When farmland is returned to the wild, it shows a striking ability to hold onto the carbon that built up in farm soils over time, retaining this stock for 30 years or more, a unique, long-term study shows.
The new finding challenges a long-held view that farmlands release large amounts of CO2 when they are rewilded. At issue are two opposing features of nitrogen fertilizer. On one hand, nitrogen fertilizer drives pollution and can increase emissions from the soil. On the other hand, by nourishing plant growth it also increases biomass, which in turn captures more carbon that is stored in the ground.
So if nutrient flows suddenly stop, the thinking goes, it’s possible that biomass will decline and release the soil’s stored carbon into the atmosphere—making retired farms a carbon source, not a sink.
To investigate, the researchers turned to a tract of Minnesota land that for the past 42 years has been retired from farming, and is used as a base for experiments. For theirs, the researchers tracked soil carbon and nitrogen levels across two different types of plots on the reclaimed land: one set that had been continuously fertilized over the 42 years, and a second set that was fertilized for the first 10 years and then left alone for the remaining three decades.
The team found that the plots with continued fertilizer application had higher quantities of soil carbon compared to the plots that were fertilized for just the first ten years. This suggests that continued application of nitrogen can elevate carbon levels in the soil.
But the most interesting discovery was this: on the plots that received fertilizers for the first ten years only, the amount of carbon and nitrogen locked in the soil over that period stayed stable, remaining at elevated levels over the following 32 years, the team found. They think this may be because the initial injection of fertilizer encourages more plant growth, which infuses the soil with carbon-rich biomass that is in turn stabilized by the increased plant cover, locking it in for decades.
The takeaway? Provided it remains untilled, retired farmland soils can hold onto the carbon benefit of applied fertilizers for decades, and will become a persistent carbon sink—not a source.
These experiments took place in sandy soil, and it’s possible that results will differ across soil types. Ongoing research in these environments is urgently needed, considering growing pressure to recultivate retired farmlands globally, the researchers say.
By going with the assumption that retired farmlands emit more than they sequester—which this study turns on its head—we risk unlocking a potentially huge carbon store by disturbing these rare, retired lands.
Borer et. al. “Multidecadal persistence of soil carbon gains on retired cropland following fertilizer cessation.” Nature Geoscience. 2025.
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