Rice to the Rescue: Reversing land subsidence and carbon emissions
Thousands of years ago, the plants of California’s Delta began sucking carbon out of the atmosphere and storing it in the soil. Today, roughly the weight of the Golden Gate bridge in carbon dioxide is being kept below ground. But as the equilibrium between dry land and water in the Delta is in flux, that stored carbon is being released back into the atmosphere.
Not only does this mean that the Delta is becoming a potent greenhouse gas producer, but the land is subsiding at scary rates. Delta science researchers reviewed our existing understanding of current rates and agricultural practices and put forth a recent publication that makes pointed suggestions to restore a healthy equilibrium and keep carbon where it is best, out of our warming atmosphere and safely below ground.
Science-in-Short is a quarterly podcast introducing scientists working on emerging topics in the San Francisco Estuary watershed. The podcast is written and produced by Ashleigh Papp with editing support from Ariel Rubissow Okamoto and the Estuary News Group, and music created by Peter Rubissow. Science in Short is funded by the Delta Stewardship Council.
FULL TRANSCRIPT
You’re listening to Science in Short, a quarterly podcast introducing you to scientists working on emerging topics in the San Francisco estuary watershed. I’m Ashleigh Papp.
Interviewer Ashleigh Papp: California’s Sacramento-San Joaquin Delta is a vast network of water. Most of its waterways are shallow and wind through low islands, while others open up into larger bays that eventually feed into the Pacific Ocean.
This expansive ecosystem, just over one thousand square miles in total, is part of one of the largest estuaries on the West Coast. Water from the Delta and its watershed supplies the Central Valley’s agricultural economy and even parts of Southern California, and over 500 plant and animal species can be found here.
But in addition to supporting the human and natural worlds, the Delta is also working in a way that’s hidden to the naked eye. Below the aquatic surface, the plant roots and Delta soil are holding 200 teragrams of sequestered carbon, or about 2.5 billions times the weight of the Golden Gate Bridge. For thousands of years, the plants in this area have been working like a giant “sink” or vacuum, sucking carbon from the air and storing it in the soil.
This process, known as carbon sequestration, has become a key focus in delta science research. Because in our current warming, carbon-filled atmosphere, this ecosystem’s ability to store or release carbon today will have profound impacts on society’s access to healthy, clean air tomorrow.
Hydrologist Deverel: My name is Steven Deverel. I’m a principal hydrologist here at HydroFocus. I’ve been involved with carbon issues related to subsidence in the Delta since about 1990. I would say I work with ecosystems, and the relationship of ecosystems to water and how that fits into our overall sustainability in water supply through the Delta and for the state of California.
In order to really understand the complexities of carbon storage and how it relates to water supply, let’s go back to California’s last ice age, when the Delta was formed.
Deverel: It started to form about 7,000 years ago, at the end of the last glacial period. At that time, sea level began to rise. And it rose into what is now the Delta, and so that initiated a period where basically the landscape was flooded. And so that flooded condition led to the formation of tidal marsh. And so there was this tidal marsh that existed throughout the Delta, that was a primary landscape feature.
And in that tidal marsh with ongoing sea level rise, the vegetation was basically anoxic. Even though there was the tidal situation, water moved in and out, the vegetation grew and decomposed under very low oxygen conditions, which resulted in the accretion of biomass over time. And in some places, it was up to 50 or 60 feet of material. As a result of that combination of anoxic or low oxygen conditions and the sea level rise, that was the peat soil that formed.
Worldwide, peatlands occupy about 3% of land surface, but their biomass and soil contain nearly 20% of all terrestrial organic carbon. So in the scheme of carbon storage, peat is pretty much a powerhouse. And around 7,000 years ago, Deverel estimates that roughly 5 billion cubic meters of peat accumulated as the Delta flooded. Which means a lot of carbon was sucked out of the atmosphere by the peat and moved into the soil below.
But, in the late 1800s, gold was discovered in California. And the Delta landscape changed …
Deverel: People began to see that these rich, organic soils in the Delta really had a lot of promise for agriculture. And so what happened was that they began to build dikes around the wetlands and began to drain the soil.
In addition to almost immediately changing the landscape of the Delta — from flooded to not, the newly-exposed soil quickly went from covered by water and anoxic — to open and full of oxygen … and therefore, was exposed to a whole new category of microbes.
Deverel: And so as you expose that to oxygen, the microbes are basically consuming that carbon and releasing it as carbon dioxide.
In this transfer of carbon, from soil to air, the microbes began releasing a lot of carbon back into the atmosphere and decreasing the amount of mass in the soil … So much so, that these days, land subsidence is becoming a big issue in the Delta.
Deverel: Subsidence is really defined as the lowering of the land surface. The primary mechanism for subsidence in the Delta is the loss of the organic soil that happens through the oxidation. So, for example, you know, the wetlands that were created over the last 7,000 years were pretty close to sea level before they were drained for agriculture. Now, we have islands where the land surface elevation is as deep as 25 to 30 feet on these islands.
In a publication from the journal San Francisco Estuary & Watershed Science, Deverel and a team of researchers investigated management opportunities for the Delta … Because not only is the land sinking, but harmful greenhouse gasses are being released back into our atmosphere.
Deverel: There’s this hand-in-hand subsidence, carbon relationship that’s evolved over time. So a lot of the work has involved measuring carbon fluxes with different kinds of land uses and the current agricultural practices in the Delta. And that’s a lot of what was in that paper and then developing strategies for stopping those carbon emissions, which corresponds to reducing, stopping or reversing the effects of subsidence.
As the land continues to sink, the Delta is becoming a carbon-producer, which is bad for our environment … And the levee system is being compromised, which is really bad for California’s water supply. Through investigating the history of the Delta and the current way that the land and water are being used, the team came up with three suggested interventions:
- First, reverse subsidence by restoring certain parts of the Delta back to wetlands and allowing peatlands to reestablish.
- Second, reconnect the Delta to tidal water, which will help to build back the land mass and nutrients via marine water flowing in and out.
- The third recommendation is to rewet agricultural fields and instead of planting crops that need dry land, switch to something that can grow in a flooded plain. Something … like rice.
Deverel: We know rice stops subsidence, we’ve got good good data to show that. It can provide a greenhouse gas emissions reduction, depending on what the situation is.
These three proposed changes, altogether, create something like a patchwork or quilted Delta, filled with mini multi-use ecosystems.
Deverel: One of the things we’ve worked on with The Nature Conservancy and Metropolitan Water District [of Southern California], both of those organizations own islands in the Delta, is this mosaic of land uses that can stop subsidence and/or reverse subsidence and generate enough income to support the farming activities.
Another interesting solution that’s in the works already is the idea of a floating island.
Deverel: We did some experiments on taking peat blocks from wetlands and putting them in some pools and floated them. And they’re still floating out there today on Bouldin Island.
So much acreage of the Delta’s tidal lands has been lost and there isn’t enough time to restore them before sea levels rise. Thus, the concept of human-made islands as a new landcover type is emerging. These floating wetlands not only help sequester more greenhouse gases, but they also provide additional benefits.
Deverel: We looked at the foodweb potential and greenhouse gas emissions reductions and some of the water quality issues and basically, they would provide the same benefits, in my opinion, that the impounded wetlands would provide, but they’re now at sea level.
With all of this information brought together and real solutions on the table, Deverel knows that there’s a lot to consider.
Deverel: The most gratifying lesson that we’ve learned really relates to this difficulty in making this transition. For all of these farmed islands, which depend on a farming income, how do you transition to a more sustainable situation where you’re not continuing to subside the land and you can still generate enough income to actually maintain the levees and keep a viable agricultural situation as well?
There’s nearly 400,000 acres of non-flooded, or dry-land, in the Delta used for agriculture. Even though this is the largest category of land use that could be managed to improve greenhouse gas emissions and subsidence, Deverel’s research suggests that only flooding certain parts of the Delta will result in the biggest return.
Deverel: It’s not everywhere in the Delta that you need to flood the land. We want to prioritize those areas in the Central Delta that have the highest organic matter content and are subsiding at the highest rates. And the idea is to prioritize those lands for conversion to rice and wetlands.
At current rates, the Delta is releasing nearly 1% of the state’s carbon dioxide into the atmosphere each year, according to Deverel’s research. Given the link between sequestered carbon being released, the land subsiding, and the state’s water supply being threatened, the interventions suggested from this research seem worthy of immediate attention.
Deverel: It seems like that’s been an evolving process, where there is this growing recognition that if we’re going to have a Delta that’s sustainable from a water supply standpoint and an ecosystem standpoint, we need to really take some giant steps towards reversing the effects of subsidence.
Science in Short is a quarterly podcast written and produced by me, Ashleigh Papp! With editing support from Ariel Rubissow Okomoto and the Estuary News Group, with funding from the Delta Stewardship Council. For Maven’s Notebook, I’m Ashleigh Papp. Thanks for listening!
