Soil management practices can contribute to net carbon neutrality in California

Stabilizing climate requires reducing greenhouse gas (GHG) emissions and storing atmospheric carbon dioxide (CO2) in land or ocean systems. Soil management practices can reduce GHG emissions or sequester atmospheric CO2 into inorganic and organic forms. However, whether soil carbon strategies represent a viable and impactful climate mitigation pathway is uncertain. A specific question concerns the role that land-management practices and soil amendments can play in realizing California’s ambition for carbon neutrality by 2045. Here we examine the carbon flux impacts of soil conservation (i.e., compost, reduced tillage, cover crop) and enhanced silicate rock weathering (EW) practices at different areal extents of implementation in cropland, grassland, and savanna in California under two climate change cases. We show that with implementation areas of 15% or 50% of private cultivated land, grassland, and savanna in California, soil conservation practices alone can contribute 1.40.72.1 % ( −1.8−0.9−2.7 Mt CO2eq y−1) and 4.62.36.9 % ( −6.0−3.0−8.9 Mt CO2eq y−1) of the additional emissions reduction needed (beyond previous targets) to meet the 2045 net neutrality goal (−129.3 Mt CO2eq y−1), respectively, on an average annual basis, including climate uncertainty. Including EW in these scenarios increases the total contributions of management practices to 4.12.55.6 % ( −5.2−3.2−7.3 Mt CO2eq y−1) and 13.58.218.6 % ( −17.5−10.7−24.2 Mt CO2eq y−1), respectively, of this reduction. This highlights that the extent of implementation area is a major factor in determining benefits and that EW has the potential to make a real contribution to net reduction targets. Results are similar across climate cases, indicating that contemporary field data can be used to make future projections. With EW there remains mechanistic uncertainties, however, such as rock dissolution rate and environmental controls on weathering products, which require additional field research to improve understanding of the technological efficacy of this approach for California’s 2045 carbon neutrality goal.


Introduction
Atmospheric concentrations of carbon dioxide (CO 2 ) continue to rapidly increase primarily due to fossil fuel combustion and land use and land cover change (LULCC) (IPCC 2021).Limiting global warming to 1.5 • C to avoid increasingly dangerous climate change will require reducing net CO 2 emissions to zero (by reducing greenhouse gas (GHG) emissions and sequestering carbon) and aggressively mitigating emissions of other GHGs by 2050 (Rogelj et al 2018).California (CA) has been a leader in championing climate change mitigation policy through the CA Global Warming Solutions Act of 2006(AB 32 2006) and later commitments (SB 13862016, SB 32 2016) to reduce gross GHG emissions by 40% and 80% below 1990 levels by 2030 and 2050, respectively.In 2018, a more ambitious target was set to achieve statewide carbon neutrality by 2045 (Executive Order B-55-18) and was recently codified in law (AB 1279(AB 2022)).
The majority of GHG emissions reductions must come from the energy, industrial, and transportation sectors, but LULCC will play a key role because the land sector produces ∼22% of global GHG emissions (Pathak et al 2022).Land-based strategies for GHG emissions reduction have been explored at the global (Griscom et al 2017, Mayer et al 2018), national (Fargione et al 2018, Drever et al 2021), and subnational scales (Cameron et al 2017, Simmonds et al 2021), and have been included in all scenarios targeting the 1.5 • C warming limit.Land-based emissions reductions can be achieved through avoidance or through management practices that reduce emissions or sequester carbon.Alternative methods include bioenergy carbon capture and storage and direct air capture, but land management has been reported as the least expensive pathway for mitigating climate change, both in global (Griscom et al 2017) and sub-national modeling analyses (e.g.$11 per tonne of CO 2 in CA (Baker et al 2020)).
Most land-based practices for mitigating climate change address organic carbon and have received more attention than practices addressing inorganic carbon.The net carbon flux impact of soil conservation practices (i.e., those affecting organic carbon; e.g., cover cropping, reduced tillage, and compost application), compared to conventional practices, has been estimated to range from 0.53 Mg CO 2 e ha −1 y −1 to −5 Mg CO 2 e ha −1 y −1 (increase in soil carbon) (Cameron et al 2017, Baker et al 2020, Graves et al 2020, Di Vittorio et al 2021).These increases are low compared to other land-based practices (e.g.woody restoration), thus requiring application across large areas to have significant impact (Di Vittorio et al 2021).In contrast, enhanced silicate rock weathering (EW) accelerates the accumulation of inorganic carbon in land or aquatic systems.Like organic carbon practices, accumulation of inorganic carbon may reduce emissions or sequester carbon, depending on the system.While EW field studies are rare, laboratory EW experiments and modeling suggest that under certain conditions it could have a relatively high rate of carbon accumulation on a per-area basis (Beerling et al 2018, Stler et al 2018).
EW of magnesium-and calcium-rich silicate rocks and minerals is a chemically and physically mediated process that can convert two moles of gaseous CO 2 to two moles of bicarbonate, per mole of base cation released (equation ( 1)).The fate of bicarbonate determines the net carbon accumulation rate: it is either transported to the ocean with residence times of potentially millions of years, or it precipitates as carbonate at half the bicarbonate conversion rate (equation ( 2)), in which case the residence time depends on site conditions (Koutsoukos and Kontoyannis 1984, Beerling et al 2018, Zamanian et al 2021), (2) Reactive surface area, temperature, water availability, and soil chemical properties are key variables in EW reaction rates.Simulations and laboratory studies have shown that dissolution and accumulation rates increase in controlled environments as grain size decreases (total surface area increases) (Hangx and Spiers 2009, Kelland et al 2020, Amann et al 2022).This suggests that finely pulverized rock may be applied to agricultural lands to capture CO 2 .Furthermore, modeling indicates that the benefits of ultra-fine grains may not outweigh the additional costs of reducing grain diameter from 100 µm to 10 µm (Kantzas et al 2022).A recent study of EW in croplands reported that −25 to −100 Gt CO 2 could be converted globally over 50 years through sustained implementation, depending on the extent and suitability of the implementation area.The US contribution was −0.13 and −0.42 Gt CO 2 y −1 , requiring 10% and 55% of cropland area, respectively (Beerling et al 2020).EW also has potential co-benefits, such as increased plant nutrient availability and yield (Kelland et al 2020), improved soil physical properties, reduced cropland N 2 O emissions (Blanc-Betes et al 2021), and reduced ocean acidification (Beerling et al 2018).The efficacy of EW has also been demonstrated under drought conditions (Holzer et al 2023).However, the magnitude of carbon accumulation and the long-term effects of continual rock amendment to agricultural soils are not well understood.Furthermore, a lifecycle assessment is needed to determine the availability of toxin-free rock substrate relative to farmland location and the associated GHG costs of production, transport, and application.
The State of CA includes natural and working lands (NWLs) management as contributing to its overall emissions reduction goals (SB 1386 2016) while meeting other needs such as wildfire mitigation (CA 2019).CA's 2030 NWL Climate Change Implementation Plan (CA 2019) estimates net GHG benefit for two suites of land management practices.Importantly, the small extent of alternative agricultural management limited benefits of both cases (Simmonds et al 2021).For example, 417 636 ha were dedicated to alternative agricultural management in the most ambitious scenario (CA 2019), yet there were over 4 Mha of cropland in 2010 (CALAND 2020).Another modeling study treated 1.3 Mha of croplands with soil conservation and conversion to grassland and 4.3 Mha of grassland with compost amendment to provide a 10 year mean annual benefit of −3.9 Tg CO 2 e y −1 .They also estimated that an emissions reduction of −125 to −150 Tg CO 2 e y −1 from the previous targets is needed to achieve carbon neutrality in 2045 (Baker et al 2020).CA's 2019 GHG emissions were estimated to be 418 Tg CO 2 e y −1 (CARB 2021).
We examine the climate change mitigation potential of CA-field-tested soil conservation practices and EW in CA using the CA NWLs carbon and GHG model (CALAND; Di Vittorio and Simmonds 2019, CALAND 2020).We focus on private cropland and rangeland (grazed grassland, savanna, and woodland) as the most likely areas for these practices to be applied and explore uncertainties in implementation area, weathering fate of mineralized CO 2 , and climate.Specifically, we address the following research questions: what are the potential carbon flux impacts of EW and soil conservation practices in CA? How much can these practices contribute to meeting California's 2045 target of statewide carbon neutrality?

Model description
The CALAND model simulates annual, terrestrial, net, carbon fluxes and stocks for 941 CA land units and eight carbon pools, including wood products and their decay.Species-specific carbon emissions (CO 2 , CH 4 , and optional black carbon) from biomass decay and burning (wildfire, controlled burning, bioenergy production) are also tracked.CALAND 3.0.1 (Di Vittorio and Simmonds 2019, CALAND 2020) has 16 distinct land management practices, plus variations, including soil conservation (cover cropping, reduced tillage, and compost application for cropland; compost application for rangeland).This study uses a modified version of CALAND 3.0.1 that adds rock amendment practices (text S1).We analyze the effects of land management on net ecosystem carbon fluxes by subtracting baseline outputs from management scenario outputs.Negative values represent increases in soil carbon due to alternative management that affects only soil CO 2 fluxes in the model, and indicate emissions reduction or carbon sequestration depending on whether the system is a carbon source or sink, respectively.

Scenario development and simulations
To explore the potential of soil management in CA cropland and rangeland to contribute to the state's GHG reduction goals, we developed 18 alternative land management scenarios and one baseline scenario that represents conventional or no management (table 1).All scenarios were simulated from 2010 through 2050, and alternative management was applied from 2021 through 2050.All scenarios assume mean levels of initial carbon densities and fluxes.We varied three factors: (1) management practice, (two of three options) (2) implementation area (three designations), and (3) per-area effects of each management practice based on their uncertainty (three levels) (tables 1, S1, text S1).The available practices are soil conservation (including compost amendment in rangeland), rock amendment, and both.Soil conservation is applied each year in cropland, compost is applied every 10 years in rangeland, and rock amendment is applied every three years.The annual average per-area effects of the two individual practices were estimated using 100% of the respective land type areas (to maximize spatial heterogeneity; table 2) and three uncertainty levels (table 1).Twelve scenarios correspond to treating 15% or 50% of privately owned cropland, grassland, and savanna (table 2) with soil conservation only or with both practices, at three uncertainty levels (table 1).These two extents were chosen to reflect the likelihood of implementation, uncertainty in source material availability, logistical challenges of application in woodland, and alignment of different practice areas for comparison.Plant and manure compost availability in CA has been estimated to be sufficient for application to 550 000 ha per year (DeLonge et al 2013), which is similar to 15% of private grassland and savanna (541 109 ha).More than tripling this area allows for considerable expansion of source material availability and does not assume that all grassland and savanna are grazed.Constant land type areas for determining implementation area were calculated as the average projected CALAND area from 2021 to 2050, and simulations were performed with constant 2010 CALAND land cover.These 12 scenarios were extended to 2070 to quantify when CA carbon neutrality may be reached if the state's previous emissions targets were met.Soil conservation and compost amendment input uncertainties are ±2 * SD from the mean carbon flux of each practice, where SD is the variability in the effect of each practice.These values (without climate change effects) were derived from CA-specific estimates of net soil carbon change due to compost amendment or the average of soil conservation practices relative Table 1.Summary of the management scenarios for sequestering soil carbon or reducing emissions in California cropland, grassland, savanna, and woodland.Land is segregated by private or non-private ownership.All scenarios were simulated using the CALAND model from 2010 to 2050, with management prescribed from 2021 to 2050, except for the baseline (first) scenario that had no management (or conventional management for cropland).The private land scenarios were extended to 2070.The prescribed management areas are based on fractions of the average total land type areas from 2021 to 2050 with projected land use-driven land cover change (table 2).All scenarios were duplicated for RCP 4.5 and RCP 8.5 climate change projection.Table 2. Summary of California land-type specific and total land areas prescribed for the management scenarios listed in table 1.A simplified equation summarizes the model estimate of total annual net soil carbon flux (Mg C y −1 ) in each land category (region-ownership-land type combination) as the product of the total land category area and the climate-scaled, area-weighted average soil carbon flux (Mg C ha −1 y −1 ) (equation (3)), where C x,r,o,l is the simulated absolute net soil carbon flux (Mg ha −1 y −1 ) in year x, located in region r, ownership o, and land type l; a r,o,l is the total land category area (ha by r, o, l), f r,o,l,x is the fraction of land category managed in year x; c r,o,l,x is the climate scalar in each land category (r, o, l) in year x, ȲB,r,o,l is the constant average annual baseline net soil carbon flux (Mg ha −1 y −1 ) in each land category (r, o, l), and ȲM,r,o,l is the constant average annual net change in soil carbon flux due to alternative management (Mg ha −1 y −1 ), which can be zero for the baseline or the mean, mean + upper bound, or mean − lower bound values.S1).The CALAND input for this practice's impact (−1.9 ± 0.11 Mg CO 2 e ha −1 y −1 over 10 years) (table S1) was derived from a single modeling study of long-term changes in organic carbon densities at three CA grassland sites following a single compost application (Ryals et al 2015) (see methods).In contrast, the combined temporal and spatial variability over the first three years at the same sites (Ryals and Silver 2013) was much greater than the modeled uncertainty.The short-term field impact (−2.35 ± 3.46 Mg CO 2 e ha −1 y −1 ), the mean of which is 24% greater than our mean input (by magnitude, table S1), was extrapolated in another study to estimate greater soil carbon increases and uncertainty due to compost amendments in CA grassland (Cameron et al 2017).We used the longer-term modeling data to represent the net impacts of compost amendment in rangeland because they more accurately reflected our 10 year reapplication period, and we accepted the smaller, modeled uncertainty associated with declining annual soil carbon increases over 10 years (Ryals andSilver 2013, Ryals et al 2015).A recent study with additional field sites also shows declining soil carbon increases over time for a single compost application (Mayer and Silver 2022).Additional studies with longer-term measurements may increase these modeled uncertainties.

Net carbon flux impacts of individual practices on a per-area basis
While rock dissolution rate is a major source of uncertainty for EW, we assumed repeated rock amendment every three years with 100% dissolution during this period and focused on the fate of CO 2 as our primary source of uncertainty for simulations (figure 1 and table S1).This is a conservative approach that acknowledges the limited availability of experimental data (Almaraz et al 2022).Previous modeling studies have assumed 100% dissolution annually (e.g.Beerling et al 2020), while laboratory experiments indicate incomplete dissolution in the first year and declining amounts of rock dissolution over time (e.g.Amann et al 2022).In our framework the annual effects are distributed evenly across the dissolution period and thus our results scale linearly with the amount of annual rock dissolution.Hence, assuming 100% dissolution each year with annual reapplication would triple our results.The fate of CO 2 as carbonate or bicarbonate also contributes uncertainty to the estimates.Our mean estimate represents 50% carbonate and 50% bicarbonate, while the lower and upper accumulation bounds represent 100% carbonate and 100% bicarbonate, respectively.These results suggest that rock amendment practices be developed to maximize dissolution and bicarbonate production in order to maximize per-area benefits.Our findings highlight the need for additional field trials and model development with the aim of linking fundamental geochemistry to meteorological, soil, and management conditions to improve EW projections in California.

Statewide changes in net carbon flux due to soil management
To estimate changes in net carbon flux due to soil conservation across CA, we simulated the impacts of aggregate practices in cropland and compost amendment in grassland and savanna for two proportions of private land area (15% and 50%) from 2021 to 2050 (tables 1 and 2, figure 2(a)), under two climate change cases.The results scale approximately linearly with implementation area and interannual variability reflects the spatially explicit interaction between climate change and management.Average annual flux changes ranged from −0.9 to −2.7 Tg CO 2 e y −1 and −3.0 to −8.9 Tg CO 2 e y −1 for 15% and 50% private land area, respectively.Another study reported that similar practices could change fluxes by −3.9 Tg CO 2 e y −1 when applied to 1.5 Mha of cropland and 4.3 Mha of rangeland in CA (Baker et al 2020).This is within the range of our 50% private area scenarios, which have less implementation area (2 Mha of cropland and 1.8 Mha of savanna and grassland) but are based on CA-specific data while their analysis is based on a national farm model.
Scenarios with additional rock amendment and the 50:50 fate of mineralized CO 2 increase the magnitude of changes by a factor of 2.9 at both extents and climates (figure 2, table S2).For example, the mean RCP4.5 flux changed from −1.8 to −5.2 Tg CO 2 e y −1 with the addition of rock amendments on 15% of private land.We assume that EW is independent of soil conservation because EW affects inorganic carbon while soil conservation affects organic carbon and potential interactions between these practices are unknown.The long-term stability of bicarbonate depends on it leaching through the soil and travelling through rivers to the ocean (Beerling et al 2018).Material remaining in the soil may precipitate as carbonate minerals with lifespans of thousands to millions of years in natural soils, but with shorter lifespans in agricultural soils (Zamanian et al 2021) because fertilizer could reduce soil pH sufficiently for carbonate dissolution to favor H 2 CO 3 that can re-emit CO 2 to the atmosphere (Koutsoukos and Kontoyannis 1984).

Contributions of soil management to CA net carbon neutrality
The potential for soil conservation and rock amendment to contribute to CA's 2045 carbon neutrality goal depends on the implementation area and average annual per-area flux impacts of each practice (figures 3 and 4).The relative importance of these two factors depends on the range of per-area impacts (figure 3), which is partly based on data availability.We estimate that by 2045 an additional emissions reduction of −129.3Tg CO 2 e y −1 , with respect to previous targets, is required to meet the overall neutrality goal.Soil conservation on 15% and 50% private area of respective land types (tables 1 and 2) provides average annual flux changes of −1.8 −2.7 −0.9 and −6.0 −8.9 −3.0 Tg CO 2 e y −1 , respectively, under RCP 4.5, corresponding to 1.4 2.1 0.7 % and 4.6 6.9 2.3 % of the additional reduction required to meet the neutrality goal, respectively (figure 4).The RCP 8.5 results are nearly identical (table S5).Compost amendment on grassland and savanna contribute most to overall emissions reduction because their mean, annual, per-area impacts (figure 1) are more than double those from soil conservation on cropland and the respective areas are similar.
Adding rock amendment to scenarios more than doubles the magnitude of flux change (figure 4(b)).Scenarios with all practices implemented on 15% and 50% private area of each respective land type change carbon flux by −5.2 −7.2 −3.2 and −17.4 −24.1 −10.7 Tg CO 2 e y −1 , respectively, under RCP 4.5.These values correspond to 4.0 5.6  2.5 % and 13.4 18.6 8.2 % of the additional reduction required to meet the neutrality goal, respectively (figure 4(b)).Assuming California reduces fossil fuel and other industrial emissions following the trajectory set by the 2030 and 2050 targets, extended simulations of the mean scenarios show that the combined practices could help achieve carbon neutrality by 2060 or 2059 for 15% or 50% private area, respectively, regardless of climate case.While this does not meet the 2045 target, silicate rock amendments have the potential to contribute to overall emissions reduction.Field studies are needed to determine realistic dissolution rates, the fate of sequestered CO 2 , and dependence on rock types, soil physical and chemical properties, and management, particularly with regards to tillage and water management.Thus, focusing research efforts on optimizing the rock sequestration potential on the most suitable land is warranted.
Regardless of practice or uncertainty, it is clear that increasing implementation area of these practices increases their impacts (figure 3).Our scenarios do not maximize the physical area because there are many physical, political, social, and economic barriers that limit implementation extent of these practices.However, our results can be scaled linearly with increasing area to estimate the effects of broader implementation, with caution.Cropland is inherently managed and 55% of potential rangeland in California is grazed, 46% percent of which is public land (Brown et al 2018; based on savanna, grassland, desert, shrubland, and conifer/hardwood woodlands).This provides a large base for alternative management, although compost amendment is most suitable for grass-dominated rangeland.Availability of source material for these practices will limit implementation area and may become more or less across the simulated uncertainty range of per area emissions benefits and potential treatment areas of cropland, grassland and savanna (figure 1).The data shown were derived from scenarios with 100% of each land type area treated and simulated from 2021 to 2050 under RCP 4.5 climate change.Note the different axis scales on each plot that reflect different levels of contribution and different available land areas (with the area limits determined by the scenarios).The uncertainty range for each soil-based strategy was generated using model inputs for the mean net soil carbon flux of each practice ±2σ of the estimated effect of each practice, with the exception of the rock amendment inputs which correspond to the fate of the mineralized CO2 as 100% carbonate (minimum effect) or 100% bicarbonate (maximum effect).limited over time as demand grows.However, additional practices could boost implementation area; biochar soil amendment has been shown to increase soil organic carbon content at rates comparable to those of soil conservation (e.g.Blanco-Canqui et al 2020) and to improve soil health (Amonette et al 2021).Furthermore, incentives for alternative management in agricultural lands may be limited.For example, an ambitious CA Department of Food and Agriculture scenario estimates only 33 724 ha of agricultural land to be under alternative management annually through 2030 (CA 2019), which is only 5.8% of cropland under soil conservation in our 15% private land scenario.Ultimately, our 50% private land scenario may not be an accurate upper estimate of implementation, but it is reasonable given our current knowledge of governing factors.

Conclusions
Alternative soil management practices have the potential to contribute to CA's 2045 carbon neutrality goal, with implementation area a major factor in determining the magnitude of contribution.The effects of alternative management are similar across climates, indicating that contemporary field data can be used to make future projections.EW, which increases inorganic carbon storage, is a promising contributor to this goal but mechanistic uncertainties, such as rock dissolution rate and environmental controls on weathering products, need to be further quantified through additional field research to improve understanding of the technological efficacy of this approach.
This study utilizes the best available data and a novel model to make estimates and characterize uncertainties, but several factors contribute to additional uncertainty that may not be represented here.Limited field data for many practices, including those not in this study, contribute to uncertainty in regional carbon flux estimates due to the inherent variability of soils, climate, management, and interactions across CA.Availability of source materials for amendment practices is highly uncertain and may limit implementation areas very differently than presented here.Climate uncertainty is based on only two cases from one Earth system model, and how the carbon accumulation rate of EW responds to changing conditions is not well known.Climate extremes can also reduce potential emissions reductions in particular years.Additional factors, such as economics or life cycle emissions, may also affect contributions to overall emissions reductions.Fortunately, the CALAND model has been shown to provide robust results at the landscape level given uncertainties in initial carbon state, carbon fluxes, and land use/cover change.

a
Proxy for a range of regenerative agricultural practices, such as no-till, reduced tillage, cover crop, compost, but excluding practices that involve land cover change.b Application rate of 14 Mg C ha −1 y −1 , C:N = 11:1, 10 y repeat frequency.c Compost applied to woodland only for the 'All' ownership scenarios.d Application rate of 40 Mg ha −1 of ground basalt with 15% alkalinity applied once every three years, with complete dissolution over three years.

Figure 1 .
Figure 1.Simulated 30 year mean annual net soil carbon flux impacts on a per area basis of individual soil management practices from 2021 to 2050 in cropland, grassland, savanna, and woodland in California under Representative Concentration Pathways 4.5 and 8.5.Negative values represent reduced net soil carbon emissions or increased net soil carbon sequestration relative to a baseline of no treatment.Error bars are the uncertainty bounds for each technology, with the upper and lower bounds corresponding to the 30 year mean annual soil carbon flux difference from baseline of two additional scenarios per management-land type combination.For the soil conservation and compost application uncertainty bound scenarios, the inputs for the absolute net soil carbon flux were the mean net flux for the practice ±2σ of the estimated effect of the practice.The rock application uncertainty bound scenarios were run with inputs for the absolute net soil carbon flux corresponding to the fate of the mineralized CO2 as 100% carbonate (minimum benefit) or 100% bicarbonate (maximum benefit), while the mean scenario represented the fate as 50% carbonate and 50% bicarbonate.

Figure 2 .
Figure 2. changes in annual soil carbon flux in California soils, relative to a baseline of no management, with (a) compost applications in grasslands and savanna and soil conservation in cultivated lands, and (b) the additional practice of applied rock amendment in these land types.Negative values represent decreased soil carbon emissions or enhanced soil carbon sequestration relative to a baseline of no management.Two climate cases (Representative Concentration Pathways 4.5 and 8.5)and two implementation extents (15% private, 50% private) are shown.The shaded areas are the uncertainty bounds for each scenario, which were generated using model inputs for the mean net soil carbon flux of each practice ±2σ of the estimated effect of each practice, with the exception of rock amendment for which the central estimate represents the fate of the mineralized CO2 as 50% bicarbonate and 50% carbonate and the uncertainty corresponds to 100% carbonate (minimum benefit) or 100% bicarbonate (maximum benefit).

Figure 3 .
Figure 3. Potential contributions of individual soil conservation (a)-(c) and rock amendment (d)-(f) practices (relative to a baseline of no alternative management) to meeting California's 2045 statewide carbon neutrality goal (−129.3Tg CO2e y −1 )across the simulated uncertainty range of per area emissions benefits and potential treatment areas of cropland, grassland and savanna (figure1).The data shown were derived from scenarios with 100% of each land type area treated and simulated from 2021 to 2050 under RCP 4.5 climate change.Note the different axis scales on each plot that reflect different levels of contribution and different available land areas (with the area limits determined by the scenarios).The uncertainty range for each soil-based strategy was generated using model inputs for the mean net soil carbon flux of each practice ±2σ of the estimated effect of each practice, with the exception of the rock amendment inputs which correspond to the fate of the mineralized CO2 as 100% carbonate (minimum effect) or 100% bicarbonate (maximum effect).

Figure 4 .
Figure 4. California statewide carbon neutrality goal in 2045 (thick dashed black line) and emissions reduction targets for 2020, 2030, and 2050 (thin dashed black line) compared to the estimated emissions reductions for practices implemented at varying area extents in cropland, grassland and savanna, as added to the 2020, 2030, and 2050 target projection.Note that RCP 4.5 and RCP 8.5 are indistinguishable from each other at this scale.The shaded areas are the uncertainty bounds for each scenario (and are also not clearly visible at this scale) which were generated using model inputs for the mean net soil carbon flux of each practice ±2σ of the estimated effect of each practice, with the exception of the rock application inputs which correspond to the fate of the mineralized CO2 as 100% carbonate (minimum benefit) or 100% bicarbonate (maximum benefit).

Table 3 .
Summary of the input uncertainties for changes in soil carbon flux due to alternative management.
(Simmonds et al 2021)term (30 year) impacts of individual soil management practices across CA's landscape on a per-area basis, we analyzed CALAND simulations for 100% of cropland, grassland, savanna, and woodland, with and without each practice implemented from 2021 to 2050, under two climate change cases (table 1 and figure1).Consistent with a previous study(Simmonds et al 2021), the different climate cases produced similar results with the main effect of adding interannual (figure 2, tableS1) and spatial variability to a no climate change case.
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