Can compost use impact deep-soil carbon storage?
Atmospheric carbon dioxide (CO2) levels have reached a 3-million-year high. The overload has been building up since the Industrial Revolution hit its stride about 200 years ago. One culprit is conventional farming, which releases stored soil carbon, (C) relies on synthetic input, and degrades soils. The use of fossil fuels — which adds even more carbon to the air — is another major contributor to the increase.
Impacts include reduction in carbon storage capacity due to topsoil loss, pollution, and climactic changes linked to global warming.
While some say CO2 levels are a non-issue, others believe the opposite and are working to reduce atmospheric carbon by cutting greenhouse gas emissions and sequestering more carbon.
Proposed solutions include ideas like basalt rock injection and ocean storage. But one of the simplest and most cost-effective options already in play is to simply return carbon to soils.
But there is a catch. Since soil disturbance releases carbon, farmland dedicated to annual crops offers little to no upper layer storage potential. Unless the farm — organic or conventional — has adopted no-till practices, the soil is disrupted every year during planting. Therefore, acres that will remain undisturbed for long periods of time are the best candidates for long-term carbon storage, a.k.a. carbon farming.
This translates into a need for conversion of annual grains and other crops to perennials to maximize soil carbon storage potential. But developing high-yielding perennial grains is a process that will take time.
Applying compost to both annual and perennial acres is something that can be done now.
Long-term soil carbon storage has two primary pathways — plant-based via deep-rooted perennials (like some trees and grasses) and soil amendment. Compost leads the pack as the amendment of choice, offering a plethora of soil- and crop-enhancing benefits in addition to carbon storage.
The carbon sequestration benefit of compost use is two-fold. It adds compost’s inherent carbon content to the soil. But it also improves soil productivity, increasing above- and below-ground biomass, which stores more carbon. This positive impact can persist for many years from just one compost application.
Microorganisms and insects like earthworms and ants also influence carbon storage.
Current research emphasis seems to focus on C storage potential in agriculture. That’s because worldwide, farmers crop 4.62 billion acres — a treasure-trove of carbon storage potential.
But it should be noted there are many more non-ag acres that can offer long-term, deep-soil carbon storage, too. No need to wait for researchers to develop new perennial grains or convince farmers to make the switch from annuals to perennials.
In the U.S. alone, millions of acres could take and store compost-applied carbon today:
- 40 million acres of lawns
- 50 million acres of managed turf including 700,000 athletic fields and 17,000 golf courses
- 7 million acres of public lands under the jurisdiction of the Bureau of Land Management (FY 2017)
- Umpteen million acres in government-controlled roadside easements, utility rights-of-way, local and state parklands, and other managed greenspace, both public and private.
While it may not be practical or possible to add compost to all those acres, the potential for long-term, compost-based carbon storage in the U.S. alone is … well … pretty big. And the deeper the storage, the longer the retention time. Even annually tilled acres can offer C sequestration if that carbon can find a pathway to deeper soils below the plow layer.
But how does compost-applied carbon migrate from upper soil levels to deep-soil storage?
Below the plow layer — deep-soil carbon storage
A “plow layer” is the layer of soil disturbed when a plow (plough) is dragged through a field. Depending on the type of plow used and its settings, the layer depth can range from a typical 8 inches to 20 inches or more.
The trick is to facilitate the movement of carbon from the plow layer to deeper soils where it can lie locked up and undisturbed for centuries.
Typically, the root systems of trees and perennial grasses grow deeper than annuals. As the root systems of plants bury themselves in soil, they do more than just carry carbon in their tissues.
Downward growth also creates passageways for water and a transportation route for microbes.
Critter burrows create pathways for water and microbes, too. Well below the plow line, termite colonies can be found as deep as 6 meters. Some types of ants live at depths of 3-4 meters. (1 yard = .91 meters)
Nightcrawlers will work their way down several meters into the soil, bringing organic matter with them. Their castings (excreta) have a carbon-to-nitrogen ratio of 12-15:1 and include beneficial microbes. A healthy soil layer above ensures plenty of available carbon for below-the-plow-layer earthworm storage.
C storage depends on many factors
It is important to remember neither the C cycle nor microbial metabolic processes operate in isolation. These functions are influenced by many factors like:
Climate plays a role, too. Globally, cool-wet regions tend to have the highest concentrations of soil carbon and deserts the lowest. One of the more disturbing aspects of a warming planet is that more soil-bound carbon may be released to atmosphere as temperatures rise.
Effective solutions will require wholistic approaches, but compost use continues to rank among the top options due to its affordability, universal applicability, ease of use, and immediate availability.
Bottom line: The potential for deep-soil carbon storage exists, even on annually tilled cropland, through management programs designed to improve soil health and encourage symbiotic facilitators like nightcrawlers.