Balancing non-CO2 GHG emissions and soil carbon sequestration in U.S. rice paddies: implications for natural climate solutions

The U.S. rice paddy systems play an increasingly vital role in ensuring food security, which also contribute massive anthropogenic non-CO2 (CH4 and N2O) greenhouse gas (GHG) emissions with expanding cultivation area. However, the magnitude and spatiotemporal variations of net GHG balance, i.e., trade-offs between SOC (Soil Organic Carbon) sequestration and non-CO2 GHG emissions, in paddy soils and the drivers of their changes, have not yet been rigorously assessed. Integrating an improved agricultural ecosystem model with a meta-analysis of multiple field studies (a total of 322 site-year data representing 43 locations for U.S. rice paddies and 3402 site-year data representing 1113 locations for global rice paddies), here we found that U.S. rice paddy was a rapidly growing net GHG emission source, increased 138% from 3.73 ± 1.16 Tg CO2eq yr-1 in the 1960s to 8.88 ± 2.65 Tg CO2eq yr-1 in the 2010s. CH4 emission made the most significant contribution (10.12 ± 2.28 Tg CO2eq yr-1) to this increase in net GHG emissions in the 2010s, but increasing N2O emissions, accounting for ~2.4% (0.21 ± 0.03 Tg CO2eq yr-1), cannot be ignored. SOC sequestration could offset about 14.0% (1.45 ± 0.46 Tg CO2eq yr-1) of the climate-warming effects of soil non-CO2 GHG emissions in the 2010s. The intensification of land use/cover area, atmospheric CO2,synthetic N fertilizer usage and the application of manure were the dominant factors that induced net GHG emissions aggravation in rice paddies. Climate change was also critical in aggravating ~21% of soil N2O emission and ~10% of soil CO2 release in the 2010s. However, adopting no/reduced tillage resulted in a substantial decrease of ~10 % in net soil GHG emissions, and non-continuous irrigation exhibited the potential to mitigate around 39% of soil non-CO2 GHG emissions. Overall, the cost of this net GHG emissions for achieving increased U.S. rice yield markedly declined from 1960 to 2018, resulting in 0.84 ± 0.18 kg CO2eq ha-1 of net soil GHGs on average for each kilogram of grain produced in the 2010s. There would be a great potential to reduce emissions intensity in the mid-South U.S., especially the Mississippi Delta region, through optimization of the synthetic N fertilizer and manure ratio, reduction of tillage practices, and implementation of non-continuous irrigation. Our findings underscore the importance of net CO2 GHG mitigation in U.S. rice paddies for achieving net zero-emission and climate-friendly rice production.Unit: CH4 (g C/m2); N2O (g N/m2); CO2 (g C/m2)