<p>Humans have increased atmospheric carbon dioxide (CO₂), causing major changes in global climate while concurrently increasing the supply of biologically limiting nutrients especially nitrogen (N). Despite myriad negative effects on ecosystems and human health, nutrient pollution can increase the storage of soil carbon (C) in grasslands and retired farmland, potentially reducing atmospheric CO₂. However, the persistence of nutrient-induced soil carbon gains remains a knowledge gap at the heart of a potential policy dilemma: whether reducing nutrient pollution could lead to the release of soil carbon that accumulated under high nutrient supply. Here we use a four-decade experiment conducted on retired, marginal cropland to demonstrate that nutrient addition increased soil C storage after intensive tilling, and that these soil C gains persisted for at least three decades following fertilizer and tilling cessation. This occurred despite plant biomass rapidly returning to pre-fertilization levels and plant community composition recovering from the effects of fertilization. These results demonstrate that nutrient-induced increases in soil C can persist for decades following reduced nutrient pollution as long as the soil remains untilled.</p>