Net greenhouse gas balance in U.S. croplands: How can soils be a 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
(N2O) 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 (DLEM) for regional simulations and used
field observations of SOC sequestration rates and N2O and CH4 emissions to
calibrate, validate, and corroborate model simulations. Results show that
U.S. agricultural soils sequestered 13.2±1.16 Tg CO2-C yr-1 in SOC (at a
depth of 3.5 m) during 1960-2018 and emitted 0.39±0.02  Tg N2O-N
yr-1 and 0.21±0.01  Tg CH4-C yr-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±11.46  Tg CO2-eq yr-1, thus contributing to climate
warming. 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. Our study highlights the importance of concurrently quantifying
SOC-sequestered CO2 and non-CO2 GHG emissions for developing effective
agricultural climate change mitigation measures.