Irrigating a Dryland Soil Carbon Sponge
Posted on Dec. 11, 2023 / Irrigation / Subscribe 0
By: Sonny Gamponia & Barry Solomon
The classic Köppen System of Climate Classification revised its description of the area along the leeward coast of Maui in 2023. Previously classified as a Tropical Savanna Climate, it is now considered a Hot Semi-Arid Climate. The classification is based on rainfall data but can be affected by changes in the dominant vegetation in the area. Whatever the reasons for the change in classification, the seasonal transformations in the native dryland ecosystems are dramatic, especially under drought conditions.
Figure 1. Left photo: Rainy season 2022-2023,
Maturing shrubs can change the structure of ecosystems, leaving bare areas vulnerable to invasive species when the rains return. Research studies on shrublands in the semiarid soils in New Mexico found that the moisture depth is held in the top 4 inches. Mulch, ground cover, and low canopies can help hold the moisture accumulated during the dry season.
Volunteers from ReTree Hawai’i are collaborating on a citizen science project at Keālia Pond National Wildlife Refuge. They are planting in bare areas along a proposed nature trail, creating a diverse plant community. This collection of native groundcovers and low shrubs serves as a “soil carbon sponge”, absorbing moisture, slowing evapotranspiration, and making the soil more absorbent during the rainy season.
The term “soil carbon sponge” comes from a concept proposed by Australian soil microbiologist and climate scientist Walter Jehne, who describes how the void between rocks and soil particles can be filled with roots, microbes, and humus to hold moisture. The idea is gaining traction, evidenced by the 2020 Netflix documentary Kiss the Ground, narrated by Maui resident Woody Harrelson. These soil carbon sponges can affect an underground microclimate, allowing roots and microbes to interact. Native roots and microbes are the keystone ingredients of a soil carbon sponge. It attracts a series of underground predators and prey, called the soil food web.
Photosynthesis needs to be active for roots to produce enzymes at their root tips. These enzymes (called exudates) attract bacteria and fungi. The microbes consume the sugars, which signals whether the plants need calcium, nitrogen, potassium, or zinc. The microbes harvest the minerals from the surface of the soil particles in exchange for more sugar. In this manner, roots can put moisture back into the soil even when there’s no rainfall. The Keālia Pond project uses historical native Hawaiian plants with the climate and local soil conditions on Maui.
Figure 2. Left panel: These native plants remain green, even during a drought and low rainfall. ‘Akulikuli can photosynthesize at night. It also holds droplets from passing light showers. ‘Ohelo kai, also known as Hawai’i desert thorn, is a thornless endemic that is very adaptable in dunes, wetlands, and drylands. Right panel: These dry season survivors are some of the hardiest survivors in dryland soils. Their leaves shrink or hold moisture in their stems during the dry season and bounce back in the rainy season.
When feasible, the transplants are cultivated with compost tea to establish a microbial community. This miniature ecosystem is then transplanted within an irrigation frame to initiate the soil carbon sponge. Each soil carbon sponge is grown in a 4-foot-wide drip irrigation frame connecting inline drip irrigation tubing. Transplants consisting of 60% active photosynthesizers and 40% dry season survivors are irrigated over two growing seasons.
Traditional irrigation formulas are designed to optimize crop yield or maintain a consistent look for landscaping. The standard formula calculates gallons per day by assigning quotients for plant species in square feet, climate, and evapotranspiration efficiency. Habitat restoration irrigation demands a different approach to agriculture and landscaping. This irrigation plan is based on local rainfall targets. Rainfall in inches is converted to gallons per square foot. At Keālia Pond it rains once a week during the rainy season and every other week during the dry season. During a drought, there is only a trace of rain once a month or no rain at all. The irrigation amounts are applied to different stages of growth over two growing seasons.
Figure 3. A: Baseline measurements: 1) Choose rainfall targets for each stage of growth; 2) Convert rainfall to gallons or cups per week; 3) Apply the calculations to 1 square foot. This plan shows how the irrigation is tapered as the soil carbon sponge matures.
Each irrigation area is connected to a flow meter and hose timer. Depending on water pressure, drip irrigation times may start at 20 to 30 minutes and taper to 10 to 15 minutes. By contrast, a 5/8-inch garden hose with a sprinkler covering a comparable area would use 1,020 gallons of water an hour.
Details for the calculations, spreadsheets, and photos of soil carbon sponges can be found in a photo journal for this citizen science project at the Kealia Restoring Soil website.
A USDA soil report states that 1% organic matter such as roots, microbes, and humus can hold up to 25,000 gallons of water. Replacing an acre of kiawe and buffelgrass with that much water can reduce the frequency and severity of wildfires. Expanding soil sponges along the leeward coast could increase soil moisture to a large enough scale, potentially transforming the current hot semi-arid climate back into a tropical savanna.
References:
Beck, H.E., McVicar, T.R., Vergopolan, N., et al. 2023. High-resolution (1 km) Köppen-Geiger maps for 1901-2099 based on constrained CMIP6 projections. Scientific Data 10: 724. https://doi.org/10.1038/s41597-023-02549-6
Jehne, W. 2015. Restoring water cycles to naturally cool climates and reverse global warming, paper presented at the Global Cooling Earth Org, Biodiversity for a Livable Climate’s Conference, Restoring Water Cycles to Reverse Global Warming. Medford, MA: Tufts University, October 16-18, 2015. http://www.globalcoolingearth.org/cooling/.
Kurc, S.A. & Small, E.E. 2004. Dynamics of evapotranspiration in semiarid grassland and shrubland ecosystems during under summer monsoon season, Central New Mexico. Water Resources Research 40(9): W09305. https://doi.org/10.1029/2004WR003068
Solomon, B.D. & Gamponia, S. 2023. Using native roots and microbes for local landscapes. Hawaii Landscape 74, https://hawaiiscape.com/blog/id/17
Solomon, B.D. & Gamponia, S. 2023. Restoring health to degraded soils with native plants and microbes. Hawaii Landscape 71: 22-24.
USDA Forest Service. 2015. Healthy soils: the promise for the future. Redding, CA: Soils Report, USDA Forest Service Shasta-Trinity National Forest.
Sonny Gamponia,
0 Comments