What if you knew that doubling your forage and increasing long-term carbon sequestration was as simple as spreading a thin layer of compost on pastures? Would you do it?
In 2008, my friends John Wick and Peggy Rathmann began a study to determine how much carbon is sequestered in soil as a result of managed, intensive grazing. Working with UC Berkeley’s Whendee Silver and the Silver Lab, they began by taking a look at the state of soils in their area. They gathered soil samples from 35 sites on 22 ranches in Marin County, California to get a baseline look at the state of soil carbon sequestration in the area. They were particularly interested in the more stable fractions of soil carbon – the kind that remains in the soil for twenty, forty and even hundreds and thousands of years.
They were surprised to find some samples had much more of this stable carbon than the rest of the samples. These high carbon samples all came from dairy farm pastures. What made them different from the rest was the farmers’ practice of spreading dairy cow manure on the pastures they came from. It seemed that the addition of large quantities of manure was increasing carbon sequestration.
This discovery changed the course of their research. Initially they had planned to look at whether keyline plowing and/or managed, intensive grazing were helpful for increasing long-term soil carbon sequestration. But these dairy farm soil samples gave them another option to look into. They added compost to their research protocol, an alternative similar to manure, but more stable and without the issue of methane off-gassing.
Not all Soil Carbon is Sequestered Long-Term
If we want to make a difference to the climate, we want to meet the definition of carbon sequestration soil scientist Rattan Lal uses:
“Carbon sequestration implies transferring atmospheric CO2 into long-lived pools and storing it securely so it is not immediately re-emitted.”
Not all soil organic carbon meets this definition.
Most soil organic carbon is very active. Found in plant tissues and the bodies of microorganisms in the soil, most of it cycles back to the atmosphere in a few months to several years. This is labile or the light fraction of soil organic matter. Here’s how it works:
Thanks to photosynthesis, soil organic carbon increases dramatically in the spring as plants green up, and increases even more when there’s good spring moisture. When plants go dormant in the winter, CO2 is respired back to the atmosphere.
Seen from space, the change of seasons almost looks like the earth is taking a deep breath in spring, and then exhaling again as fall and winter come on:
Below ground, there’s a whole world of micro-organisms. They eat carbon, supply nutrients to the plants and to each other, and respire CO2 back into the atmosphere as a waste product. In the process of all the soil microbes eating plant tissues and each other, some of the soil carbon becomes trapped inside microscopic particles of minerals. Think of CO2 as the tasty center inside a chocolate truffle. This is “occluded carbon:” trapped particles that can stay in the soil for decades.
It’s this trapped carbon that meets the definition of carbon sequestration. And it turns out, a thin layer of compost is very good at creating this kind of carbon.
With more than a decade of data, here’s what they found.
The purpose of John and Peggy’s project was to find out what could be done to increase carbon sequestration on grazing lands. Darren Doherty, a specialist in keyline plowing systems, suggested that keylining their ranch could increase carbon sequestration. So a keyline test was part of this research project. John and Peggy also wanted to know if their adaptive managed grazing would do the trick, so they added it as one of the treatments being monitored.
Neither keyline plowing or grazing have shown increases in carbon sequestration since the experiment began in 2008. There were improvements on the grazed lands as a result of managed grazing, including reductions in problematic annuals, increases in native perennials, and an increase in the soil’s ability to absorb and hold precipitation.
It was the compost that stood out. They spread a half inch of compost over test plots and then monitored results. In response to this added fertilizer, forage production increased by 50%, and when they analyzed the soil, they found they had added a metric ton of occluded carbon per hectare (2.47 acres) to the soil. The next year — with no further treatment — the soil had captured another ton of carbon, the next year another ton, and so it has continued. In fact, thirteen years later, forage production continues to be better thanks to compost, even through the severe California drought, and this durable carbon continues to be added to the soil.
Based on the results, the researchers have looked at how much compost a grazier should spread and how often. Their models show that 1/4″ would provide the same results, and compost would only need to be applied every twenty years.
But what about the contribution of greenhouse gases from making the compost and then trucking it to the pasture, and from the cows grazing all this forage? Does it offset the benefits of compost? Project researchers did a complete life cycle analysis and found that the compost treatment was still beneficial.
What does this mean for graziers interested in being successful AND helping prevent climate change?
“We have 3.5 billion hectares of grasslands [globally],” Wick said. “The most conservative estimate shows that if we do this with 2.7 billion hectares it would get atmospheric carbon down to 350” parts per million, the level that might prevent human-driven global warming from escalating. “There is the capacity to do this while producing food, fuel and fiber. And helping wildlife.”
Of course, we can’t spread compost over all of these grasslands for a variety of reasons, including the potential response of weeds or invasive grasses to such a high dose of nutrients. Researchers are already looking at that to provide us with some answers. But we can do it at home, making a difference to the planet and our pocketbooks. John and the Marin Carbon Project have been working with the Natural Resources Conservation service on technical notes for the practice and financial assistance to graziers interested in applying it. Currently it is covered under the EQIP program.
The American Carbon Registry, which oversees the creation of carbon offset protocols and the registration of carbon offset projects, has approved a protocol, Compost Additions to Grazed Grasslands, which was researched and proposed by Marin Carbon Project and its partners. Using this protocol allows graziers to sell offset credits for sequestering carbon on grazed grasslands.
But carbon markets are still very young and it’s difficult to sell carbon credits. In fact, a 2020 look at the markets by the Marin Carbon Project found that the current price of carbon on the voluntary market is still insufficient to support implementation and verification of carbon farm plans for soil carbon sequestration. That means, if you don’t get financial assistance, you’ll need to put a pencil to the cost vs the benefit of 50% more forage to your operation.
Visit Marin Carbon Project for access to many of the scientific papers published on the study and to learn more about carbon farming.