance, nutrient cycling and retention, and N₂O and CH₄ emissions from soils, all of which affect the net GHG consequences of biochar amendments. In addition, research to produce full life cycle analyses that consider the alternative fates/uses of feedstocks from which the biochar is produced (Paustian et al., 2016a) are needed for a comprehensive assessment of net carbon removal potentials from biochar amendments.

Monitoring of Forest Carbon Stock Enhancement Projects

For private and public forestlands, USFS should develop a plan to monitor recommended carbon stock–enhancing activities, conduct statistical sampling of total ecosystem carbon stored in a subset of projects, and develop local “climate impact factors” that also account for biophysical effects. It is time consuming and expensive to directly measure the effects of many small projects on net GHG emissions; therefore, approaches that could achieve accurate average estimates for aggregates of projects (based on remote sensing and validated expansion factors) are needed to reduce transaction costs. Additional needs are to monitor leakage (which is a global phenomenon) and to use the monitoring system to attribute observed changes in carbon removal to management activities vs increasing CO₂ or climate change. Monitoring leakage could require a new LiDAR satellite dedicated to mapping global forestry activities. Knowledge and monitoring of lateral transfers of carbon from land to inland waters are lacking, and these transfers are not currently detected by remote sensing or operational field inventories. The substantial capacity of current remote sensing and field monitoring could be built on to address the additional needs described here. For example, the USFS Forest Inventory and Analysis (FIA) program is funded at approximately $70M/y, collects continuous field data on status and trends of U.S. forests, and collaborates with NASA to develop methods to integrate remote sensing data with field data. Internationally, the status of monitoring is highly variable, with many countries lacking field measurements and capacity to implement monitoring programs. However, there is a significant international aid effort to improve capacity in forest monitoring at the country scale, as well as to advance research on global monitoring capability using satellites. The research cost for the United States only is approximately $1.0M/y for 3 years for system development, and continuous operation would cost approximately $4.0M/y to staff a small office to analyze data, coordinate field checks, and develop reports. A significant contribution to improving international forest monitoring and reporting, including detection of leakage worldwide, would require about 10 to 20 times the amount needed in the United States.

A National On-Farm Soil Monitoring System

USDA should fully implement a national on-farm soil monitoring system on existing National Resource Inventory (NRI) points on cropland and grassland. Full buildout to approximately 5,000-7,000 NRI locations with soil sampling and analysis carried out at intervals of 5-7 year (on an annual rotating basis similar to the FIA system) is recommended. Similar systems already exist in many countries, including in the European Union, Australia, New Zealand, and China (van Wesemael et al., 2011). This system would provide an ongoing data stream to improve national-scale soil carbon inventory systems and reduce uncertainties. Such a monitoring system has the potential for broader utility for tracking and assessment of soil health and to support long-term sustainability of soils for the U.S. agricultural economy, which has a gross domestic product of more than $135 billion from farm output alone. The cost for this system is $5M/y as an augmentation of USDA's existing NRI system.

Model-Data Platform for Quantifying and Forecasting Carbon Removals by Soils

For implementation of soil carbon sequestering practices at Gt/y scales, improved platforms for quantifying soil carbon storage accurately and cost effectively are needed (Figure 3.6). Such systems should integrate data from a repository of existing and new experimental field sites to inform process-based land use/ecosystem models that can be independently validated using on-farm soil monitoring networks. Spatial data layers of model drivers (e.g., weather, soil maps, topography), together with comprehensive remotely sensed activity data on management practices (e.g., crop species, presence/absence of cover crops, tillage system, irrigation, vegetation productivity) provide the main data inputs to the model. Potential crowd-sourcing of farm management information (e.g., nutrient management) from the land users themselves can provide data that cannot be readily obtained from remote sensing. Such a system is scalable to estimate national-scale outcomes of CO₂ removals over time as a function of land use policies (e.g. for national GHG inventory reporting) as well as to provide information on field- and local-scale dynamics that can inform policies based on carbon markets and/or sustainable product supply chains supported by agricultural industries. Many component parts of such a system (e.g. field experiment networks, remote sensing data) already exist and could be leveraged. Funding of $5M/y for a 3-year development period followed by annual operating costs of similar magnitude would support system integration, data-model fusion, model development, decision support systems, visualization and communication.

FIGURE 3.6

Conceptual design for a data-model platform for quantifying carbon removal to soils.

Forest Demonstration Projects

Conventional forestry practices have been implemented worldwide for decades, so demonstrations are readily available. One area requiring new demonstrations is the frontier technology of increasing preservation of harvested wood products, particularly, how to improve the disposal and collection processes of wood products after their useful life. This research is linked to the basic research needed to improve landfill design for for harvested wood product preservation. The cost for USDA and EPA to conduct this research is $1.5M/y for 3 projects for 3 years. Associated with the basic research of improving landfill design to improve preservation of harvested wood, it is necessary to implement several demonstration projects representing different environmental conditions to facilitate scaling up this new technology. The cost for this research done by USFS and EPA is $1.5 M/y for 3 projects for 3 years. Another promising activity requiring demonstrations of potential impacts on carbon removal is forest restoration, which has been described for fire-prone areas as increasing carbon removal in the long term as a result of short-term reduction in carbon stocks of overstocked stands; for areas converted from natural vegetation to unsustainably managed forests that prematurely lose ability to sequester carbon; and for degraded forests that would respond to interventions to improve regeneration and stocking. The cost for USFS and partners to conduct this research is $1.5M/y for 3 projects for 3 years.

Agricultural Systems Field Experiment Network

Field experiments to rigorously evaluate region-specific best management practices for soil carbon sequestration (and net GHG reductions), in comparison to conventional practices, should be established in a coordinated network across the major U.S. agricultural soil types, involving land-grant universities and other research institutions with the relevant expertise in ecosystem carbon and GHG dynamics. Sites selected should complement existing experimental sites in USDA Agricultural Research Service's (ARS) GraceNET and Long-term Agricultural Experiment networks and relevant sites within NSF's Long-Term Ecological Research (LTER) and National Ecological Observatory Network (NEON) programs. The new network should have a coordinated set of measurement protocols and methods, including whole system carbon balance (i.e., eddy covariance) and other GHG (e.g., N₂O and CH₄) flux measurements as well as precision soil carbon stock and stock change measurements. Data sharing and archiving to support modeling and meta-analysis should be priority and field sites should include significant extension, outreach, capacity building, technology transfer, and demonstration to provide information to producers, extension agents, crop consultants, agency personnel, and other stakeholders. Sites should be designed for the field experiments and demonstrations to run for a minimum of 12 years. This research should be conducted by land-grant universities and USDA at about 10-15 sites at a cost of $600K/y per site for a total of $6-9M/y.

Forest Carbon Project Deployment

Although scientific uncertainty about the amount of carbon sequestered locally from improved forestry practices is low, uncertainty about far-field impacts caused by the long-term effects of climate change and the economic feedback is high. To reduce this uncertainty, the following research is needed:

(1)

Improvement in economic models that estimate leakage (i.e., reduced harvest in one location causing deforestation in another) and the impact of converting cropland to forest on food prices. Biological research is also needed to develop understanding of the impacts of significant impacts of carbon removal on biodiversity and of safeguards to ensure a sustainable biosphere. This research and related costs are discussed in the section on the research agenda for integrated assessment modeling in Chapter 4.

(2)

Improvement in LCA methods to evaluate substitution of wood products for other materials that could expand wood use, especially if these displace structural materials such as steel and cement, which require large carbon emissions to produce. Substituting wood for other material typically results in a net reduction of GHGs; however, tools to quantify the net reductions in GHGs are not readily available or validated. The research needs for LCA and related costs are discussed in Chapter 4.

(3)

Social sciences research on how landowners respond to incentives and the utilization of Extension Service system at the land-grant universities, to reach more landowners and practitioners who work on small parcels of forest to improve the equity of participation in assistance programs that tend to favor larger landholders. Only a small percentage of forest landowners respond to incentive programs or price signals by changing land management practices. Very little research has been conducted on these topics (USDA and NSF; $1.0M/y for 3 projects over 3 years).

(4)

Research on GHG and social impacts of reducing traditional uses of biomass for fuel, which involves households and small entities using wood biomass for heating and cooking. Globally, increasing durable wood products or increasing commercial use of biofuel could reduce consumption of biomass for traditional fuel use, which would have a ripple effect on types of energy supplied (USDA and NSF; $1.0M/y for 3 years).

(5)

Social science research on responses to economic and behavioral incentives to reduce meat consumption and food waste. This research and related costs are included in the research agenda for integrated assessment modeling in Chapter 4 (BECCS).

Scaling Up Agricultural Carbon Sequestering Activities

Current adoption of existing best management practices for soil carbon sequestration are low (e.g., cover crop are used on <5 percent of U.S. cropland; Wade et al., 2015) and barriers to scale-up, including economic and behavioral changes (e.g., value proposition, risk management, motivation), information needs, and technology transfer, are poorly understood. Economic and behavioral research along with continuation and expansion of pilot emission reduction and carbon removal projects, such as through the USDA Natural Resource Conservation Service's Conservation Innovation Grants, can provide needed empirical knowledge on which barriers are most limiting and on how to design the most effective policies and education programs to promote agricultural carbon removal activities at scale.

Regional Life-Cycle Assessments

Any program to offer incentives to landowners for adopting carbon removal and storage should be preceded by a regional assessment of life cycle emissions, costs, co-benefits and negative impacts, including estimates of leakage and the degree of permanence, which can be undertaken by federal land management agencies (e.g., USDA Forest Service and USDA Agricultural Research Service) and/or university researchers, with a review of the findings by an independent scientific board. This research and related costs are discussed in the LCA section of Chapter 4.