The Water-Carbon-Nitrogen Nexus in Cotton Agroecosystems: A Lever for Achieving Water Sustainability and Enhanced Ecosystem Services

Cotton production faces a mounting challenge: the crop's high water demand collides with intensifying water scarcity under climate change. Traditional approaches to water management have pursued efficiency gains through irrigation technologies and drought-tolerant varieties, yet they typically treat water in isolation from the biogeochemical cycles that ultimately shape agroecosystem performance. This review synthesizes current understanding through the lens of the Water-Carbon-Nitrogen (W-C-N) nexus, framing water management as the central lever that orchestrates coupled carbon and nitrogen processes in cotton systems. We systematically examine how irrigation practices regulate carbon assimilation and soil organic carbon dynamics while simultaneously controlling nitrogen transformations and losses, thereby co-determining water use efficiency (WUE), nitrogen use efficiency (NUE), greenhouse gas emissions, and nitrate leaching. The analysis then evaluates eco-engineering strategies—including regulated deficit irrigation, integrated drip fertigation, soil amendments, and intercropping—for their capacity to optimize W-C-N couplings. Evidence shows these practices can enhance water productivity, mitigate environmental impacts, and build soil health in concert, rather than trading one benefit for another. The review's primary contribution lies in synthesizing the mechanistic feedbacks that govern synergies and trade-offs within the nexus, thereby providing an integrated framework for evaluating management options. We identify critical research gaps in rhizosphere processes, cross-scale modeling, and ecosystem service trade-offs, and argue for a paradigm shift: from isolated water conservation toward integrated, nexus-informed water-smart management that designs for multifunctionality. Such an approach is essential to build cotton systems resilient enough to deliver productivity, sustainability, and ecosystem services under global change. • Proposes the Water-Carbon-Nitrogen (W-C-N) nexus as a systemic framework for water-smart cotton agroecosystems. • Identifies irrigation as the master lever that links coupled carbon and nitrogen cycles to ecosystem service outcomes. • Synthesizes how eco-engineering practices (e.g., drip fertigation, deficit irrigation) optimize the nexus for synergistic benefits. • Reveals critical trade-offs and feedback loops among water use efficiency, carbon sequestration, and nitrogen losses. • Outlines a research agenda for translating nexus science into climate-resilient farming practices.