Fertilizer boosts soil’s ability to lock away carbon

The 180-year experiment at Rothamsted — the world’s longest-running agricultural trial — has revealed that long-term application of nitrogen and phosphorus fertilisers can significantly increase the amount of carbon stored in farmland soils, helping to mitigate climate change.

An international team of scientists led by Zhejiang University, China, together with partners at Rothamsted and the University of Bangor, analysed samples from the Broadbalk Classical Experiment, which has been growing winter wheat continuously since 1843, and found that plots receiving nitrogen and phosphorus fertilisers contained up to 28% more soil organic carbon than those left unfertilised.

The study, published in Nature Geoscience, combined radiocarbon tracing, metagenomics and metabolomics to offer new insights into how fertilisers influence the complex chemistry and microbial life that control carbon storage in soil.

“Soil organic carbon is critical for climate regulation sustainable food production and soil health, but its decline in many agricultural soils has been deeply concerning,” said Dr Andy Gregory, one of the co-authors of the study. “Our findings show that long-term mineral fertilisation can actually enhance soil carbon sequestration, provided it’s managed carefully to minimise other unwanted impacts.”

The research found that nitrogen and phosphorus acted in distinct ways. Phosphorus alone boosted microbial activity and respiration — processes that release carbon — meaning that although microbial biomass increased, relatively little of it was converted into stable, long-lived forms of carbon. Nitrogen fertilisation, by contrast, improved the efficiency with which microbes transformed plant material into more persistent “mineral-associated” carbon.

When applied together, nitrogen and phosphorus fertilisers produced the strongest effect: enhancing plant growth, promoting the conversion of short-lived “labile” carbon into more stable forms, and increasing both the quantity and durability of carbon stored in the soil.

A global meta-analysis by the team found similar patterns elsewhere. Across dozens of long-term fertilisation trials worldwide, nitrogen and phosphorus were associated with average soil carbon increases of 21% and 13%, respectively. The benefits appeared to fade in the first decades of use, before strengthening again after about 30 years — suggesting that soil carbon gains from fertilisation build slowly over time.

The findings, the authors say, underscore the importance of long-term research and careful nutrient management in designing climate-friendly farming systems.