Balancing Non-CO2 GHG Emissions and Soil Carbon Change in U.S. Rice Paddies: A Retrospective Meta-Analysis and Agricultural Modeling Study

U.S. rice paddies, critical for food security, are increasingly contributing to non-CO₂ greenhouse gas (GHG) emissions like methane (CH₄) and nitrous oxide (N₂O). Yet, the full assessment of GHG balance, considering trade-offs between soil organic carbon (SOC) change and non-CO₂ GHG emissions, is lacking. Integrating an improved agroecosystem model with a meta-analysis of multiple field studies, we found that U.S. rice paddies were the rapidly growing net GHG emission sources, increased 138% from 3.7 ± 1.2 Tg CO₂eq yr⁻¹ in the 1960s to 8.9 ± 2.7 Tg CO₂eq yr⁻¹ in the 2010s. CH₄, as the primary contributor, accounted for 10.1 ± 2.3 Tg CO₂eq yr⁻¹ in the 2010s, alongside a notable rise in N₂O emissions by 0.21 ± 0.03 Tg CO₂eq yr⁻¹. SOC change could offset 14.0% (1.45 ± 0.46 Tg CO₂eq yr⁻¹) of the climate-warming effects of soil non-CO₂ GHG emissions in the 2010s. This escalation in net GHG emissions is linked to intensified land use, increased atmospheric CO₂, higher synthetic nitrogen fertilizer and manure application, and climate change. However, no/reduced tillage and non-continuous irrigation could reduce net soil GHG emissions by approximately 10% and non-CO₂ GHG emissions by about 39%, respectively. Despite the rise in net GHG emissions, the cost of achieving higher rice yields has decreased over time, with an average of 0.84 ± 0.18 kg CO₂eq ha⁻¹ emitted per kilogram of rice produced in the 2010s. The study suggests the potential for significant GHG emission reductions to achieve climate-friendly rice production in the U.S. through optimizing the ratio of synthetic N to manure fertilizer, reducing tillage, and implementing intermittent irrigation.