By Julia Frisbie
posted May 6, 2021
We buried our lawn five years ago under a layer of cardboard and then compost, planted a living mulch the first season, and have been mulching with arborist chips and rotating poultry through it, and growing a happy riot of annuals and perennials in it ever since. You can see the difference in this picture between our backyard soil (front right) and the soil in the adjacent easement (back left), but now after a soil test we have quantitative data. Using this data to estimate how much carbon we’ve sequestered was the most fun I’ve had with math in months!
We sent in two soil tests to the University of Massachusetts, with the city easement functioning as a control because it’s all in grass, and our yard was all in grass before we moved in five years ago. (It would have been better if I’d tested the same spot 5 years ago, but I didn’t, so the easement is our best point of reference for a comparison.) The backyard had 22.4% soil organic matter (measured by LOI, or “lost on incineration”… basically, how much of its mass can burn up) and the easement soil had 10.6%. So the backyard’s soil is 11.8 percentage points higher than our control plot.
I based my calculations on research by R. Lal of Ohio State University’s Carbon Management and Sequestration Center. His article “Abating climate change and feeding the world through soil carbon sequestration” was published 2013 in an anthology called Soil as World Heritage. Lal estimates that 21 tons of carbon are sequestered per additional 1% of soil organic matter per hectare.
Our backyard measures 74 feet x 40 feet, which is 2,960 square feet. There are 107,639 square feet in one hectare, so our backyard = 0.0275 hectares. Multiply 21 tons by 0.0275 = 0.5775 tons of carbon sequestered in our yard per added 1% of organic matter. Multiply that by our 11.8, and we get 6.8145 tons (x 2,000 = 13,629 pounds) of carbon sequestered within our backyard in the past 5 years.
For reference, a mature tree absorbs about 48 pounds of carbon per year (Source: European Environment Agency), so our backyard activity has sequestered about 284 tree-years of carbon. Also for reference, nearly 20 pounds of carbon dioxide are produced from burning one gallon of non-ethanol gasoline (source: US Energy Information Administration), so our backyard activity has offset about 681.45 gallons of gasoline. (With a 12 gallon tank, that’s 57 fill-ups.)
On the one hand, it’s a drop in the bucket. On the other… in this little place… it makes a world of difference. And it’s profoundly do-able for anyone who wants to sequester carbon in their yard. A movement for “Climate Victory Gardens” has published some helpful ideas for backyard-scale carbon sequestration.
Gabe Brown is a second-generation rancher in North Dakota, and since 1991, he’s seen an increase in soil organic matter from 1.9% to 6.1%. In his excellent book Dirt to Soil, he identifies five principles for soil health that he uses to manage his 5,000 acres:
- Limit disturbance. Tillage releases soil carbon into the atmosphere and causes topsoil to erode away. Don’t do it. Chemical disturbances such as synthetic fertilizer, herbicides, pesticides, and fungicides also qualify as a type of disturbance.
- Keep the soil covered. Bare soil is an emergency. Covering your soil prevents erosion and provides habitat for microorganisms.
- Diversity. Grow as many different types of plants (and host as many different types of animals!) as you possibly can. Brown sows cover crop mixes with up to 70 different species!
- Living Roots. Include perennials and cover crops in your garden plans so that there are living roots in the soil for as much of the year as possible.
- Integrated animals. Intermittent predation by animals stimulates flushes of plant growth which pulse more carbon into the soil. Gabe uses cattle to rotationally graze his cover-cropped acres.
In conversation with these principles is emerging research from ecologists on the Intermittent Disturbance Hypothesis. Robin Wall Kimmerer, in her less-well-known book Gathering Moss, explains “diversity of species is highest when the disturbance occurs at an interval between the extremes. Ecologists have shown that in the complete absence of disturbance, superior competitors… slowly encroach upon other species and eliminate them by competitive dominance. Where disturbance is very frequent, only the very hardiest species can survive the tumult. But in between, at intermediate frequency, there seems to be a balance that permits a great variety of species to flourish.” As more research is published on regenerative agriculture practices, I bet we’ll learn that the disturbance caused by Gabe Brown’s cattle herd moving across the landscape actually supports the increased diversity of each acre they graze.
What I like about Brown is that he comes from a pretty standard (extractive, damaging) American farming perspective, and ends up becoming a champion of regenerative agriculture. We can do the same in our yards. We can convert monoculture lawns to polyculture garden beds, keep soil covered, plan for living roots in the soil year-round, provide gentle intermittent disturbance with animals, and cultivate maximum diversity.
Below, you can see many of these principles at work in my backyard: I’ve got plastic mulch I scavenged at The Predecessors under my zucchinis. I’ve removed the mulch from under my cabbages, kale, and tomatoes, and undersown a diverse cover crop mix, which is just beginning to germinate. Nearby in a temporary enclosure, the chickens are busy making compost and appropriate-scale disturbance in a bed that’s about ready to turn over to the next crop. Flowers peek out around the edges. Carbon sequestration is a joy!
Julia Frisbie has been gardening in Coast Salish territory for six growing seasons, and is thankful to learn from plants, animals, and people who have been here much longer. She’s grateful to her mom, Anne Kayser, for cultivating her curiosity, and also to Robin Wall Kimmerer for writing the book Braiding Sweetgrass, which transformed her relationship with the more-than-human world. Follow Julia’s micro-farm on Facebook, Instagram, and/or TikTok.