New irrigation management in paddy fields reduces methane emissions and increases water use efficiency

Proper water management in rice fields is crucial for productivity, water use efficiency (WUE), and greenhouse gas (GHG) emissions. We aimed to evaluate the effect of water management systems on these agronomic and environmental subjects in a subtropical paddy rice. A field experiment was conducted over three growing seasons (GS-1, GS-2 and GS-3). The water management evaluated were: (i) continuous flooding throughout the whole season, from four-leaf phenological stage (V4) to harvest (CF); (ii) intermittent flooding (IF), where irrigation by flooding starting at V4, with water layer until seven-leaf stadium (V7), interrupted until the panicle initiation stage (R0), with water re-entering and maintaining the water layer until harvest; and (iii) saturated soil (SS), where the soil was maintained saturated. On average over the three harvests, compared to the CF, which presented an average productivity in the three harvests of 8.50 Mg ha^-1, the SS system provided a 9 % reduction in rice yield, which is equivalent to 0.77 Mg ha^-1. Across three growing seasons, compared to continuous irrigation (4914 m³), the reduction in water consumption in alternative systems was, on average, 1179 m³, which is equivalent to 23 %. IF and SS reduce water use by 18 and 30 % compared to CF, respectively. In the three seasons, the intensity of CH₄ peaks was lower in the alternative irrigation systems, than the inundation irrigation system. Averaged over three growing seasons, there was no difference due to water management on the seasonal emissions of N₂O, but seasonal emissions of CH₄ were average 58 % lower in alternative irrigation (IF and SS), than in the CF system. In comparison to CF system (4476 kg CO₂ eq. ha^-1), the alternative water regime promoted a decreased in 63 % in SS system (1655 kg CO₂ eq. ha^-1) and of 48 % in IF system (2322 kg CO₂ eq. ha^-1). Intensity Index of GHG emissions (YpGWP = pGWP/rice yield) were 62 % lower with SS system (205 kg CO₂ eq. Mg^-1 rice) and 45 % lower with IF system (292 kg CO₂ eq. Mg^-1 rice), than continuous irrigation system (533 kg CO₂ eq. Mg^-1 rice). Thus, IF and SS reduce global warming potential by ∼50 %. Despite new irrigation systems may strongly decrease GHG emissions and water use in subtropical paddy rice systems, it is crucial advances on water management aiming to avoid penalty in rice yields, which is a main reason for their lack of adoption by rice farmers.