Abstract
Water scarcity and declining soil fertility are critical constraints to sustainable crop production in semi-arid regions of the western United States. Diversification through cereal–pulse intercropping and rotation offers an opportunity to improve yield stability, nutrient-use efficiency, and agroecosystem resilience under limited irrigation. This dissertation integrates multi-year field experiments (2020–2023) conducted at the University of Idaho Aberdeen Research and Extension Center to evaluate how irrigation regimes and crop diversification strategies influence aboveground yield, grain quality, soil phosphorus dynamics, and root system plasticity in cereal-based systems.In Chapters 2 and 3, funded by the Idaho Wheat Commission, spring wheat was intercropped with pea and canola under two irrigation regimes – full (100% ET) and deficit (50% ET). Results showed that intercropping maintained grain yield and biomass production comparable to monocropping under drought-stressed conditions while improving land equivalent ratio (LER > 1.0) and water use efficiency. Wheat–pea intercropping enhanced nitrogen contribution and drought tolerance, whereas wheat–canola systems offered additional weed suppression benefits. Furthermore, intercropping stabilised wheat grain quality and end-use traits under water stress, sustaining protein content, gluten performance index (GPI), and solvent retention capacity (SRC) values comparable to well-watered controls. The findings demonstrated that diversified wheat systems can mitigate yield and quality losses under limited irrigation.
Chapter 4, supported by the USDA-ARS Pulse Crop Health Initiative, investigated phosphorus (P) use efficiency in barley-legume intercropping and rotation systems. Across four cropping years, intercropping and rotation significantly enhanced soil Olsen P concentrations in the upper soil layers (0–15 cm) and minimized subsoil depletion. Grain total P and phosphorus use efficiency (PUpE) increased in barley–lentil and barley–pea systems compared to monocropped barley. The calculated PUpE index confirmed that legume inclusion improved P acquisition under both full and deficit irrigation, indicating that diversification enhances nutrient cycling and soil fertility in calcareous semi-arid soils.
Chapter 5 examined root morphological, biochemical, and isotopic responses of barley–legume systems to irrigation regimes. Root traits such as fine root length density (FRLD), fine root mass density (FRMD), and root surface area density (RSAD) were strongly influenced by cropping system and depth, reflecting adaptive plasticity across soil layers. Stable isotope analyses revealed higher δ¹³C enrichment under drought-stressed irrigation, indicating improved intrinsic water-use efficiency, while δ¹⁵N signatures confirmed enhanced biological nitrogen fixation in legume-inclusive systems. Modelling of vertical root distribution (β and D₅₀) showed that diversified systems developed deeper and more functionally efficient rooting profiles under water stress. This work was funded by USDA-ARS Pulse Crop Health Initiative and Idaho Barley Commission. This research demonstrates that cereal-pulse diversification under contrasting irrigation regimes enhances productivity, maintains grain and end-use quality, improves nutrient-use efficiency, and fosters adaptive root plasticity. The integration of above- and belowground analyses provide a comprehensive understanding of how diversification and irrigation interact to strengthen agroecosystem resilience. These findings offer a scientific foundation for developing climate-smart, resource-efficient cropping systems that support sustainable intensification of agriculture in water-limited environments.