A review of 422 scientific studies by Rothamsted Research has found that conservation tillage is not a uniform climate win: reduced and zero tillage consistently cut greenhouse gas emissions, but mulching can increase them in some farming systems.

The analysis assessed how the four core principles of conservation agriculture — reduced or no tillage, mulching, crop rotations and appropriate fertiliser use — influence emissions of the three major greenhouse gases: carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4).

The verdict is nuanced. Which principles a farmer adopts matters, but so does how they are implemented and the environment — rainfall and soil type — in which they are used.

What did reduced tillage achieve?

Reduced and zero tillage showed the most consistent potential for lowering greenhouse gas emissions of any practice examined.

The review found strong evidence that reduced tillage lowers CO2 emissions, and it can also reduce N2O. There is some evidence it enhances soil uptake of methane.

The mechanism is straightforward: less soil disturbance protects soil structure and slows the release of carbon stored in soils.

Where does mulching backfire?

Mulching — retention of crop residues and use of cover crops — delivered its well-established benefits for soil protection, water retention and soil health.

But the review found evidence it can increase greenhouse gas emissions in some systems. Decomposing organic materials create conditions that stimulate microbial activity, raising production of CO2, N2O and methane.

The researchers say mulch type, quantity and management need careful consideration when designing climate-smart systems.

Why did fertiliser results vary?

The study evaluated organic amendments, integrated soil fertility management, slow-release fertilisers and nitrogen-use efficiency approaches. Results were highly variable — some strategies cut emissions, others raised them, depending on environmental conditions.

Dr. Grace Kangara, lead author and researcher at Rothamsted Research, said: "Approaches promoting the appropriate use of fertilizer, as the fourth proposed principle of conservation agriculture, showed mixed effects on greenhouse gas emissions."

She added: "This shows that environmental factors like rainfall and soil type create complex challenges for reducing emissions in temperate and tropical farming systems."

One driver stood out: nitrogen application rate remains a key determinant of N2O emissions, reinforcing the case for precision nutrient management.

Rotations mattered too. Systems including legume crops tended to show lower CO2 emissions than continuous cereal-based rotations, while cereal-after-cereal sequences were more often linked to increased emissions. The findings add to growing evidence that diversified cropping supports sustainability while reducing reliance on synthetic fertiliser.

What are the knowledge gaps?

The study identified substantial gaps in global research, particularly in Africa, where relatively few long-term studies have measured greenhouse gas emissions under conservation agriculture.

The authors called for greater investment in long-term field experiments and more standardised reporting of greenhouse gas measurements to improve comparability of future research.

Rather than treating conservation agriculture as a single climate solution, they argue individual practices must be evaluated separately and in combination, with local conditions factored in.

The researchers describe the findings as "important evidence" for farmers, researchers and policymakers developing climate-smart systems as agriculture faces the dual challenge of feeding a growing population while cutting emissions. Farmers adopting conservation tillage for its carbon benefits should next scrutinise their residue and nitrogen management, where the emissions balance is most likely to tip the wrong way.