Net greenhouse gas balance in U.S. croplands: How can soils be part of the climate solution?

Agricultural soils play a dual role in regulating the Earth's climate by releasing or sequestering carbon dioxide (CO₂ ) in soil organic carbon (SOC) and emitting non-CO₂ greenhouse gases (GHGs) such as nitrous oxide (N₂ O) and methane (CH₄ ). To understand how agricultural soils can play a role in climate solutions requires a comprehensive assessment of net soil GHG balance (i.e., sum of SOC-sequestered CO₂ and non-CO₂ GHG emissions) and the underlying controls. Herein, we used a model-data integration approach to understand and quantify how natural and anthropogenic factors have affected the magnitude and spatiotemporal variations of the net soil GHG balance in U.S. croplands during 1960-2018. Specifically, we used the dynamic land ecosystem model for regional simulations and used field observations of SOC sequestration rates and N₂ O and CH₄ emissions to calibrate, validate, and corroborate model simulations. Results show that U.S. agricultural soils sequestered $13.2\pm 1.16$ Tg CO₂ -C year^-1 in SOC (at a depth of 3.5 m) during 1960-2018 and emitted $0.39\pm 0.02$ Tg N₂ O-N year^-1 and $0.21\pm 0.01$ Tg CH₄ -C year^-1 , respectively. Based on the GWP100 metric (global warming potential on a 100-year time horizon), the estimated national net GHG emission rate from agricultural soils was $122.3\pm 11.46$ Tg CO₂ -eq year^-1 , with the largest contribution from N₂ O emissions. The sequestered SOC offset ~28% of the climate-warming effects resulting from non-CO₂ GHG emissions, and this offsetting effect increased over time. Increased nitrogen fertilizer use was the dominant factor contributing to the increase in net GHG emissions during 1960-2018, explaining ~47% of total changes. In contrast, reduced cropland area, the adoption of agricultural conservation practices (e.g., reduced tillage), and rising atmospheric CO₂ levels attenuated net GHG emissions from U.S. croplands. Improving management practices to mitigate N₂ O emissions represents the biggest opportunity for achieving net-zero emissions in U.S. croplands. Our study highlights the importance of concurrently quantifying SOC-sequestered CO₂ and non-CO₂ GHG emissions for developing effective agricultural climate change mitigation measures.