The 180-year-long Broadbalk Classical Experiment at Rothamsted -- the world's longest-running agricultural trial -- has shown that long-term use of nitrogen and phosphorus fertilisers can substantially increase soil carbon storage, offering a potential tool to help mitigate climate change.
An international team led by Zhejiang University, in collaboration with Rothamsted and the University of Bangor, analysed samples from plots continuously growing winter wheat since 1843. They found that soils receiving nitrogen and phosphorus contained up to 28% more organic carbon than unfertilised plots.
Published in Nature Geoscience, the study used radiocarbon tracing, metagenomics, and metabolomics to uncover how fertilisers affect the soil's chemical composition and microbial communities, shedding new light on the mechanisms controlling carbon storage, according to a press release.
"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 study revealed that nitrogen and phosphorus influence soil carbon in different ways. Phosphorus alone stimulated microbial activity and respiration, releasing carbon, so while microbial biomass increased, only a small portion became stable, long-lasting carbon. Nitrogen, on the other hand, enhanced microbes' ability to convert plant material into persistent, "mineral-associated" carbon.
When applied together, nitrogen and phosphorus had the greatest impact: they boosted plant growth, helped transform short-lived "labile" carbon into stable forms, and increased both the amount and longevity of carbon stored in the soil.
A global meta-analysis of long-term fertilisation trials confirmed similar trends. Worldwide, nitrogen and phosphorus fertilisation increased soil carbon by an average of 21% and 13%, respectively. Gains tended to be modest in the first decades but strengthened after roughly 30 years, indicating that carbon accumulation under fertilisation is a slow, long-term process.
The authors highlight that these findings emphasize the value of long-term research and careful nutrient management in developing farming systems that support climate mitigation.