Alternate wetting and drying enhanced the yield of a triple-cropping rice paddy of the Mekong Delta

In the Mekong Delta, triple rice cropping is widely spread. The rice cultivation is characterized by a short cropping period and nearly year-round flooding. Putting the soil under an oxidative condition through changing the water regimes during cropping periods might have a potential to increase the rice yield. To test this potential, an experiment was conducted at a typical triple-cropping farmer’s alluvial paddy in a central area of the delta for five years, 15 consecutive cropping seasons. We adopted an intermittent irrigation technique known as alternate wetting and drying (AWD). Under AWD, we irrigated to a depth of 5 cm above the soil surface only when the field water level dropped to 15 cm below the soil surface, with some exceptions. As a control, we prepared a treatment under which the paddy was continuously flooded (CF) except 2 weeks before expected harvest. We also prepared three straw treatments: all raw straw return, all straw-ash return, and no straw return to the paddy. For other field management, we adopted locally conventional methods. Results show that the straw treatment effects on the rice yield were not significant. For water treatments, the soil redox potential under AWD remained higher, in most cases, than that under CF; and the rough rice yield, harvest index, and percentage of ripened grains under AWD respectively became 8.9%, 4.4%, and 3.5% higher (<i>p</i> &lt; 0.01) than those under CF, with lower N contents in rice grains and 43% less irrigation water use. The increase in rice yield by AWD was found with an increase in harvest index and a decrease in aboveground biomass and nitrogen content, indicating that it was due to the promotion of nutrient translocation in rice by AWD. The effect of AWD was enhanced particularly in the winter–spring cropping season, during which the nighttime temperature was the lowest. This piece of evidence that the rice yield of the delta can be increased simply by introducing AWD has the potential to encourage farmers to take voluntary actions that will also lead to a reduction in methane emission from rice paddies.

在湄公河三角洲（Mekong Delta）地区，三季稻种植模式已广泛普及。该区域水稻栽培具有生育期短、近乎全年淹水的典型特征。通过在种植周期内调整水管理方式使土壤处于氧化状态，或具备提升水稻产量的潜力。为验证这一潜力，研究团队在三角洲中部典型三季稻种植户的冲积型稻田中开展了为期5年、涵盖15个连续种植季的田间试验。本研究采用干湿交替灌溉（alternate wetting and drying, AWD）技术：仅当田间水位降至土壤表面以下15 cm时，才灌水至土壤表面以上5 cm，仅少数情况例外。对照组设置为除收获前2周外全程保持淹水（continuous flooding, CF）的处理模式。同时设置3类秸秆处理方案：全部新鲜秸秆还田、全部秸秆灰还田，以及不向稻田施用秸秆的空白对照。其余田间管理措施均采用当地常规操作。研究结果表明，秸秆处理对水稻产量无显著影响。就水管理处理而言，干湿交替灌溉（AWD）下的土壤氧化还原电位在多数场景下高于全程淹水（CF）处理；AWD处理的稻谷产量、收获指数以及结实率分别较CF处理提升8.9%、4.4%和3.5%（p < 0.01），同时稻米籽粒氮含量更低，灌溉用水量减少43%。此外，AWD处理带来的水稻产量提升伴随收获指数升高、地上部生物量与氮含量降低，表明该增产效果源于AWD促进了水稻体内养分转运。尤其在夜间温度最低的冬春种植季，AWD的增产效果更为显著。本研究证实，仅通过引入干湿交替灌溉技术即可提升湄公河三角洲的水稻产量，这一发现或可激励农户主动采用该技术，同时还能减少稻田甲烷排放。