If you use traditional farming techniques of plowing the ground and planting seeds, the answer is zero net carbon sequestration, but in the 21st century new soil science has been developed that drastically changes the answer.
There are 3 aspects you need to consider:
• Visible growth above the ground
• Hidden growth in the plants roots
• Soil Organic Carbon (SOC) exuded by the roots
Visible growth above the ground will likely be burned or allowed to rot within a few years and thus has no long term sequestration capability in my possibly naive understanding of hemp products.
Plant roots also decay within a few years and thus have no net sequestration capability.
SOCs exuded by the roots is the potential jackpot and that is where 21st century science kicks in. Prior to 1996 no one, and I mean no one, knew plants could exude SOC and have it build up for decades in the soil.
Hemp is on the list of mycorrhizal plants at http://www.rootnaturally.com/PlantListMycorrhizal.pdf
That means it can establish a symbiotic relationship with Arbuscular Mycorrhizal fungi (AMF). AMF can be purchased as a low cost soil enhancer and Pennington Seed, as an example, just came out with a lawn fertilizer that incorporates AMF.
If a hemp farmer uses traditional plowing and tilling techniques the AMF in the soil will be killed and adding more via enhanced fertilizer will be a waste of time.
If the hemp farmer uses modern no-till planting techniques and targets enhancing the soil with AMF a healthy AMF population can be established. Because AMF lives on the roots of mycorrhizal plants and hemp grows very deep roots a deep healthy population of AMF can be established.
Everything I’ve written above was known prior to 1996, but to be honest no one really cared. AMF is ubiquitous globally. Who cares if farmland has a healthy population or not?
In 1996 something wonderful was discovered and it applies not just to hemp, but to all the 85% of plants that are mycorrhizal. But hemp can have roots as deep as 2 meters (6 feet) if allowed to grow for 2 years, that makes if far more capable of sequestering carbon via root exudates than most lawn grass as an example.
Glomalin was discovered in 1996. Since then it has been proven to be both the key to rich healthy soil and the key to carbon sequestration in soil. By weight, natural soil can have as much as 10% glomalin, but that is extremely rare in natural soil outside of perennial grasslands and tallgrass prairies.
Unfortunately farmland that is in active use tends to be 30–80% depleted in SOC relative to pristine grasslands and most of that depletion is caused by a lack of glomalin in the soil. You may be aware that farmers often leave their land fallow to allow the nutrients to build up for a year before growing a crop. During that year (or more) of the land lying fallow the soil nutrients build up, then the crop pulls nutrients out of the soil when it grows and the process is repeated. In soil low on glomalin, rain water also aggressively removes nutrients by dissolving them. Glomalin aids in forming soil aggregates that then form a protective barrier around the soil pore spaces that protects nutrients from being leached away by rain water. Instead they are trapped in these micropores like a sponge.
If you’ve ever heard of strip farming, it is a method of farming where the land is divided into strips. Only half the strips grow a crop each year and the other half lies fallow for a year. Thus only 50% the land is actually growing a crop each year. Strip farming is very common in some parts of the country due to the poor soil. My grandfather raised wheat in both Nebraska and Colorado. He used strip farming methods in both states.
With new knowledge gained in the last 2 decades, soil scientists now know how to permanently improve the health of degraded soil and allow 100% production from the land. One of the key steps is to stop plowing farmland.
It can take 2 or 3 years to build up enough SOC in the soil to eliminate the need for adding amendments such as manure or left over crop stalks via plowing them into the soil. Thus farmers need to be educated about this new no-till farming technique and a profitable use of their land needs to be found as it effectively lies fallow for 2 to 3 years as the SOC density is build-up sufficiently.
The method hasn’t yet made it into general use but we can pray it happens over the next decade or two because healthy soil can sequester carbon on a massive scale and it can build up the carbon storage amount in decades, not thousands of years as believed by soil scientists prior to 1996.
Now that it is legal in the US, growing hemp on the land during the first few years of converting the land from traditional plowed agricultural use to new modern no-till farming may be a profitable way to manage the transition as hemp is well known for growing well in degraded soil. And it can be harvested without killing off the plants.
So, if glomalin is the magic ingredient that healthy soil is built out of, where does it come from?
There is only one known source of glomalin, but by the grace of God it makes glomalin in huge quantities and then sloughs if off deep in the soil. If you don’t know the word slough, snakes slough off their skin as they grow. Dogs often slough off hair, but we call it shedding. Humans slough off skin continuously, but our skin isn’t durable after we slough it off so we don’t notice it.
Glomalin is extremely durable and lasts from 7 to 42 years in soil after being sloughed off before it deteriorates. When it does deteriorate it does so into extremely long lived molecules called stable humic polymers which can survive for centuries.
The only known source of glomalin is AMF (Arbuscular Mycorrhizae fungi). That is why establishing a healthy population of AMF in soil is important and hemp is excellent at doing so because of the deep roots it can grow if allowed to.
AMF in farmland is in general highly depleted because AMF is damaged by plowing the land. In our urban areas it is also damaged by leveling land, etc. Thus human natural tendency to control their environment depleted one of the most important biological species on the planet.
But, as I said before AMF is ubiquitous in undisturbed soil. Take a shovel out to a natural grassland and dig up some soil complete with grass and roots and you are likely to have at least some AMF.
So now we know how to establish a population of AMF in soil and how to plant hemp such that we don›t kill off the AMF in the process, but how much carbon can it sequester?
Let’s assume we start with farmland that is 50% depleted in SOC (such farmland is unfortunately readily available). For the first planting amend the soil the traditional way and plow in manure or other nutrient rich amendments, remembering we will never plow that land again.
As the hemp grows (and AMF incorporated fertilizers are used) a healthy AMF population is established, the AMF will take over the job of ensuring the soil is rich in SOC and other nutrients.
Soil weighs approximately 2 tonnes (metric tons) per cubic meter and there are about 4,000 square meters per acre. Since hemp roots of 2 year old plants extend 2 meters deep, that is about 8,000 cubic meters of soil for hemp to sequester carbon in. That soil weighs about 16,000 tonnes per acre.
6–8% soil organic material is a realistic long term goal for farmland. 50% of that mass is carbon, or 3–4% of the soil being carbon is a realistic long term goal. It turns out carbon declines in density with depth in a more or less linear way. Thus if the top 20 cm is 3–4% carbon, the overall percentage for the top 2 meters is probably in the 1.5–2.0% range. Thus, we easily get roughly 250 tonnes per acre of potential carbon sequestration, but remember it is only 50% depleted so about 125 tonnes per acre of potential additional carbon sequestration.
It will take about 30 years of no-till farming of hemp on a plot of land to raise the carbon content from roughly 2% to roughly 4% so we are looking at 4.2 tonnes per year per acre of potential carbon sequestration, or up to 15 tonnes CO2e per year per acre of carbon sequestration.
The US per capita carbon footprint is about 16 tonnes CO2e per year, so an acre of hemp grown with techniques optimizing carbon sequestration can offset up to 1 person’s carbon footprint.
If a portion of the hemp itself is used to make biochar, then that biochar added to the above system, then the biochar tonnes produced would be additive to the above carbon. However, if the hemp itself was allowed to rot as compost, it is a good amendment, but almost all would return to the atmosphere as CO2 or CH4.
If I scale this up to potential global cropland. I’ll start discussing the US first. There are approximately 1 billion acres of agricultural land in the US. Of that 300 million acres is used to produce a crop in any given year. That doesn’t include land lying fallow, failed crops, pasture, or range land.
Since the US has 300 million acres of cropland and 300 million people, regenerative farming as described above has the potential to 100% offset US C02 emissions. That doesn’t include range land. There are regenerative grazing techniques that could allow the US’s roughly 500 million acres of range land cumulatively to sequester roughly the same as farm land (not per acre), so the US has the potential to be very carbon negative by using regenerative agriculture.
Globally there are 4 billion acres of cropland and 6 billion acres of rangeland. Combined, they can easily transition the earth into a negative carbon emission scenario.
Some Facts To Consider
The 2007 US farm survey results don’t break down how land is filled, but the 2012 survey does.
1 billion acres of US agricultural land (10 billion globally), including pasture, range land, and timber
300 million acres of US productive cropland (4 billion globally)
100 million acres of US cropland is plowed/tilled traditionally (no idea globally)
100 million acres of US cropland is reduced tilled (strip or seasonal tillage)
100 million acres of US cropland is no-till (as of 2012, also 300 million globally as of 2010)
I found a separate reference that said 21% of US cropland (60 million acres) is under continuous no-till management as of 2017. I’m not clear what the difference is between that 60 million acres and the 100 million acres under no-till in 2012.
The UN 4 per 1000 initiative (www.4p1000.org) claims CO2 accumulation in the atmosphere can be 100% offset by increasing the carbon content of just the top 30–40 centimeters of soil. Thus 1/6th of the numbers I used above.