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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 legumes or cropping systems. • WSE, WUE, and wheat yield responses to different crop rotations were assessed. • Summer fallow was inefficient in storing precipitation water. • Replacing the summer fallow with cover crops did not increase WSE. • Planting cover crops helped to increase system WUE during normal or wet years. • Winter wheat yield response to cover crop biomass production was negatively linear. [ABSTRACT FROM AUTHOR]

This investigation was part of a project to evaluate green manure forage legumes as relay intercrops for sweet corn (Zea mays L.) and chile (Capsicum annuum L.) in north-central New Mexico. The study was carried out under irrigated conditions in 1994 on a Fruitland sandy loam (coarse-loamy, mixed, calcareous, mesic Typic Torriorthent) to evaluate the 15N recovery by forage sorghum (Sorghum bicolor L.] Moench) and soil after the incorporation of lSN-labeled alfalfa Medicago sativa L. Nitro') or hairy vetch (Vicia villosa Roth. 'Madison'). Two application rates of alfalfa (2.7 or 4.0 Mg dry matter ha[sup -1]) or hairy vetch herbage (3.2 or 6.4 Mg dry matter ha[sup -1]), were incorporated into soil microplots. Additional treatments consisting of 56, 112, and 168 kg N ha[sup -1] (as urea) were used to compare soil inorganic N concentrations with those released from the legumes. Nitrogen-15 recovery by sorghum tops was 16% of the [sup 15]N applied, in two harvests. Of this, 80% was recovered at the first harvest. In soil, [sup 15]N recoveries were 50 and 3% of the input at the 0-0.3 and 0.3-0.6 m depths, respectively. Maximum inorganic N concentrations were found 14 d after amendment application. At the end of the growing season, inorganic N concentrations in all treatments were essentially the same as the background (8 mg N kg[sup -1]). The low [sup 15]N recoveries by crop plus soil and inorganic N concentrations in soil at the end of the season suggest that about 12% and 35-58% of the N applied as green manure was lost from the system for alfalfa and hairy vetch, respectively. [ABSTRACT FROM AUTHOR]