Water-Conserving Practices in Arkansas Rice Production to Improve Soil N Availability and Reduce Greenhouse Gas Emissions.

Rice is commonly grown in the U.S. under continual flood from early vegetative growth until shortly before harvest.  Alternate wetting and drying cycles (AWD) offer potential savings in water use.  In a two-year, three-site field study in Arkansas, AWD improved water use efficiency (kg grain m^-3 water applied) by 22 to 43%.  It also reduced in-season greenhouse gas emissions by 38 to 90% depending on crop rotation (continuous rice or rice-soybean) and duration of drying cycles (vegetative growth only versus full-season).  Methane emissions fell by 43 to 94%, and N₂O emissions rose only slightly due to careful water management and rate of N fertilizer inputs. Grain yield of continuous rice trended downward compared to rice-soybean across all aeration treatments, consistent with earlier studies.  Within continuous rice, grain yield decreased by 5% with season-long AWD compared to the flood control.  Yield did not decrease with AWD during vegetative growth only, which we attribute to a late-season increase in available soil N as measured in ¹⁵N microplots.  At harvest, continuous rice had taken up 20 to 47 kg N ha^-1 more in this treatment than in the continuous flood control or in full-season AWD, due mostly to the soil N flush.  The size of this N flush evolved during the study, possibly reflecting a gradual accumulation of higher quality soil organic matter.  The rice-soybean rotation decreased soil phenols by 25% compared to continuous rice, consistent with earlier work that linked soil phenols to inhibited cycling of soil N.  But the effects of AWD on soil phenols were inconsistent.  Combinations of crop rotation and AWD offer growers a range of options to meet their desired balance of grain yield, water savings, and reduction of greenhouse gas emissions. They might also provide producers with a strategy to better use available soil N.