How Biochar Feeding Could Turn Cattle Manure into a Carbon Sink

Researchers have found that most biochar included in dairy cow feed travels through the animals’ digestive systems and is present again in manure, while retaining much of its original carbon structure.

This durability may allow a major agricultural emissions source to become a carbon sink, transferring carbon dioxide from the atmosphere into soils on working farms.

Biochar survives digestion intact

Dark particles extracted from dairy cow manure offered the strongest evidence that the feed additive withstood digestion.

Iva Lucill Walz of Agroscope tracked these particles throughout feeding periods and found that most of the added biochar was excreted with little alteration.

Recovery rates repeatedly ranged from 70 to 90 percent, while the recovered material kept the compact carbon structure associated with long-term persistence.

Establishing that biochar can survive digestion was just the initial stage. Whether this method can substantially cut agricultural emissions depends on how much biochar remains and how stable it is after passing through the animal.

Biochar recovered in manure

Laboratory analysis found that biochar – a carbon-rich substance made by heating biomass without oxygen – mostly moved through the cows and was retrieved from their manure.

During the feeding trials, researchers recovered 70 to 90 percent of the biochar after the cows had digested their feed.

“Our research shows that feeding biochar to cattle can create a cascading environmental benefit,” wrote the researchers.

Digestion removed the less robust components, leaving a harder residue that is likely to last longer after manure is applied to fields.

Stable carbon stays in soil

The carbon contained in biochar is useful only when it remains stored after manure is spread across farmland.

Soil microbes normally break down leftover plant matter, returning its carbon to the atmosphere as carbon dioxide in months or years.

Biochar’s structure withstands this breakdown, allowing carbon sequestration and keeping carbon out of the atmosphere for decades.

Persistent storage still relies on soil conditions and manure-management practices, meaning stable particles require careful handling on farms.

Biochar improves water retention

When manure is spread, the surviving biochar is returned to fields, where it becomes incorporated into soil around plant roots.

The porous particles can retain water and nutrients, which may help soils remain productive during dry periods.

A global meta-analysis associated biochar additions with greater soil water retention in dozens of experiments across varied conditions.

Better retention of water and nutrients may change manure odours and the movement of dissolved nutrients through soil, with implications for crop performance and water quality downstream.

Emissions from cattle digestion

Farm emissions of methane and nitrous oxide – a greenhouse gas produced as nitrogen breaks down – continue to increase, particularly from cattle digestion and manure.

Microbes in a cow’s first stomach ferment fibre, producing methane as a waste gas during digestion.

An international climate assessment identifies ruminant digestion as a major global source of agricultural methane.

Biochar feeding trials have produced mixed results for methane, while one cattle trial reported only small and inconsistent reductions.

Measuring biochar in cattle manure

A more difficult issue underpinned the findings: farmers and researchers need dependable ways to quantify biochar in manure.

The researchers assessed three laboratory methods and found that all could estimate biochar concentrations in dung to within plus or minus one percent.

Dichromate oxidation, a chemical treatment that removes readily degradable carbon, produced the most precise results across samples.

Reliable measurement is essential because carbon schemes require evidence; inaccurate figures could reduce climate claims to speculation.

Production quality affects durability

Biochar does not always perform in the same way, as both its feedstock and production conditions determine the material placed in the bag.

In pyrolysis, plant material is heated with very little oxygen. Depending on the process, producers can make either a dense, stable char or a weaker material with lower durability.

The team’s trial used feed-grade biochar produced from wood chips, ensuring that digestion began with a material intended to be durable.

That initial standard makes biochar selection crucial, as lower-quality material may fail to deliver results or could even create problems.

Animal health still matters

Farmers are unlikely to put biochar in feed unless it benefits animal health and maintains the safety of milk or meat.

A wide-ranging review reported mainly positive indications, although many trials found no distinct effect on performance.

In rare cases, biochar seemed to hinder absorption of fat-soluble nutrients including vitamin E, prompting concern about feeding it over long periods.

Clean manufacturing is also vital for safety, because contaminated char might introduce heavy metals or other toxins into the ration.

Future research directions

Long-term field studies will determine whether digested biochar genuinely remains in soil after exposure to weather, ploughing and microbes.

Soils vary in whether they move particles deeper, wash them away or bury them, so carbon monitoring must continue for years.

Credit schemes also need farmers to record inputs and outputs, making laboratory techniques relevant to practical on-farm verification.

Without this proof, biochar feeding may remain a specialist practice, despite the promise currently suggested by its chemistry.

Feeding biochar to cattle connects two systems that are generally treated separately: animal diets and soil carbon storage through manure.

Future studies need to examine different biochars under real field conditions, while monitoring animal outcomes and greenhouse gases simultaneously.