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

Authors: YOU, YONGFA - Auburn University; TIAN, HANQIN - Boston College; PAN, SHUFEN - Auburn University; SHI, HAO - Chinese Academy Of Sciences; LU, CHAOQUN - Iowa State University; BATCHELOR, WILLIAM - Auburn University; CHENG, BO - Auburn University; HUI, DAFENG - Tennessee State University; KICKLIGHTER, DAVID - Woods Hole Marine Biological Laboratory; LIANG, XIN-ZHONG - University Of Maryland; LI, XIAOYONG - Chinese Academy Of Sciences; MELILLO, JERRY - Woods Hole Marine Biological Laboratory; PAN, MAIQING - Auburn University; Prior, Stephen - Steve; REILLY, JOHN - Massachusetts Institute Of Technology

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2023
Publication Date: 12/29/2023
Citation: You, Y., Tian, H., Pan, S., Shi, H., Lu, C., Batchelor, W.D., Cheng, B., Hui, D., Kicklighter, D., Liang, X., Li, X., Melillo, J., Pan, M., Prior, S.A., Reilly, J. 2023. Net greenhouse gas balance in U.S. croplands: How can soils be part of the climate solution?. Global Change Biology. 30:e17109. https://doi.org/10.1111/gcb.17109.
DOI: https://doi.org/10.1111/gcb.17109

Interpretive Summary: This study quantified variations in net soil GHG balance in U.S. croplands (1960 to 2018) using a model-data integration approach. U.S. croplands were a net carbon sink with an average SOC sequestration rate of 13.2±1.16 Tg C year-1 but a net source of N2O and CH4 with average emission rates of 0.39±0.02 Tg N year-1 and 0.21±0.01 Tg C year-1, respectively. In terms of GWP, the simulated national average net GHG emission rate was 121.9±11.46 Tg CO2-eq yr-1. Non-CO2 GHG (N2O plus CH4) emissions surpassed sequestered SOC. Sequestered SOC offset ~28% of climate-warming effects from non-CO2 GHG emissions and this offset increased over time. Increased nitrogen fertilizer use was the dominant factor contributing to increased net GHG emissions (1960-2018) and explained ~47% of total changes. In contrast, adoption of conservation practices (e.g., reduced tillage) and rising atmospheric CO2 attenuated net GHG. This work highlights the need to consider both SOC sequestration and non-CO2 GHG emissions when examining role of soils in addressing climate change and the great potential of CSA management practices (e.g., reduced tillage and optimized N fertilization) in mitigating net GHG balance. To curb climate change, future work could focus on predicting long-term impacts and mitigation potential of various CSA management practices on net GHG balance and crop production under different climate scenarios, with the ultimate goal of achieving carbon neutrality and sustainable agriculture.

Technical Abstract: Agriculture plays a dual role in regulating the Earth’s climate through releasing or sequestering carbon dioxide (CO2) in soil organic carbon (SOC) and releasing non-CO2 greenhouse gases (GHGs) such as nitrous oxide (N2O) and methane (CH4). While previous studies have focused on individual GHGs or SOC, little is known about trade-offs between SOC-sequestered CO2 and non-CO2 GHG (N2O plus CH4) emissions. Herein, we used a model-data integration approach to quantify the magnitude and spatiotemporal variations of the net soil GHG balance (i.e., sum of SOC-sequestered CO2 and non-CO2 GHG emissions) in U.S. croplands driven by multiple environmental changes during 1960-2018. Specifically, we used the Dynamic Land Ecosystem Model to perform 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. croplands sequestered 13.2±1.16 Tg CO2-C yr-1 in SOC 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 121.9±11.46 Tg CO2-eq yr-1, and therefore contributing climate warming. Non-CO2 GHG emissions surpassed SOC-sequestered CO2 in U.S. croplands during the study period. The sequestered SOC offset ~28% of the climate-warming effects resulting from non-CO2 GHG emissions, and the proportion of 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, and explained ~47% of total changes. In contrast, adoption of agricultural conservation practices (e.g., reduced tillage intensity) and rising atmospheric CO2 attenuated net GHG emissions from U.S. croplands. Our study highlights the importance of simultaneous quantification of SOC-sequestered CO2 and non-CO2 GHG emissions for developing effective agricultural climate change mitigation measures and assessing regional climate impacts.