Carbon Sequestration : An Ecosystem Service Provided by NCOS
Recent months in our area have been dominated by dramatic events, notably the Thomas fire in December and the torrential downpours this month. These events are possibly the result of human induced climate change associated with the increase in carbon dioxide in the atmosphere from the burning of fossil fuels. The North Campus Open Space project (NCOS) helps reduce our local carbon footprint through a number of mechanisms: growing perennial plants such as salt marsh species and perennial grasses; integrating stable forms of carbon into the soil profile; providing for non-vehicular access to open space areas; and creating a self-sustaining habitat that does not require irrigation or mechanized maintenance (e.g. mowing). Click here for an explanation of the carbon cycle.
Plants grow by pulling carbon dioxide from the air through photosynthesis and converting it to leaves, stems and roots. Salt marsh plants such as pickleweed, jaumea, alkali heath and salt grass are adapted to growing in intermittently flooded wetlands where natural sediment flows interact with plant growth to incrementally increase the elevation of the ground in line with slowly rising sea levels. Consequently, the carbon-rich roots form part of the soil and remain in the wet, anoxic (low oxygen) environment where decomposition rates are slow, so carbon is preserved and taken out of the atmosphere long term. Although trees also sequester carbon, when they die the wood is often burned or decomposes on the surface to re-release the carbon to the air, but wetland soils generally stay buried and moist and thus hold that carbon into the future.
With funding from the California Climate Initiative Cap and Trade funds managed by the California Department of Fish and Wildlife (CDFW), CCBER and Geography professor, Jennifer King, are studying how fast the soil/plant horizon in Devereux Slough has been accreting (growing) over the past several decades in order to make predictions about the rate of salt marsh carbon storage in NCOS. This work is based on careful coring of the soils in the existing Devereux Slough and analysis of the carbon content of two-centimeter slices of those cores. These soil samples are dated by depth using signals from Cesium 137 isotopes that were deposited during bomb tests in the early 1960s. Initial results are just in, and it appears that this unique, intermittently open estuary, has been accreting, or rising, at rates similar to tidal marshes studied in San Francisco Bay and elsewhere in California. On-going work using Lead isotopes and assessments of the carbon content of those slices will allow us to determine the rate of carbon sequestration of the system and to make some predictions about future sequestration. We will be able to test those predictions using the ‘feldspar’ plots that have been established at multiple elevations in the salt marsh. These small squares of white clay (feldspar) create a time marker for 2017 allowing us to measure the rate of accretion as the soils develop on top of the marker over time. We will measure by taking small soil cores in 5 or 10 years hence and measuring the depth of soil above the white marker.
Figure 1. Feldspar plots at NCOS (left) and Devereux Slough (right).
CCBER has planted more than 50,000 salt marsh plants that will grow rhizomotously (e.g. spread out) to form the new salt marsh while simultaneously sequestering carbon and creating a system that can keep up with baseline levels of sea level rise. We have designed the project with a range of elevations for salt marsh plants, providing a buffer in case the rates of accretion cannot keep pace with actual levels of sea level rise associated with human-induced climate change. Stay tuned as we develop more accurate estimates for the tons of carbon we anticipate that the 22 acres of NCOS salt marsh will sequester over the next 100 years!
Integrating carbon into the soils
Because the soils excavated from the filled wetland were relatively low in nutrients and organic matter, we incorporated short and long-lived carbon into the newly created upland soil profile in the restored mesa. The short-lived carbon inputs were in the form of compost, and the long-lived input is known as Biochar (Figure 2). Biochar is created by burning slash wood from logging operations at high temperatures and low oxygen levels to create a very stable form of carbon that can persist for as long as 1000 years without breaking down. For more information about how biochar is produced and how it functions in the soil, see the Biocharproject website. At NCOS, 200 tons of purchased biochar were buried one foot below the soil surface, resulting in the storage of at least 175 metric tons of carbon equivalent according to estimates from carbon-accounting agencies. There is some loss associated with water weight and decomposition.
Figure 2. Biochar piles ready to be spread at NCOS (left), and varying combinations of biochar and compost being spread at different depths in experimental plots (right).
Soil carbon can increase plant growth
The physical structure of the carbon compounds created through the biochar creation process are complex and provide important functions in newly developing soils that support micro-organisms, nutrient storage and porosity which all support long-term plant growth which, in turn, increases the carbon sequestration potential of the site through carbon taken up by perennial plants and stored in roots. CCBER plans to establish more than 12 acres of perennial grasslands dominated by California’s state grass: Stipa pulchra or Purple needle grass (Figure 3). The deep, filamentous roots of this species help store carbon deep in the soil over the long term. Students are helping with the baseline analysis of soil carbon and measurements of plant growth to document the beginning of this long term process.
Figure 3. Stipa pulchra (purple needle grass) grassland.
Supporting vehicle-free recreation and sustainable land management
The NCOS project includes more than 2.5 miles of trails that will provide connections from the neighborhood, and bus and bike routes which will link people to the full 652 acres of Ellwood Mesa and Devereux Slough open space area. This resource encourages and facilitates non-vehicular travel, exercise, and nourishing experiences in nature. Finally, the goal of NCOS is to restore native plant communities that are adapted to the local coastal environment without need for irrigation or regular mechanical maintenance. These aspects are in contrast to the site’s previous function as a heavily irrigated golf course used by motorized golf carts and maintained by regular mowing and trimming.
Through all these mechanisms, NCOS will help sequester carbon into the future. The restoration project construction component, required to undo the impacts of the wetland destruction that happened in the 1960’s, has contributed carbon to the atmosphere, and CCBER is quantifying that by documenting fuel use and hours of operation by heavy equipment and will produce a report summarizing the carbon balance of the project compared to a no-project alternative.