Water storage and use efficiencies of rainfed winter wheat-summer green manure systems of the US Southern Great Plains

Conservation practices such as green manures and no-tillage can reduce soil erosion, enhance soil biological activity, increase water storage in soils, and provide nitrogen (N) in support of winter wheat (Triticum aestivum L.). However, green manures utilize soil water during the summer months, and such water depletion may negatively impact production by wheat in ecoregions with limited water resources. This 5-year study quantified the impacts of tillage systems and green manures on water storage efficiency (WSE) during summer, water use efficiency (WUE), and grain yield of winter wheat. The study was conducted during 2011-2016 in the United States (US) Southern Great Plains (SGP). Three legumes as green manures (cowpea, pigeon pea, and soybean) were compared with summer fallow and three levels of applied inorganic N (0, 45 and 90 kg N ha-1) at wheat planting, under conventional and no-till cultivation. Summer WSE was significantly (P < 0.05) reduced in response to green manures; summer fallow was also inefficient in storing soil water. Grain yield and WUE of wheat were reduced in response to green manures compared to summer fallow during dry years but there were no differences during wet or normal rainfall years. No till significantly increased WSE under all treatments, and reduced evapotranspiration during a subset (either summer or wheat growing season) of years but did not affect amounts of soil water. Our results demonstrate replacing summer fallow in continuous systems of winter wheat with green manures have limited effects on improving water availability or efficiencies in water use by wheat. More-effective approaches for using green manures to support winter wheat in the SGP include opportunity cropping during years with adequate moisture to support double-cropping or utilizing more water-efficient le-gumes or cropping systems.