Warming the World’s Croplands: Coupled Carbon–Nitrogen Cycling, Management Interactions, and the Durability of Soil Carbon Sequestration

Croplands hold about 83 Pg of organic carbon (C) in their topsoil, yet most evidence on how soil C responds to warming comes from nitrogen (N)-limited forests, grasslands, and tundra. Decades of fertilization have given cropland soils a C–N balance with no close natural equivalent. Drawing on cropland field-warming experiments and quantitative syntheses, we argue that soil C sequestration under warming can only be judged when N is part of the accounting, and that management, not climate alone, decides the outcome. In a global synthesis of mostly East Asian experiments (mean warming about 2 °C), total soil respiration rose by about 15% and nitrous oxide (N2O) emissions by 13%; respiration includes roots, so this is not net C loss. Mineralization depends on the pool exposed, its physical protection, and substrate quality, whereas N losses as N2O or leached nitrate follow soil moisture closely. In N-limited soils, warming drives microbial N mining and C loss. Chronic fertilization may weaken this response in croplands, but no cropland warming experiment has tested this directly. Because the N2O penalty can offset or exceed the C gained, cropland soils are a finite, uneven, and reversible climate sink, best assessed by measuring C and N together.