Understanding and assessing discrepancies in nutrient budgets

In the pursuit of sustainable nutrient management, growing efforts have been devoted to quantifying and understanding nutrient budgets for a wide range of systems and on multiple spatial scales. However, the wide application of the nutrient budgeting approaches is accompanied by differing definitions and quantification methods. While these definitions and methods serve various research or application purposes, the lack of consistency in the system definition and its budget terms can potentially lead to misinterpretation or confusion by stakeholders and hinder comparisons among studies and experience sharing among researchers and regions. To address this knowledge gap, we have made three initial attempts: We systematically reviewed major challenges, as well as opportunities, in defining, quantifying, and applying nutrient budgets. The review synthesizes existing nutrient budgets under a framework with five systems (i.e., SoilPlant system, Animal system, AnimalPlantSoil system, AgroFood system, and Landscape system) and four spatial scales (i.e., Plot and Farm, Watershed, National, and Global scales). Based on the review, we proposed a CAFE framework for unpacking nutrient management challenges across systems. Focusing on nitrogen (N) budgets in crop systems, we investigated the theoretical differences, as well as connections, among three major methods for N use efficiency (NUE) quantification (i.e., N difference, 15N tracer, and N balance methods). The investigation revealed the influence of soil legacy effects and offered recommendations for selecting the proper approach. To quantitatively assess discrepancies in current crop N budgets, we led the first-of-its-kind inter-comparison project for the N budgets on a national scale, involving 10 major research groups around the world. We identified the most uncertain N budget terms by country, many of which range as widely as their median, and offer recommendations for improving estimates and reducing uncertainties. A benchmark N budget dataset is derived to facilitate future assessments of biogeochemical models and inform N management. These efforts reveal large discrepancies in current crop N budget assessment, and call for broader collaboration to assess nutrient budgets at various system and spatial scales.