Global Potential to Increase Soil Carbon Storage by Reducing Rotational Fallow in Semiarid Regions

Intensification of cropping systems improves crop productivity and soil organic carbon (SOC) storage by maintaining or enhancing existing SOC stocks. We compiled published data on SOC changes in agricultural soils globally from experiments evaluating the impact of bare-fallow reduction to determine the change in SOC storage that results from management change. Overall, the intensification of cropping systems by eliminating fallow led to an average increase in SOC stocks that were 3.2 (±0.3) Mg C ha^-1 than in cropping systems with bare-fallow. To account for variation in fallow frequency among study sites, we estimated the SOC change on a per year of fallow reduction basis and found the difference in SOC stocks was 443 (±34) kg C ha^-1 for each year of fallow reduction. Soil texture influenced the amount of SOC change, with average differences of 552 (±85), 406 (±38), and 430 (±92) kg C ha^-1 yr^-1 for fallow reduction in fine-, medium-, and coarse-textured soils, respectively. The rate of SOC storage declined over time with SOC increases of 615 (±74), 433 (±45), and 360 (±60) kg C ha^-1 associated with fallow reduction for < 10 years, 11 to 20 years, and > 21 years, respectively. Soil type and aridity index had an impact on SOC storage when comparing crop systems with and without fallow. Countries with significant amounts of bare fallow could promote intensification of cropping systems by reducing bare fallow as part of their nationally determined contributions to the Paris Agreement. Canada has reduced the annual area of fallow from 1990 to 2022, resulting in a cumulative gain of SOC storage of 66.3 Mt C. Based on global statistics of annual fallow area, a significant reduction in this practice is feasible on a global scale with cumulative changes in SOC storage of 0.54 Gt C for a period of 20 years.