How Much Soil Carbon Can a Hectare Gain Per Year? A Benchmark for Removal Rates

A number arrives in an email: this project sequesters 1 tonne of CO₂ equivalent per hectare per year. Or 0.2. Or 5. The sender wants to know whether that is good. Usually the number itself is sound, and what it lacks is context. Soil carbon rates are small changes to a very large stock. The same absolute figure can therefore be an excellent result on one field and an ordinary one on the next.

Five pieces of context make a rate interpretable. They are the unit, the baseline stock, the sampling depth, the control, and the time since the practice started. With those attached, the number becomes checkable. This article supplies the benchmark, so that a rate can be compared with what the literature reports instead of with intuition.

The evidence is encouraging. These practices do build soil carbon, and the ranges below are gains measured in field experiments, some of them running for more than a century. The gains are smaller and slower than the figures in general circulation, and they need a precise description before they mean anything. The effect itself is well established. The co-benefits for soil function are larger and more certain than the carbon.

This article is the soil companion to the cross-system benchmark for tree carbon. The two are misread for different reasons. A tree rate is misread mostly because of stand age and density. A soil rate is misread mostly because of the stock, the depth and the comparison. The method is the same for both: find the envelope first, then ask where the number falls within it, and why.

A rate only means something as a fraction of a stock

Soil is the largest terrestrial carbon pool. Globally, the top meter contains between 1,500 and 2,400 gigatonnes of carbon, and agriculture has already released about 140 to 150 gigatonnes of it2. A spatial reconstruction of twelve thousand years of land use puts this soil carbon debt at 116 gigatonnes of carbon over the top two meters. Cropland and grazing land account for roughly equal shares, and the loss rate rose sharply over the last two centuries21. That is the size of the deficit. It also explains why the annual numbers are so small: a field must refill a very large container with a very thin stream.

At field scale, a cultivated topsoil typically contains between 30 and 120 tonnes of carbon per hectare to 30 cm. A gain of one tonne of CO₂ per hectare per year is 0.27 tonnes of carbon. Against a stock of 55 tonnes, that is about 5 parts per thousand. Against a stock of 120 tonnes, it is about 2 parts per thousand. The measurement and the effort are identical, yet the relative gain differs by a factor of two, only because of the starting stock.

The same numbers look different on a laboratory report. A hectare of topsoil to 30 cm, at a bulk density of 1.3 g/cm³, weighs about 3,900 tonnes. A stock of 55 tonnes of carbon in that soil corresponds to a soil test reading of 1.41 percent carbon. A good year adds 1 tonne of CO₂, and the reading becomes 1.42 percent. A successful year therefore changes the second decimal place of a soil test. After a decade of the practice, the reading moves from 1.41 to 1.48 percent. That change is finally large enough to see against the variability of a field.

Most of the difficulty in this subject follows from that one fact. It makes the measurement expensive. It is the reason careful researchers reach different conclusions from the same trials. It also means that a rate quoted after two seasons is a projection, not an observation. The same fact explains why the practices are worth doing anyway. The agronomic effects of the extra organic matter appear long before the carbon accounting can resolve it.

Units are the first source of error. Carbon and carbon dioxide differ by a factor of 3.67, so “one tonne” without a unit is ambiguous by nearly fourfold. Soil science usually reports tonnes of carbon. Carbon crediting standards and corporate greenhouse gas inventories report tonnes of CO₂ equivalent. The tool below converts between the two. It expresses the rate as a fraction of the stock you enter. It then places the rate on a scale from 0 to 25 per mille, with the “4 per 1000” aspiration marked.

Four per mille is the yardstick, and reaching it takes about double the carbon input

The most useful single reference point is the “4 per 1000” target launched at COP21. Its aspiration is to increase soil organic carbon stocks by 0.4 percent a year. As a global accounting device it has drawn criticism. As a yardstick for a single field it is excellent, because it is a fraction and not an absolute amount. A survey across twenty regions concluded that best management practice can reach 4 per mille and beyond. Rates up to 10 per mille are achievable where the topsoil starts with less than 30 tonnes of carbon per hectare. These high rates occur mostly in the first twenty years after a change in practice1. Soils already at equilibrium cannot add more.

What the target demands in practice is usually left out. Sixteen long-term experiments at Rothamsted, with 114 treatment comparisons over 7 to 157 years, met or exceeded 4 per mille in most comparisons. Most of those successes came from treatments that no working farm would sustain. Farmyard manure applied every year at 35 tonnes of fresh material per hectare supplies about 3.2 tonnes of organic carbon. In the first two decades, it produced increases of 18 and 43 per mille a year. Lower or intermittent applications gave 3 to 8 per mille. The authors concluded that achieving the goal across ordinary agriculture faces severe limitations. The reasons are practical, not biological. There is not enough manure, the easy practices are already adopted, and the effective ones are uneconomic or compete with food production14.

Modeling the question from the input side gives a consistent answer. One study ran an ensemble of six soil carbon models across sixteen European long-term experiments. Raising stocks by 4 per mille a year for thirty years required an extra 1.5 tonnes of carbon input per hectare per year. That is about 119 percent more than the control15. In round terms, reaching the target means doubling what the soil receives. Any claimed rate therefore needs a plausible account of where the extra biomass came from.

Most practices land between 0.1 and 0.8 tonnes of carbon per hectare per year

With that yardstick in place, the practice-level numbers stop looking arbitrary. The table below gathers the ranges reported by the syntheses cited in this article, in both units. Every figure is a topsoil rate relative to a stated control, not a project figure. The spread within each row is usually wider than the difference between rows.

The clearest single source for European cropland is a review of more than 700 records across twelve carbon-farming practices. Its topsoil rates, in tonnes of carbon per hectare per year, are 0.40 for cover crops and 0.52 for organic amendments. Residue retention gives 0.14 and improved rotations 0.21. Reduced soil disturbance gives 0.24, silvoarable systems 0.21, and conversion of cropland to permanent grassland 0.796. Grasslands have their own synthesis. Improved grazing, fertilization, sown legumes, irrigation and conversion from cultivation all raise soil carbon. The rates range from 0.105 to somewhat over 1 tonne of carbon per hectare per year7. At global scale, one estimate puts the achievable grassland potential at 2.3 to 7.3 gigatonnes of CO₂ equivalent a year for biodiversity restoration. Improved grazing management offers only 148 to 699 megatonnes8.

Two entries in the table need a caveat from the start. Temperate agroforestry averages 0.21 tonnes of carbon per hectare per year in the topsoil across 61 observations. The standard deviation is 0.79, so the variation between studies is nearly four times the mean, and silvopastoral systems showed a slight loss9. Organic farming averages 0.45 tonnes of carbon per hectare per year across 74 studies. Restricted to zero-net-input systems with measured bulk densities, the difference falls to 0.07 and is no longer statistically significant10. For both practices the headline number is real, but it does not mean what it appears to mean.

A worked example: the cover-crop disagreement

Cover crops are the most instructive example in the field, because the disagreement is public, recent and entirely about method. The widely cited meta-analysis compiled 139 plots at 37 sites, over periods of up to 54 years. It found soil carbon rising by 0.32 plus or minus 0.08 tonnes of carbon per hectare per year, at a mean depth of 22 cm3. For a decade, policy documents and project designs have used that figure.

A 2023 reanalysis screened the same body of field studies. All 37 had sampled to 30 cm or less, and none had compared treatments on an equivalent soil mass basis. The authors then excluded trials that reported concentrations instead of stocks, used inappropriate controls, ran three years or less, or grew cover crops as cash crops. Six trials remained. Four showed non-significant trends, one was negative and one was positive, for a mean of 0.03 tonnes of carbon per hectare per year4. The same literature produced a figure ten times smaller than the accepted one.

The original authors published a rebuttal. They argued that the screening was too severe and discarded valid evidence5. The defensible position today is that the true value lies somewhere in that range and depends heavily on context. For anyone reading a project document, the useful lesson is the mechanism of the disagreement. Neither side fabricated anything. The gap came entirely from sampling depth, stock accounting method and choice of control. Those three choices matter more than the practice itself.

Depth, soil mass and control explain most of the disagreement between papers

Sampling depth is the largest lever. Around 55 percent of the carbon in the top meter lies below 30 cm13. Practices that concentrate residues at the surface move carbon upward as much as they add it. A systematic review of 101 long-term tillage trials found the benefit of reduced disturbance confined to the top 30 cm. The European review found that, over 0 to 50 cm, the topsoil gain from reduced disturbance cancelled out entirely, leaving no net climate benefit166. A shallow sample does not measure a smaller effect. It measures a different quantity.

The second lever is the stock calculation. Management changes bulk density, so a fixed-depth sample compares different masses of soil before and after. Correcting to an equivalent soil mass removes that artifact, and the correction is substantial. Ignoring it overstated the gain from reduced tillage by 15 to 47 percent, depending on the comparison16. The bias does not always run in the same direction. In tropical land-use change, the uncorrected calculation understated the effect by 28 percent17. Fixed-depth accounting is therefore not a conservative choice. It is wrong in a direction that only the correction itself can reveal.

The third lever is the control, and it is the one most often chosen for convenience. A practice compared with bare fallow looks transformative. The same practice compared with ordinary local management looks modest. Compared with the grassland that preceded it, it may look like a loss. The European review shows this clearly. Set-aside scores 0.75 tonnes of carbon per hectare per year against a cropland control and −0.39 against a grassland one. Poplar plantations score 0.25 on cropland and −0.85 on grassland6. The comparison, not the practice, determines the sign of the answer.

The tool below keeps a single hypothetical field constant and varies only those three choices. Nothing about the soil changes. The reported rate moves by more than a factor of ten, and it can change sign.

A gain below the detection limit is a design problem, not a small result

A fourth measurement question lies behind the other three, and it is the one most often skipped. Could this design have detected the change at all? Every monitoring campaign has a detection limit: the smallest difference it can separate from sampling noise. Three things determine that limit: the variability of the field, the number of samples and the time between campaigns. The practice itself plays no part. Soil carbon moves slowly against a large and noisy background. Ten-year resampling intervals are therefore the usual compromise, and anything within three to five years is considered challenging2.

Consider a field with a within-field standard deviation of 11 tonnes of carbon per hectare, sampled at thirty points. That design cannot resolve any change smaller than about 8 tonnes per hectare. A practice that delivers 1 tonne of CO₂ per hectare per year adds 2.7 tonnes of carbon in a decade. At that sampling intensity, the gain stays invisible for roughly thirty years. The 0.2 rate from the top of this article fares worse. It stays below the detection limit even with three hundred samples over thirty years.

A low number therefore needs careful reading. A quoted rate of 0.2 tonnes of CO₂ per hectare per year is not simply a modest result. On most fields it falls inside the noise band of an ordinary design. A campaign that reports it as a measured gain has probably not measured anything. The accurate description is not “a small increase.” It is “no detectable change, and here is the smallest change the design could have detected.” The first wording invites a claim. The second invites a better design.

Accrual saturates, so the first year is never the thirty-year mean

Soil carbon does not accumulate linearly. When the input to a soil rises, the stock moves toward a new steady state, quickly at first and then ever more slowly. It slows because losses rise as the stock grows. The cover-crop meta-analysis modeled its own data and placed the new equilibrium 155 years out. The total gain at equilibrium is 16.7 tonnes of carbon per hectare3. Almost every quoted annual rate comes from the steep early part of that curve.

Behind the curve lies a hard upper limit. Mineral-associated organic carbon is the fraction that persists. Globally it totals 899 gigatonnes of carbon to one meter. Yet it fills only 42 percent of its mineralogical capacity in surface layers, and 21 percent deeper down. Distance from that capacity predicts how efficiently a soil accrues carbon. Across 103 accrual measurements, soils at a tenth of their capacity gained carbon roughly three times faster than soils at half of it18. Later work argues that carbon input, and not mineral surface, is the binding constraint in most soils. In that work, maximum loadings fall from 160 grams per kilogram in coarse soils to 75 in fine ones19. Either way, the direction is the same. The emptier the soil, the faster it fills, and the fuller it gets, the slower it goes. This capacity question deserves its own article, soil carbon saturation, which explains what determines a soil's upper limit and how close a given field already is to it.

Saturation matters to anyone who reports a removal over thirty years, in a crediting project or in a corporate greenhouse gas inventory. Accounting that ignores saturation overstates the contribution of soil carbon to mitigation by 53 to 81 percent out to 210020. The tool below runs the standard first-order approach to a new steady state. Choose a first-year rate and a response speed, then compare the annual rate with the running mean as the two diverge.

Two outputs matter. The front-loading ratio is the first-year rate divided by the thirty-year mean. It measures the error made by quoting an early measurement as a durable rate. The share of the total gain delivered by year twenty shows how much of the opportunity a twenty-year project captures. In most plausible settings, the early years deliver the result and the later years are close to flat. A single figure repeated across a project lifetime is therefore the wrong shape of claim.

The largest per-hectare numbers are avoided losses, not gains

Everything above concerns new carbon in mineral soil. There, the envelope tops out near one tonne of carbon per hectare per year, and most rates fall well below it. On organic soils the arithmetic changes completely. Croplands on drained lowland peat emit 23.1 plus or minus 10.4 tonnes of CO₂ per hectare per year23. That is about six times the best accrual rate on a neighboring mineral soil. It is also more than twenty times the 1 tonne of CO₂ that counts as a good result. Water table depth overrides every other control on those fluxes. Each 10 cm reduction in effective water table depth cuts the net warming effect by at least 3 tonnes of CO₂ per hectare per year. The effect continues until the water table is within 30 cm of the surface22.

This is why global assessments place protection alongside accrual. Soil carbon accounts for about a quarter of the mitigation potential of natural climate solutions. Forty percent of that quarter comes from protecting existing stocks, not from rebuilding depleted ones. Soil makes up 72 percent of the wetland potential, against 9 percent of the forest potential24. For a project portfolio or a company's sourcing region, the consequence is direct. Organic soils anywhere in the footprint dominate the balance. No amount of cover cropping on the mineral fields will offset a drained peat field left as it is.

Land-use change produces the other large numbers, in both directions. Converting lowland tropical forest to oil palm, rubber or cacao plantations loses up to half the stored soil carbon. The authors therefore proposed revising the IPCC Tier 1 change factor from 1.0 to 0.6 for oil palm and cacao agroforestry33. Conversion of primary forest to cropland loses 25 percent of the stock, and conversion to perennial crops loses 30 percent17. Reversals are slow. In German agricultural soils, cropland converted to grassland took 83 years to reach its new equilibrium. Grassland converted to cropland took 180 years. Fields today still gain or lose around 0.26 tonnes of carbon per hectare per year because of decisions taken a century ago32.

Four deductions stand between a gain and a claim

A soil carbon gain becomes a climate benefit after four deductions. Most well-run projects pass them with a comfortable margin. Making the deductions explicit brings any problem to light early, while the design can still change, and not at audit.

The first deduction is relocation. Organic amendments produce some of the highest recorded rates. Olive orchards that receive mill pomace reach up to 5.3 tonnes of carbon per hectare per year, with apparent sequestration efficiencies above 100 percent12. An efficiency above 100 percent is a warning sign. More carbon appeared than was applied, which usually means that carbon moved from somewhere else and was counted here. Manure and compost that would otherwise go to a neighboring field are a transfer, not a removal, and the Rothamsted authors state this plainly14.

The second deduction is nitrous oxide. Most practices that add carbon also add nitrogen, and N₂O is roughly 273 times more potent than CO₂ per tonne. The resulting emissions are large enough that the greenhouse gas benefit of soil carbon storage cannot be assessed on carbon alone28. Mineral fertilization is the extreme example, with no distinct effect on topsoil carbon and a real emissions penalty11.

The third deduction is reversibility. Soil carbon that took twenty years to accumulate can be released in a season of plowing. Scientific and policy vocabularies diverge sharply on this point. Policy treats soil carbon as uniformly vulnerable, while the science describes a continuum of residence times29. A review of private soil carbon certificates concluded that permanence cannot currently be guaranteed. It also found unresolved problems with additionality, leakage and long-term accountability30. None of this makes soil carbon worthless. It makes a non-permanent removal a different claim from a permanent one. A carbon credit and a corporate inventory should both describe and account for it as such.

The fourth deduction is baseline drift, introduced above. It is the easiest of the four to forget, because it is invisible in the field. A soil still responding to a land-use change made generations ago gains or loses carbon on its own. In the German inventory data, the rate is around 0.26 tonnes per hectare per year32. Measured against zero, that drift is attributed to whoever farms the field now. Measured against a control with the same history, it disappears. That is the whole reason the control matters.

In light of these deductions, the global estimates fall within a narrow band. The technical potential for soil carbon sequestration is estimated at 2 to 5 gigatonnes of CO₂ a year26. Near-complete adoption of best practice gives an upper limit of 4 to 525. A realistic cropland-only figure is 0.28 to 0.43 gigatonnes of carbon a year, once manure availability and adoptable area are taken into account27. Overstating these numbers does its own damage. Inflated estimates give cover for delaying emissions reductions31. The argument for building soil carbon is also strong enough on soil-function grounds that it does not need inflating34.

That last point deserves more weight than it usually gets. Most practices in this benchmark improve water holding capacity, aggregate stability and nutrient supply. Those returns arrive whether or not a single tonne is ever credited or reported. They are also easier to demonstrate. Yield stability and workability improve within a few seasons, while a stock change takes a decade to emerge from its own noise. A program built on agronomic value, with carbon as a co-benefit, copes better with a difficult season than one built the other way around. It is the approach Ekodama usually recommends.

So: is one tonne a lot?

One tonne of CO₂ equivalent per hectare per year is 0.27 tonnes of carbon. On a typical arable topsoil with 55 tonnes of carbon per hectare, that is about 5 per mille. It lies above the 4 per 1000 aspiration and comfortably inside the envelope for cover crops, improved rotations or organic amendments. It is a good result if a whole-profile measurement on an equivalent soil mass, against a like-for-like control, confirms it. The number is plausible, and at the better end of plausible.

A rate of 0.2 tonnes of CO₂ per hectare per year is 0.055 tonnes of carbon, or about 1 per mille of the same stock. The number is real, because many practices deliver it. It is also almost undetectable, because separating it from zero would take decades of careful resampling. The number is not wrong. It lies below the resolution of most monitoring programs, which is a different problem and one worth naming.

A rate of 5 tonnes of CO₂ per hectare per year is 1.36 tonnes of carbon, roughly 25 per mille. That exceeds the mean of every mineral-soil meta-analysis cited here, yet it is achievable. Four situations routinely explain it. The soil started severely depleted, the system received heavy organic amendments from off-site, a land-use change occurred, or the measurement is on an organic soil. Any of these makes the number credible. If none applies, the usual explanations are a unit slip between carbon and CO₂, a shallow sample, or a first-year rate quoted as a durable one.

The checks below condense the article into a portable list. Their purpose is to confirm a number, not to find fault with it, and most of the time they do so quickly. Their main value is to show which of the five pieces of context are still missing.

The deliverable a client should ask for is a bracketed range with its provenance, not a single figure. The provenance names the practice, the baseline stock, the depth, the method, the control, the duration and the deductions already made. That is harder to put on a slide than “one tonne a year,” and it is the version that passes due diligence. For a range built from a model instead of measurements, the companion piece on modeling soil carbon change explains what the models can and cannot support.

Ideas clave