Toward circular and sustainable nitrogen (N) management in the US and globally

Abstract

Over the past century, human activities have more than tripled the creation of nitrogen (N), enabling unprecedented gains in agricultural productivity. Yet inefficient N use has generated widespread air and water pollution, greenhouse gas (GHG) emissions, and ecosystem degradation, undermining food-energy-water (FEW) sustainability. Despite substantial efforts to improve farm-level nutrient management, persistent nutrient pollution indicates that N management challenges extend beyond individual farms and sectors. Therefore, a full life-cycle perspective, tracing N production, use, accumulation, and loss, is essential for identifying inefficiencies, revealing trade-offs, and designing effective intervention pathways. In Chapter 1, using the coupled human and natural systems (CHANS) model, I quantified terrestrial N budget of the contiguous US in 2012, 2017, and 2022. I found that agricultural production remains the largest contributor to nationwide N loss and accumulation, while human consumption of fuel, food, non-food consumer goods, and waste systems collectively account for 40-45% of national N loss and accumulation, highlighting intervention opportunities beyond the farm. Extending the system-wide N budget, Chapter 2 investigates the driving forces of crop nitrogen use efficiency (NUE) using county-level data in the US and China, I show that NUE responds nonlinearly to N inputs, with country-specific tipping points beyond which additional inputs reduce efficiency. With system-wide inefficiencies and key NUE drivers, Chapters 3 and 4 evaluate improvement pathways to enhance N circularity and reduce N losses. First, I develop a transparent and adaptable framework to quantify the potential of manure recycling in the US, showing that improved manure management could supply crops with N equivalent to about one-quarter of current synthetic fertilizer use, even under phosphorus constraints. Second, I assess alternative agricultural trade portfolios between China and the US, demonstrating that shifting from predominantly feed trade toward more food trade can reduce N losses and GHG emissions while maintaining food security. Because analytical insights alone do not guarantee action, the final chapter integrates stakeholder engagement to evaluate how could governance, policy incentives and international collaboration translate N management strategies into practice. Collectively, this dissertation integrates system-wide diagnostics, driver analysis, scenario analysis, and stakeholder perspectives to advance actionable strategies for sustainable N management.