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02.Soil_Organic_Carbon_sequestration.rmd
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02.Soil_Organic_Carbon_sequestration.rmd
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# | Soil Organic Carbon (SOC) sequestration
## Soil carbon
Soils constitute the largest terrestrial carbon (C) pool. Total soil carbon (C) stock comprises soil organic C (SOC) and soil inorganic C (SIC) components. SOC is the carbon component of soil organic matter (SOM), a heterogeneous pool of C comprised of diverse materials including fine fragments of litter, roots and soil fauna, microbial biomass C, products of microbial decay and other biotic processes (i.e. such as particulate organic matter), and simple compounds such as sugar and polysaccharides (Jansson et al., 2010). The global SOC stock of ice-free land contains about 1500-2400 Pg C (1 Pg = 1 Gt) in the top 1 m, 2300 Pg C in the top 3 m, and 3000 Pg C in the soil profiles (Batjes et al., 1996; Scharlemann et al., 2014; Tifafi et al., 2018; Lorenz and Lal, 2018). This represents more than the sum of carbon contained in the atmosphere and vegetation (Smith et al., 2019). Soil inorganic C comprises pedogenic carbonates and bicarbonates, which are particularly abundant in arid regions and in alkaline soils. The SIC stock is estimated at 700-1700 Pg C in the top 1-m soil layer (Lorenz and Lal, 2018) and is believed to occur predominantly in the deeper layers of temperate soils
Although soils contribute to a major share of agricultural greenhouse gas emissions (GHGs), due to the size of the soil carbon pool, even small increments in the net soil C storage represent a substantial C sink potential (Paustian et al., 2016). Carbon sequestration implies transferring atmospheric CO~2~ into long-lived pools and storing it securely so it is not immediately reemitted (Lal et al., 2018). Thus, soil C sequestration means increasing SOC and SIC stocks through judicious land use and sustainable soil management (SSM) practices. Due to the current knowledge on SOC dynamics, the global distribution, and the current knowledge on size of the SOC pool compared to the SIC pool, this technical manual will focus on SOC sequestration.
## SOC sequestration
The basic process of SOC sequestration in the terrestrial biosphere involves transfer of atmospheric CO~2~ into plant biomass and conversion of biomass into stable SOC through formation of organo-mineral complexes (Lal et al., 2018). Thus, soil carbon sequestration relies on plant photosynthesis to carry out the initial step of carbon "removal" from the atmosphere. However, rather than increasing the storage of carbon contained in plant biomass, SOC sequestration relies on management practices that increase the amount of carbon stored as soil organic matter, primarily in cropland and grazing lands. The main advantage of scaling up soil C sequestration as a biological negative emission strategy is that carbon stocks are most depleted on lands currently under agricultural management and thus this approach does not require land use conversions (e.g., to forests) nor it increases the competition for land resources. In addition, increases in SOC stocks are highly beneficial in maintaining and increasing soil health and soil fertility, which provides additional incentives for adopting SOC sequestering practices (Paustian et al., 2019).
## Factors affecting SOC sequestration
SOC sequestration is governed by the balance between the rate of C added to the soil from plant residues (including roots) and organic amendments (e.g., manure, compost), and the rate of C lost from the soils, which is mainly as CO~2~ from decomposition processes (i.e., heterotrophic soil respiration). Other forms of organic C can be lost as CH4 from anaerobic (e.g. flooded) reactions and to a lesser extent through leaching of dissolved organic C. Soil erosion can greatly affect C stocks at a particular location, but at larger scales erosion may not represent a loss process per se but rather a redistribution of soil C (Paustian et al., 2019).
Decomposition rates are controlled by a variety of factors including soil temperature and moisture, drainage (impacting soil O2 status) and pH (Paustian et al., 2019). Soil physical characteristics such as texture and clay mineralogy also impact the longevity and persistence (i.e., mean residence time) of soil C, by affecting organic matter stabilization processes, through mineral-organic matter associations (Schmidt et al., 2011; Paustian et al., 2019). In native ecosystems the rate of C inputs is a function of the type (e.g., annual vs. perennial, woody vs. herbaceous) and productivity of the vegetation, largely governed by climate (mainly temperature and precipitation) but also nutrient availability and other growth determining factors. In managed ecosystems such as cropland and grazing land both the rate of C input as well as the rate of soil C loss via decomposition are impacted by the soil and crop management practices applied. There is no one universal management practice to increase SOC sequestration (Lal et al., 2018), but in general, soil C stocks can be increased by: (a) increasing the rate of C addition to the soil, which removes CO~2~ from the atmosphere, and/or (b) reducing the relative rate of loss (as CO~2~ ) via decomposition, which reduces emissions to the atmosphere that would otherwise occur (Paustian et al., 2019).
Three key aspects need to be considered regarding the pattern of gains or losses of soil C and hence SOC sequestration (Paustian et al., 2019). The first is that with increased C inputs and/or decreased decomposition rates, soil C stocks tend toward a new equilibrium state and thus after a few decades C gains attenuate, becoming increasingly small over time. Secondly, although sequestered SOC can be highly stable, changes in management that lead to C gains are potentially reversible, i.e., if management reverts back to its previous condition, much or all of the gained C can be lost. Thus, practices that led to increased soil C need to be maintained long term. Third, mineral soils (i.e., non-peat soils) have an upper limit or "saturation level" of soil C (Six et al., 2002). While this maximum soil C concentration is well above the observed C concentration of most managed soils, carbon rich mineral soils that already have very high SOC levels (e.g., >5% C by mass) may have a propensity for further C gains.
## Estimating SOC sequestration potential
Taking into account the above mentioned factors, SOC sequestration potential after the adoption of SSM practices under specific conditions can be expressed in different ways depending on the definition of SOC baseline stocks and time towards a new equilibrium state. This Technical Manual will refer to two types of SOC sequestration: an 'absolute SOC sequestration' (SOCseq abs), expressed as the change in SOC stocks over time relative to a base period (or reference period, t0); and a 'relative SOC sequestration' (SOCseq rel), expressed as the change in SOC stocks over time relative to business as usual practices (Fig. 2.1). Thus, the 'absolute' attainable SOC sequestration can be determined for business as usual (BAU) and SSM practices (See Chapter 5), and can be either positive, neutral or negative:
\begin{equation}
\tag{2.1}
\Delta SOC_{ABS} t C ha^{-1} = SOC_{SSM/BAU \ t} - SOC_{t0}
\end{equation}
where SOC $SSM/BAU\ t$ refers to the final SOC stocks once a new equilibrium is reached or after a defined period of time (e.g. 20 years), and $SOC\ t0$ refers to the initial or base period SOC stocks ($t$=0). The 'relative' attainable SOC sequestration is either neutral or positive, can be determined as:
\begin{equation}
\tag{2.2}
\Delta SOC t C ha^{-1} = SOC_{SSM \ t} - SOC_{BAU \ t }
\end{equation}
where $SOC_{SSM \ t}$ refers to the final SOC stocks once a new equilibrium is reached or after a defined period of time (e.g. 20 years) after SSM practices are implemented, and $SOC_{BAU \ t}$ refers to the final SOC stocks under business as usual (BAU) practices at the end of the same considered period. Mean annual SOC sequestration rates (t C ha^-1^ yr^-1^; absolute or relative) can be determined by dividing SOC changes by the duration of the defined period. For more specifications on the approaches proposed in this manual, see Chapter 5.
Thus, agricultural lands may show potential for improvement in their SOC stock after the adoption of SSM practices (compared to business as usual practices), by either gaining or maintaining SOC levels. Four situations are possible: a) lands where SOC levels have reached equilibrium and it is possible to increase levels through SSM; b) lands where the SOC is increasing but can be further increased through SSM; c) lands where SOC is declining and it is possible to stop or mitigate losses in SOC levels through SSM; and d) lands where SOC is declining and it is possible to reverse this fall through SSM. These situations are depicted in Fig. 2.1.
It has been estimated that the widespread adoption of site/biome-specific SSM practices can harness a large C sink capacity in agricultural systems at a global scale: 0.4-1.2 Pg C yr^-1^ (Lal, 2004); 1.0-1.32 Pg C yr^-1^ (Smith et al., 2008); 0.4-1.1Pg C yr^-1^ (De Vries, 2017); 0.32-1.01 Pg C yr^-1^ (Batjes et al., 2019). However, the extent and rates of SOC sequestration in agricultural lands may vary greatly depending on the different land uses and practices, soil characteristics, vegetation, topography and climate, among other soil forming factors and processes (Smith et al., 2008; Minasny et al., 2017; Lal et al., 2018; Batjes et al., 2019). Sequestration rates due to management practices in croplands and grasslands are usually in the range of 0.2 - 0.8 t C ha-1 year -1 (Poepleau and Don, 2015; Kampf et al., 2016; Minasny et al., 2017; Conant et al., 2017; Paustian et al., 2016; Paustian et al., 2019).
It is therefore relevant to identify which regions, environments and systems have a greater potential to increase SOC stocks and establish priorities for research and implementation of private and public policies. In this sense, coupling SOC models to GIS (Geographic Information Systems) platforms enables the transition t from site-specific SOC stocks estimations to spatial simulations and projections (e.g. Smith et al. 2005; Milne et al., 2007; Kamoni et al., 2007; Falloon et al., 2007; Gottschalk et al., 2012; Lugato et al., 2014), allowing for the identification of conditions that increase the SOC sequestration potential.