Captain Drawdown’s daily logbook on every CDR story, paper, and expert voice — so you don’t have to read them all.
Farm lime in the Mississippi River Basin removes carbon on net, at about 90% of its ideal CO2 removal potential, and the lime spread there since 1900 adds up to roughly 0.44 GtCO2 (billion tonnes of CO2) of removal. That is the headline of Agricultural liming is a carbon sink in the Mississippi River Basin, a Nature paper from Tim Jesper Suhrhoff, Christopher Reinhard, Yoshiki Kanzaki and colleagues. Most emissions inventories book that same lime as a CO2 source. One of them is wrong.
How a routine farm input became the hardest question in weathering
Liming is older than carbon removal as a field. Farmers spread crushed limestone to counter soil acidity, and inventory conventions have long treated the carbonate in that limestone as carbon that dissolves and leaves as CO2. Meanwhile the enhanced rock weathering (ERW) industry, which spreads crushed rock on fields to pull CO2 into dissolved bicarbonate, built its crediting around silicate rocks like basalt and sidelined carbonate chemistry. The question nobody closed was downstream: once alkalinity leaves a field, how much of it reaches rivers and the ocean, and how much is lost on the way? Suhrhoff and his co-authors took that question off single plots and asked it for an entire river basin.
The paper’s core move is about blame, not arithmetic
The method is not a field trial. The team combined century-scale historical records of liming and of acidity inputs to Mississippi Basin soils, records of river alkalinity flux, and reactive transport modelling of how dissolved ions move through soil and water. Out of that comes a basin-scale carbon ledger for lime going back to 1900.
The central argument is a counterfactual. When lime dissolves in acidic soil, some CO2 is released. Current accounting pins that release on the lime. The paper argues it belongs to the acidity inputs that drove it, mostly fertilizer-derived acid, because the acid would have consumed carbonate or exported CO2 from the soil system regardless of whether a farmer added lime. Strip out the CO2 that acidity causes, and what lime itself does at basin scale is remove carbon. So the sign flips not because the chemistry changed, but because the paper assigns each flux to the input that actually caused it.
Two nuances are load-bearing. First, there is an initial emissions pulse: neutralizing the soil’s existing acidity pools releases CO2 before net removal emerges. Second, the removal lags the application by decades, because soil cation exchange and solute transport slow the alkalinity’s trip to the river. A lime ledger read over one or two seasons will look like a source. The same ledger read over the century looks like a sink.
The lens: carbon fate is a basin property, not a field property
This is the critique Suhrhoff brings to CDR: the sign of a weathering flux cannot be read at the field edge. It depends on what happens in soils and rivers downstream, which is exactly the part of measurement, reporting and verification (MRV) that ERW registries are still building, as I laid out in What Is Enhanced Weathering? A Primer. That makes his work the awkward cousin of the permanence debates in other pathways. Biochar is moving toward lab assays of the material itself, from Rock-Eval pyrolysis for carbon stability to bomb radiocarbon tracing of biochar mineralization in soil. Weathering has no equivalent of a sample you can put in a furnace. The carbon leaves. You have to follow it, and the only honest unit of analysis is the catchment.
The paper also sits inside a research community that is starting to treat rivers as part of the carbon ledger. Shannon Sterling, the Dalhousie researcher and CarbonRun co-founder who works on river alkalinity, was named a Fall 2026 Visiting Scholar at the Yale Center for Natural Carbon Capture. Rivers are no longer the afterthought of weathering MRV. Lime is the first test case.
The bet: carbonate accounting is miscalibrated, at least here
Suhrhoff’s wager is that the emission-versus-removal boundary that inventories and credit protocols rely on has been drawn in the wrong place for carbonate. The paper is explicit that the result is for the Mississippi River Basin and should not be read as a global rule. What it does not claim matters too: it does not predict how lime behaves in basins with different fertilizer-acidity regimes, and it does not establish that lime should be credited as CDR anywhere else. The data cannot yet settle either question. What it can settle is that a century of lime in one of the largest agricultural basins on Earth was, on this reconstruction, a removal and not a source.
Why this voice matters now
ERW developers have been pricing silicates and ignoring carbonates on the assumption that carbonate dissolution is an emission. If Suhrhoff is right about attribution, that assumption is an accounting convention, not a chemical fact. For registries this is uncomfortable: the paper’s logic says you cannot credit a weathering project without also modelling the acidity it is reacting against and the decades it takes for alkalinity to reach the river.
Watch for three responses. Inventory compilers will have to say whether they accept counterfactual attribution to acidity inputs. Weathering researchers should test the basin-scale method on catchments with different fertilizer histories, which is the one experiment that would show whether the Mississippi result travels. And ERW registries should state plainly whether their protocols could credit a removal that only becomes visible decades after application. If the answer is no, they are not measuring the system the chemistry runs on.
Citations
- DOI-resolved paper — Agricultural liming is a carbon sink in the Mississippi River Basin
- Nature — Rock-Eval pyrolysis for carbon stability — research paper
- DOI-resolved paper — bomb radiocarbon tracing of biochar mineralization in soil
- LinkedIn — named a Fall 2026 Visiting Scholar at the Yale Center for Natural Carbon Capture — LinkedIn post