When plants grow, they pull carbon dioxide out of the air and use it to build leaves, stems, and roots. When those plant parts die, they fall into the soil and decompose. Some of that carbon gets eaten by microbes, some escapes back into the air, and some stays locked in the soil — that locked portion is soil organic carbon.

SOC is important for two reasons. First, it improves soil health — it helps soil hold water, supports microbes, and feeds crops. Second, it stores carbon that would otherwise be in the atmosphere as CO₂. Regenerative farming practices like no-till, cover cropping, and rotational grazing are claimed to increase SOC. The question this tool asks is: how long does that stored carbon actually stay in the soil?

Think of clay and silt particles in soil as tiny magnets. Certain types of organic carbon — mostly small, broken-down molecules produced by soil microbes — stick to the surfaces of these mineral particles. Once stuck, they become chemically protected and very hard to break down. This is mineral-associated organic carbon, or MAOC.

MAOC is the durable fraction of soil carbon. It can persist in soil for decades to centuries — on average around 200 years, though this varies widely by soil type and climate. When carbon credit schemes say they are "sequestering carbon," the most valuable portion is MAOC, because it is genuinely long-lived. The downside is that mineral surfaces have a finite number of binding sites — once they are full, no more MAOC can form. This is the saturation limit that the Breure et al. slider in this tool reflects.

Not all organic carbon sticks to mineral surfaces. Larger pieces — fragments of roots, leaves, and fungal material — sit loosely in the soil without that chemical protection. This is particulate organic carbon, or POC.

POC is the temporary fraction. It is actively being broken down by soil microbes, which use it as food and release the carbon back to the atmosphere as CO₂. On average it lasts around 5–20 years before decomposing, though some studies suggest it can persist longer in cold or wet soils. Because it is loosely held, it is also the first carbon to be lost when soils are disturbed — by tillage, drought, or a change in management.

The key insight of this tool: most carbon accounting schemes credit POC and MAOC equally. But if POC is gone in 10 years, claiming a 100-year credit for it overstates the climate benefit.

Mean residence time is simply the average length of time a carbon atom spends in a given pool before it is released back to the atmosphere. Think of it like an average stay in a hotel — some guests check out after one night, some stay for months, and MRT is the average across all guests.

For POC, MRT is estimated at roughly 5–20 years — short stays. For MAOC, MRT is estimated at 45–383 years depending on soil type, climate, and depth — long stays. These ranges are uncertain, which is why this tool has sliders for both.

MRT matters for carbon accounting because a tonne of carbon stored for 10 years does not have the same climate value as a tonne stored for 200 years. Tonne-year accounting (used by CAR SEP and this tool's LSR pool-weighted scenario) adjusts for this by only crediting the fraction of storage that is actually durable relative to the accounting horizon.

Mineral surfaces can only hold so much carbon — they have a finite number of binding sites. When those sites are full, no new MAOC can form. The soil is said to be at MAOC saturation. Any additional organic carbon inputs at that point go into POC instead, where they are short-lived and vulnerable to loss.

Breure et al. (2025) mapped this saturation level across EU and UK agricultural soils. They found that the average MAOC saturation across European agricultural soils is around 68%. Soils above this threshold — common in cool, humid regions like northern England and Scotland — are at higher risk: not only are their MAOC pools near full, but they are also more likely to be losing carbon overall.

For this tool, saturation matters because a farm claiming MAOC sequestration on already-saturated soils may be overstating the amount of new carbon that can actually be durably stored. The Breure saturation slider lets you explore how much this changes the inventory number.

A carbon credit is a tradeable instrument issued by a registry (like Verra or CAR). One credit represents one tonne of CO₂ reduced or removed, verified against a specific protocol. Credits can be bought and sold, and companies use them to offset emissions they cannot reduce elsewhere.

A GHG inventory entry is different. It is an accounting record in a company's greenhouse gas report. Under the GHG Protocol LSR Standard, companies report land-based carbon removals as a stock-change: they measure the soil carbon at two points in time and report the difference. There is no registry, no tradeable credit, no buffer pool. If the carbon is lost in a future year, it is reported as a positive emission in that year.

The same tonne of soil carbon can appear in both — as a credit sold through Verra, and as a removal in the buyer's GHG inventory. But the numbers are calculated differently, and the permanence obligations are different. This tool shows all three side by side so you can see the gap.