Increasing Soil Organic Carbon to Mitigate Greenhouse Gases and Increase Climate Resiliency for California

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Rising air temperatures are projected to continue to drive up urban, agricultural, and rangeland water use, straining both surface and groundwater resources. Scientific studies have shown that managing farms, ranches, and public lands to increase soil carbon can increase soil waterholding capacity and increase hydrologic benefits such as increased baseflows and aquifer recharge, reduced flooding and erosion, and reduced climate-related water deficits. Coincident improvements in forage and crop yields are also indicated, while simultaneously sequestering carbon, reducing atmospheric greenhouse gases and mitigating climate change. This study was developed to consider the multiple benefits of increasing the organic matter content of soils across California’s working lands.

Climate change poses severe risks to working landscapes in California, including rangelands and croplands, and the ecosystem services they provide. These services include food, habitat, carbon storage, and water supply for urban and rural communities, agriculture and wildlife. A healthy landscape can increase resilience to climate change, increase water quality and net primary productivity, and buffer the impacts of environmental stress leading to forest die-off, wildfire, flood and drought.

Rangelands and croplands, including publicly and privately managed lands, represent a majority of the land base in California. Increasing soil carbon can serve as a climate adaptation strategy due to its documented beneficial effects on soil erodibility, soil-water holding capacity, soil temperature and net primary productivity. Enhancing soil carbon in working lands at large spatial scales has the potential to measurably reduce greenhouse gas levels in the atmosphere, increase the sustainability of working landscapes and ensure the provision of other ecosystem services, including water, food and wildlife habitat. Application of composted urban and agricultural organic waste materials to grasslands and croplands, in conjunction with a suite of strategic land management practices, can remove significant quantities of carbon dioxide (CO2) from the atmosphere and sequester that carbon beneficially in soils and vegetation.

map of California shaded by potential increase in soil water holding capacity in inches

Calculated change in soil water holding capacity for the study area, and decrease in climatic water deficit for 1998 in inches of water per year following application of compost to increase soil water holding capacity. (Public domain.)

Numerous scientific studies have shown that increasing organic matter (OM) in soils can have multiple benefits, including carbon sequestration and reduction of atmospheric greenhouse gases (GHG)(DeLonge et al., 2014; Ryals and Silver, 2013, DeLonge et al., 2013). Soil management strategies and active management of working lands for enhanced carbon sequestration, such as "carbon farming," have a critical role to play in helping California develop resilience to climate change while simultaneously reducing atmospheric greenhouse gases. "Carbon farming" is a systems approach to land management that involves implementing practices that can improve the rate at which CO2 is removed from the atmosphere and converted to plant material and/or soil organic matter. Carbon farming integrates ecological site assessment and mapping in conservation planning, uses dynamic ecosystem carbon models to predict and measure increases in farm-system terrestrial carbon stocks, and incorporates hydrologic modeling to evaluate potential long-term impacts to on-farm water resources. Benefits of carbon farming include improvement in soil health, increased forage and crop yields, increase in soil-water holding capacity and reduction in total landscape demand for water, carbon sequestration, reduction of atmospheric greenhouse gases (GHG) and diversion of urban and agricultural organic waste from methane-producing anaerobic disposal in landfills and manure lagoons, and from burning.

Water that stays in the watershed can serve to preserve baseflows and riparian systems during low-flow periods and can potentially serve to sustain infiltration to the groundwater system. Figure 1 illustrates the calculation, using a hydrologic model, of a reduction in climatic water deficit (CWD) of 0 to 4.5 inches per year with a 25% increase in soil water-holding capacity (WHC) associated with increased soil OM for the northern Sacramento Valley. This reduction implies greater soil moisture, less irrigation demand, an increase in net primary productivity (NPP, equivalent to actual evapotranspiration), lower fire risk, and increased drought resiliency and carbon capture capacity.

The California Natural Resources Agency (CNRA), which is conducting the California 4th Climate Change Assessment (CCA) for non-energy-related projects, and is seeking new and innovative approaches to increase resilience to climate change, has funded this project. The Berkeley Energy and Climate Institute (BECI) is serving as the administrative office for agreements and reporting.

Objective and Scope

This project will use data generated from published and ongoing field and lab trials to constrain water-balance model estimates of soil moisture and evapotranspiration in order to quantify the potential changes in WHC and carbon sequestration for all rangeland and cropland soils statewide in response to increases in SOM. This approach will rely on current soil properties and calculate maximum potential benefit of increased SOM for all grasslands, pasture and arable lands in California. Limits to soil improvements will be illustrated, as not all of these lands would benefit from increases in SOM (e.g. wetlands, vernal pools, serpentine soils). Additionally, results will be used to estimate the economic value of both no-action and management actions leading to SOM increases, with respect to system hydrology and carbon sequestration for a representative sample of agricultural crops and rangeland types.

Finally, we will identify barriers to, and incentives for, statewide farmland and rangeland carbon storage enhancement within a climate-smart land-use planning framework under current and projected climate and land-use scenarios.

The development of state-of-the-art climate and hydrological surfaces for baseline conditions and future climates at a fine spatial scale will serve to inform other California 4th Climate Change Assessment projects. Significant integration with Project 2B submittal "Soil water dynamics, carbon sequestration, and greenhouse gas mitigation potential of using composted manure and food waste on California's rangelands" will provide leveraged opportunities for further understanding and model validation.

Relevance and Benefits

The proposed work directly addresses several aspects of the USGS Science Strategy for the Decade, 2007-2017 (U.S. Geological Survey, 2007), specifically "Understanding Ecosystems and Predicting Ecosystem Change, Climate Variability and Change." In addition to uncertainties about water supply, increases in landscape stress and wildfires are becoming more prevalent in California as a result of changing climate. Land and resource managers are seeking understanding as to the most scientifically defensible strategies for successful water-supply and resource management. This study will provide refined tools and information to manage and prioritize landscapes to increase resilience to climate change.

Highlights

~ Field and model results indicate that a one-time ¼” application of compost to California’s working lands (rangelands and crop lands) leads to carbon sequestration rates in soils that are maximized after approximately 15 years, and more than offset greenhouse gas emissions stimulated by the compost amendment for at least five decades longer. Regionalization of compost applications to only 6% of rangelands in California resulted in an estimate of 8.4 – 8.7 million metric tons of CO2 equivalents at maximum sequestration, 15 years after compost amendment.

~ Increases in total soil organic matter of 3% increased the soil water holding capacity by up to 4.7 million acre-feet across all working lands in California, with hydrologic benefits greatest in locations with enough precipitation to fill increases in soil storage capacity. The benefits of increasing soil organic matter included a reduction of climate change impacts to hydrologic variables in comparison to no-action soil management. Reductions in climate impacts averaged over the state for a wet future were 1-8% in comparison to baseline, and reductions for a dry future were 1-3% in comparison to baseline, but many locations had reductions in climate change impacts of up to 50% by the end-of-century.

~ Economic valuation of benefits due to changes in soil organic matter included provisioning services associated with above-ground forage productivity, and regulating services associated with below-ground carbon sequestration and groundwater recharge. Estimated benefits from all services increased over time in the future, and analyses demonstrated a large potential for the California carbon market in the coming decades.

~ Socioeconomic and related land-use pressures pose barriers to implementing management practices to increase soil organic matter by driving conversion of rangeland and cropland to development for more greenhouse gas emission intensive agriculture. Results can be effectively used with land-use change scenarios to identify where on California’s working lands hydrologic benefits coincide with development risk, highlighting counties in California that may have locations providing resilience to climate change when strategic soil management and land conservation are combined.

~ Analyses indicate potential hydrologic benefits from soil management on Williamson Act lands are an order of magnitude greater than potential losses related to future development, totaling over 700,000 acre-feet annually state-wide in a wet climate scenario. Existing barriers to management can potentially be overcome by strengthening existing efforts/infrastructure/programs, developing flexible and diverse funding mechanisms and tailored outreach programs to landowners.

~ Increased soil organic matter can be achieved in multiple ways to increase soil waterholding capacity, forage and crop yields, increase baseflows and aquifer recharge, reduce flooding and erosion, increase carbon sequestration, and reduce climate-related water deficits, therefore developing hydrologic resilience to climate change while simultaneously reducing atmospheric greenhouse gases. Prioritized investment in California's working landscapes will yield multiple ecosystem service benefits by v targeting conservation and management actions on grasslands in locations or counties that can gain the most benefit.