To a large extent, terrestrial carbon sequestration approaches can be seen as a reversal of previous ecosystem degradation-changing land use and management to favor greater biomass and soil carbon stocks. Second, there is a significant storage capacity based on the magnitude of historical carbon stocks compared with much lower contemporary stocks. First, the “technology” for carbon capture and storage already exists, via the uptake of CO 2 by plants and its storage in longer-lived biotic carbon pools, providing opportunity for storage that is energetically and economically competitive with other carbon removal and mitigation options. The rationale for pursuing terrestrial carbon sequestration as a CO 2 removal strategy is at least three-fold. These estimates of historical losses indicate a hypothetical though highly impractical upper bound for restoring terrestrial carbon stocks through adjustments to land management. Recent estimates of the anthropogenic-induced losses of terrestrial carbon stocks are on the order of 145 Gt C from woody biomass and soils from 1850 to 2015 ( Houghton and Nassikas, 2017), 133 Gt C from soil carbon stocks over the past 12,000 years ( Sanderman et al., 2017), and 379 Gt C of biomass over the past 10,000 years ( Pan et al., 2013). 200-300 years as human population and land transformation exploded. However, historically the conversion of native ecosystems to managed land, particularly cropland, has been a large net source of CO 2 flux to the atmosphere with a significant depletion of biomass and soil carbon stocks, particularly over the past ca. 2,600 Gt C to 2 m) annual fluxes from and to the atmosphere and terrestrial ecosystems, on the order of 60 Gt C yr -1, are roughly balanced but with an average net residual carbon uptake by terrestrial ecosystems of 1-2 Gt C ( Le Quéré et al., 2016). 1,500 Gt C in soil to a depth of 1 m (ca. Global terrestrial biotic carbon stocks include ca. Rationale for Terrestrial Carbon Sequestration Achieving and maintaining increases in biological carbon storage requires active management practices that manipulate system carbon balances, for which a variety of existing as well as potential future options exist. The organic compounds added or already in the soil decompose and mineralize to CO 2. For soil OC, this means increasing the rate of input of plant-derived detritus to the soil and/or reducing the rate at which For woody biomass, this implies growing more trees with more biomass per unit area, and maintaining that biomass over a longer time span, and/or decreasing loss of woody biomass through tree mortality, fire, and harvesting operations. Thus, the standing stocks of carbon can be increased by increasing the rate of input of carbon, decreasing outputs of carbon, or both. Hence decomposition and disturbance impacts on carbon losses are included in the net values.Īs a general principle, the overall balance of these carbon stocks is driven by the difference between carbon inputs (via plant CO 2 assimilation) and carbon losses to the atmosphere (via decomposition/microbial respiration as well as from fire/combustion). While the committee recognizes that technologies to increase carbon uptake by forestry and agriculture may cause increased emissions because of decomposition and disturbance of recalcitrant carbon in soils, virtually all the data reported are for net carbon uptake, determined from stock changes over time. Harvested woody biomass used for long-lived wood products may also accumulate and persist but is also subject to release of stored carbon as product use is discontinued-unless sequestered in a landfill. More ephemeral carbon stocks, including herbaceous biomass and plant litter with short residence times (<1 y), are generally ignored in the context of carbon sequestration because they do not represent a persistent removal of CO 2 from the atmosphere. The carbon stocks of interest are those that can accumulate and persist over multidecadal timescales, namely woody biomass and coarse woody debris and soil organic matter (SOM). Biological carbon stocks are largely controlled by actively cycling processes, that is, assimilation (carbon uptake from the atmosphere) and respiration (carbon loss to the atmosphere). Terrestrial carbon sequestration is defined here as the increase in the amount and maintenance over time of organic carbon (OC) in biological stocks, driven by plant assimilation of CO 2 from the atmosphere. CHAPTER THREE Terrestrial Carbon Removal and Sequestration INTRODUCTION Definition of Terrestrial Carbon Sequestration
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