Greenhouse gas emissions altered by the introduction of a year-long fallow to continuous rice systems

Rice (Oryza sativa L.) production in California follows a norm of mono-cropping with little to no rotations or fallows. Both winter droughts, which lead to water restrictions, and spring rains, which inhibit field machinery operations, have resulted in increased fallow frequencies, where no crop is grown during the summer growing season. A 3-year field study was conducted to investigate summer and winter greenhouse gas (GHG) emissions (CH₄ and N₂O) in three treatments: continuous rice (CR), rice following the introduction of a year-long fallow (FR), and the fallow phase (F). Summer CH₄ emissions were lower in FR than CR in 2 out of 3 years, averaging a 33% reduction across years. Particulate organic carbon and mineral-associated organic carbon levels in CR and FR were similar in the year with no treatment effect on CH₄ emissions-potentially due to differences in residue management. There were negligible summer CH₄ emissions in F. Summer N₂O emissions were low for all three treatments. Summer global warming potentials (GWP) accounted for more than 96% of annual GWP in CR (13,937 kg CO₂eq ha^-1) and FR (9,236 kg CO₂eq ha^-1). For F, the winter season accounted for 94% of the annual GWP (413 kg CO₂eq ha^-1) due to N₂O emissions. Overall, this study provides a valuable resource for quantifying changes in GHG emissions when fallow periods are introduced into rice monoculture in California.