Editing IPCC:AR6/WGI/Chapter-5 (section)

===== 5.6.2.2.1 Land-based biological CDR methods =====

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Biological CDR methods, introduced in Table 5.9, seek to increase carbon storage on land by enhancing net primary productivity and/or reducing CO <sub>2</sub> sources to the atmosphere.

Forest-based methods include afforestation, reforestation, and forest management (Table 5.9). Building on previous work that emphasized the global potentials of various options, more recent advances have focused on the limits of those global potentials in light of ecological and climate risks that can threaten the long-term permanence of carbon stocks ( [[#Boysen--2017b|Boysen et al., 2017b]] ; [[#Anderegg--2020|Anderegg et al., 2020]] ). Some of those risks arise from droughts, fires, insect outbreaks, diseases, erosion, and other disturbances ( [[#Thompson--2009|Thompson et al., 2009]] ).

Sustainable forest management can help to manage some of these vulnerabilities, while in some cases it can increase and maintain forest sinks through harvest, transfer of carbon to wood products and their use to store carbon and substitute emissions-intensive construction materials ( [[#Churkina--2020|Churkina et al., 2020]] ). Forest genomics techniques can increase the success of both reforestation and conservation initiatives, accelerating breeding for tree health and productivity ( [[#Isabel--2020|Isabel et al., 2020]] ).

In response to increasing risks to permanence of carbon stocks of some types of afforestation practices and the competition for land, there has been an increasing recognition that secondary forest regrowth and restoration of degraded forests and non-forest ecosystems can play a large role in carbon sequestration ( ''high confidence'' ). The rational for this focus builds on their high carbon stocks and rates of sequestration ( [[#Griscom--2017|Griscom et al., 2017]] ; [[#Lewis--2019|Lewis et al., 2019]] ; [[#Maxwell--2019|Maxwell et al., 2019]] ; [[#Pugh--2019|Pugh et al., 2019]] ), high resilience to disturbances ( [[#Dymond--2014|Dymond et al., 2014]] ; [[#Messier--2019|Messier et al., 2019]] ), and additional benefits such as enhanced biodiversity ( [[#Strassburg--2020|Strassburg et al., 2020]] ).

The global sequestration potential of forestation varies substantially depending on the scenario-assumptions of available land and of background climate (AR6 WGIII, [[IPCC:Wg1:Chapter:Chapter-7#7.5|Section 7.5]] ). Afforestation of native grasslands, savannas, and open-canopy woodlands leads to the undesirable loss of unique natural ecosystems with rich biodiversity, carbon storage and other ecosystem services ( [[#Veldman--2015|Veldman et al., 2015]] ; [[#IPBES--2018|IPBES, 2018]] ). Comprehensive approaches to assess the effectiveness of land-based carbon removal options need to be based on the whole carbon cycle, covering both carbon stocks and flows, and establishing the links between human activities and their impacts on the biosphere and atmosphere ( [[#Keith--2021|Keith et al., 2021]] ).

A range of mechanisms could enhance CO <sub>2</sub> sequestration of forest-based methods under future scenarios, including CO <sub>2</sub> fertilization, soil carbon enrichment due to enhanced litter input, or the northward shift of the tree line in future climate projection ( [[#Bathiany--2010|Bathiany et al., 2010]] ; [[#Sonntag--2016|Sonntag et al., 2016]] ; [[#Boysen--2017b|Boysen et al., 2017b]] ; [[#Harper--2018|Harper et al., 2018]] ). There is ''low'' ''confidence'' in the net direction of feedbacks of afforestation on global mean temperature. The feedbacks are highly region dependent. For instance, afforestation at high latitudes would decrease albedo and increase local warming, while at low latitudes, the cooling effect of enhanced evapotranspiration could exceed the warming effect due to albedo decrease ( [[#Pearson--2013|Pearson et al., 2013]] ; W. [[#Zhang--2013|]] [[#Zhang--2013|Zhang et al., 2013]] ; [[#Jia--2019|Jia et al., 2019]] , SRCCL, Section 2.6.1). Both afforestation and reforestation affect the hydrological cycle through increased volatile organic compound (VOC) emissions and cloud albedo ( [[#Teuling--2017|Teuling et al., 2017]] ; [[#Kalliokoski--2020|Kalliokoski et al., 2020]] ), enhanced precipitation ( [[#Ellison--2017|Ellison et al., 2017]] ) and increased transpiration, with potential effects on runoff and, especially in dry regions, on water supply (Figure 5.36 and Cross-Chapter Box 5.1; [[#Farley--2005|Farley et al., 2005]] ; [[#Smith--2016|Smith et al., 2016]] ; [[#Krause--2017|Krause et al., 2017]] ; [[#Teuling--2019|Teuling et al., 2019]] ). Forest-based methods can either raise or lower N <sub>2</sub> O emissions, depending on tree species, previous land use, soil type and climatic factors ( ''low confidence'' ) (Figure 5.36 and Supplementary Materials, Table 5.SM.4; [[#Benanti--2014|Benanti et al., 2014]] ; [[#Chen--2019|Chen et al., 2019]] ; [[#McDaniel--2019|McDaniel et al., 2019]] ). Afforestation will decrease biodiversity if native species are replaced by monocultures ( ''high'' ''confidence'' ), while there is ''medium confidence'' that biodiversity is improved when forests are introduced into land areas with degraded soils or intensive monocultures, or where native species are re-introduced into managed land (Figure 5.36, Supplementary Materials Table 5.SM.4; [[#Hua--2016|Hua et al., 2016]] ; [[#Williamson--2016|Williamson and Bodle, 2016]] ; [[#Smith--2018|P. Smith et al., 2018]] ; [[#Holl--2020|Holl and Brancalion, 2020]] ).

Soil carbon losses from human agriculture accounted for about 116 PgC in the last 12,000 years ( [[#5.2.1.2|Section 5.2.1.2]] ; [[#Sanderman--2017|Sanderman et al., 2017]] ). With best management practices, two-thirds of these losses may be recoverable, setting a theoretical maximum of 77 PgC that can be sequestered in soils. Methods to increase soil carbon content may be applied to the restoration of marginal or degraded land ( [[#Paustian--2016|Paustian et al., 2016]] ; [[#Smith--2016|Smith, 2016]] ), but may also be used in traditional agricultural lands. A simple practice is to increase the input of carbon to the soil by selecting appropriate varieties or species with greater root mass ( [[#Kell--2011|Kell, 2011]] ) or higher yields and net primary production (NPP) ( [[#Burney--2010|Burney et al., 2010]] ). In addition, improved agricultural practices also increase soil carbon content. These include using crop rotation cycles, increasing the amount of crop residues, using crop cover to prevent periods of bare soil ( [[#Poeplau--2015|Poeplau and Don, 2015]] ; [[#Griscom--2017|Griscom et al., 2017]] ), optimizing grazing ( [[#Henderson--2015|Henderson et al., 2015]] ) and residue management ( [[#Wilhelm--2004|Wilhelm et al., 2004]] ), using irrigation ( [[#Campos--2020|Campos et al., 2020]] ), employment of low-tillage or no-tillage (W. [[#Sun--2020|]] [[#Sun--2020|Sun et al., 2020]] ), agroforestry, cropland nutrient recycling, and avoiding grassland conversion ( [[#Paustian--2016|Paustian et al., 2016]] ; [[#Fargione--2018|Fargione et al., 2018]] ). With ''medium confidence,'' methods that seek soil carbon sequestration will diminish nitrous oxide (N <sub>2</sub> O) emissions and nutrient leaching, and improve soil fertility and biological activity (Figure 5.36; [[#Tonitto--2006|Tonitto et al., 2006]] ; [[#Fornara--2011|Fornara et al., 2011]] ; [[#Paustian--2016|Paustian et al., 2016]] ; [[#Smith--2016|Smith et al., 2016]] ; SRCCL, Section 2.6.1.3, [[#IPCC--2019a|IPCC, 2019a]] ). However, if improved soil carbon sequestration practices involve higher fertilization rates, N <sub>2</sub> O emissions would increase ( [[#Gu--2017|Gu et al., 2017]] ). Some soil carbon sequestration methods, such as cover crops and crop diversity, can increase biodiversity ( ''medium confidence'' ) ( [[#Paustian--2016|Paustian et al., 2016]] ; [[#Smith--2018|P. Smith et al., 2018]] ).

Biochar is produced by burning biomass at high temperatures under anoxic conditions (pyrolysis) and, when added to soils, can increase soil carbon stocks and fertility for decades to centuries ( [[#Woolf--2010|Woolf et al., 2010]] ; [[#Lehmann--2015|Lehmann et al., 2015]] ). Biochar application improves many soil qualities and increases crop yield ( ''medium confidence'' ) ( [[#Ye--2020|Ye et al., 2020]] ; SRCCL, Chapter 4.9.5), particularly in already degraded or weathered soils ( [[#Woolf--2010|Woolf et al., 2010]] ; [[#Lorenz--2014|Lorenz and Lal, 2014]] ; [[#Jeffery--2016|Jeffery et al., 2016]] ), increases soil water holding capacity ( ''medium'' ''confidence'' ) ( [[#Karhu--2011|Karhu et al., 2011]] ; [[#Liu--2016|Liu et al., 2016]] ; [[#Fischer--2019|]] [[#Fischer--2019|B.M.C. Fischer et al., 2019]] ; [[#Verheijen--2019|Verheijen et al., 2019]] ) and evapotranspiration ( ''low confidence'' )( [[#Fischer--2019|]] [[#Fischer--2019|B.M.C. Fischer et al., 2019]] ). The use of biochar reduces nutrient losses ( ''low confidence'' ) ( [[#Woolf--2010|Woolf et al., 2010]] ), enhances fertilizer nitrogen use efficiency and improves the bioavailability of phosphorus (Figure 5.36; [[#Clough--2013|Clough et al., 2013]] ; [[#Shen--2016|Shen et al., 2016]] ; [[#Liu--2017|Z. Liu et al., 2017]] ). Biochar addition may decrease methane (CH <sub>4</sub> ) emissions in inundated and acid soils such as rice fields ( ''low confidence'' )( [[#Jeffery--2016|Jeffery et al., 2016]] ; [[#Huang--2019|Huang et al., 2019]] ; [[#Wang--2019|]] [[#Wang--2019|Wang et al., 2019]] ; [[#Yang--2019|Yang et al., 2019]] ). In non-inundated, neutral soils, CH <sub>4</sub> uptake from the atmosphere is suppressed after biochar application ( ''low confidence'' ) ( [[#Jeffery--2016|Jeffery et al., 2016]] ), and soil N <sub>2</sub> O emissions decline ( ''medium confidence'' ) ( [[#Cayuela--2014|Cayuela et al., 2014]] ; [[#Kammann--2017|Kammann et al., 2017]] ). The potential risks of introducing harmful contaminants into the soil environment are not well understood ( [[#Lorenz--2014|Lorenz and Lal, 2014]] ). With ''low confidence'' , application of biochar can have co-benefits for soil microbial biodiversity ( [[#Smith--2018|P. Smith et al., 2018]] ), while the potential trade-offs for biodiversity are due to land requirements ( [[#Tisserant--2019|Tisserant and Cherubini, 2019]] ).

Peatlands are less extensive than forests, croplands and grazing lands, yet per unit area, they hold high carbon stocks ( [[#Griscom--2017|Griscom et al., 2017]] ). Peatland restoration relies on back-conversion or building of high-carbon-density soils through flooding – that is, rewetting ( [[#Leifeld--2019|Leifeld et al., 2019]] ). High water level and anoxic conditions are prerequisites for restoring by returning drained and/or degraded peatlands back to their natural state as CO <sub>2</sub> sinks, but restoration also results in enhanced CH <sub>4</sub> emissions which are similar to or higher than the pre-drainage fluxes ( ''high'' ''confidence'' ) ( [[#Koskinen--2016|Koskinen et al., 2016]] ; [[#Wilson--2016a|Wilson et al., 2016a]] ; [[#Hemes--2019|Hemes et al., 2019]] ; [[#Renou-Wilson--2019|Renou-Wilson et al., 2019]] ; [[#Holl--2020|Holl et al., 2020]] ). In a multi-decadal time frame, the reduction in CO <sub>2</sub> emissions from rewetting more than compensates for the initial increase in radiative forcing due to enhanced CH <sub>4</sub> emissions ( [[#Günther--2020|Günther et al., 2020]] ). Rewetting drained peatlands will decrease N <sub>2</sub> O emissions ( ''medium confidence'' ) ( [[#Wilson--2016b|Wilson et al., 2016b]] ; H. [[#Liu--2020|]] [[#Liu--2020|]] [[#Liu--2020|Liu et al., 2020]] ; [[#Tiemeyer--2020|Tiemeyer et al., 2020]] ). Restored wetlands and peatlands act as buffer zones that provide infiltration and nutrient retention and offer protection to water quality ( [[#Daneshvar--2017|Daneshvar et al., 2017]] ; [[#Lundin--2017|Lundin et al., 2017]] ), particularly in nutrient-loaded agricultural catchments. Peatland restoration can also recover much of the original biodiversity ( ''medium confidence'' ) ( [[#Meli--2014|Meli et al., 2014]] ; [[#Smith--2018|P. Smith et al., 2018]] ).

The concept of BECCS rests on the premise that bioenergy production is carbon neutral – that is, as much CO <sub>2</sub> is sequestered when growing biomass as feedstock as is released by its combustion. If these emissions are also captured and stored, the net effect is removal of CO <sub>2</sub> from the atmosphere ( [[#Fuss--2018|Fuss et al., 2018]] ). Sequestration potentials from BECCS depend strongly on the feedstock, climate, and management practices ( [[#Beringer--2011|Beringer et al., 2011]] ; [[#Kato--2014|Kato and Yamagata, 2014]] ; [[#Heck--2016|Heck et al., 2016]] ; [[#Smith--2016|Smith et al., 2016]] ; [[#Krause--2017|Krause et al., 2017]] ). If woody bioenergy plants replace marginal land, net carbon uptake increases, enriching soil carbon (Don et al., 2012; [[#Heck--2016|Heck et al., 2016]] ; [[#Boysen--2017a|Boysen et al., 2017a]] , b). However, replacing carbon-rich ecosystems with herbaceous bioenergy plants could deplete soil-carbon stocks and reduce the additional sink capacity of standing forests ( [[#Don--2012|Don et al., 2012]] ; [[#Harper--2018|Harper et al., 2018]] ). Furthermore, wood-based BECCS may not be carbon negative in the first decades, initially emitting more CO <sub>2</sub> than sequestering ( [[#Sterman--2018|Sterman et al., 2018]] ). BECCS has several trade-offs to deal with, including possible threats to water supply and soil nutrient deficiencies ( ''medium confidence'' ) (SRCCL Chapters 2 and 6, and Cross-Chapter Box 5.1; [[#Smith--2016|Smith et al., 2016]] ; [[#Krause--2017|Krause et al., 2017]] ; [[#de%20Coninck--2018|de Coninck et al., 2018]] ; [[#Heck--2018|Heck et al., 2018]] ; [[#Roy--2018|Roy et al., 2018]] ). Deployment of BECCS at the scales envisioned by many 1.5°C–2.0°C mitigation scenarios could threaten biodiversity and require large land areas, competing with afforestation, reforestation and food security ( [[#Anderson--2016|Anderson and Peters, 2016]] ; [[#Smith--2018|P. Smith et al., 2018]] ). Additional risks and side effects are related to geologic carbon storage ( [[#Fuss--2018|Fuss et al., 2018]] ; see also [[#5.6.2.2.4|Section 5.6.2.2.4]] ).

In conclusion, land-based CDR methods that rely on enhanced net biological uptake and storage of carbon, have a wide range of biogeochemical and biophysical side effects. These side effects can (directly or indirectly) strengthen or weaken the climate change mitigation effect of a given method, or affect water quality and quantity, food supply and biodiversity (Figure 5.36). With the exception of weakened ocean carbon sequestration, there is ''low confidence'' in the Earth system feedbacks of these methods. Most methods are associated with a range of biogeochemical and biophysical side effects and co-benefits and trade-offs, but these are often highly dependent on local context, management regime, prior land use, and scale ( ''high confidence'' ). Highest co-benefits are obtained with methods that seek to restore natural ecosystems and improve soil carbon sequestration (Figure 5.36) while highest trade-off possibilities (symmetry with the highest co-benefits) occur for reforestation or afforestation with monocultures and BECCS, again with strong dependence on scale and context ( ''medi'' ''um confidence'' ).

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