Nutrients, such as nitrogen and phosphorus, provide vital support for human life, but overloading nutrients to the Earth system leads to environmental concerns, such as water and air pollution on local scales and climate change on the global scale. With an urgent need to feed the world's growing population and the growing concern over nutrient pollution and climate change, sustainable nutrient management has become a major challenge for this century. To address this challenge, the growing body of research on nutrient budgets, namely the nutrient inputs and outputs of a given system, has provided great opportunities for improving scientific knowledge of the complex nutrient cycles in the coupled human and natural systems. This knowledge can help inform stakeholders, such as farmers, consumers, and policy makers, on their decisions related to nutrient management. This paper systematically reviews major challenges, as well as opportunities, in defining, quantifying, and applying nutrient budgets. Nutrient budgets have been defined for various systems with different research or application purposes, but the lack of consistency in the system definition and its budget terms has hindered intercomparison among studies and experience‐sharing among researchers and regions. Our review synthesizes existing nutrient budgets under a framework with five systems (i.e., Soil‐Plant system, Animal system, Animal‐Plant‐Soil system, Agro‐Food system, and Landscape system) and four spatial scales (i.e., Plot and Farm, Watershed, National, and Global scales). We define these systems and identify issues of nitrogen and phosphorus budgets within each. Few nutrient budgets have been well balanced at any scale, due to the large uncertainties in the quantification of several major budget terms. The type and level of challenges vary across spatial scales and also differ among nutrients. Improvement in nutrient budgets will rely not only on the technological advancement of scientific observations and models but also on better bookkeeping of human activity data. While some nutrient budget terms may need decades, or even centuries, of research to be well quantified within desirable levels of uncertainties, it is imperative to effectively communicate to interested stakeholders our understanding of nutrient budgets so that scientists and a variety of stakeholders can work together to address the sustainable nutrient management challenge of this century.

Nitrogen (N) fertilization has boosted grain production during the past century, while excess N fertilization with declining N use efficiency (NUE) has led to severe pollution in many regions. To achieve the goal of sustainable food production, Technologies and Management Practices (TMPs, e.g., optimum N application rate and methods, N‐transformation inhibitors) have been developed to improve crop yield and NUE and to reduce N losses. However, it remains unclear how N fate has been changed by environmental factors and TMPs. Here, we compiled a dataset of 366 field ¹⁵N tracer observations from 74 publications worldwide and conducted a meta‐analysis to examine how environmental conditions and management practices influence the fertilizer‐N fate one growing season after the fertilization of maize. We show that the proportion of ¹⁵N taken up by aboveground biomass (NUE_15N), was significantly lower in China (33%) than that in North America (42%) and the European Union (54%). Soil organic carbon was the most critical environmental factor positively correlated with NUE_15N. Among the nine selected categories of TMPs, deep placement of fertilizer and split application increased the grain yield and decreased the fertilizer‐N loss consistently among studies. Reducing the fertilizer‐N rate could increase the NUE_15N but presents risks of yield reduction in some regions. This study demonstrates the importance of SOC and varying impacts of TMPs on the efficiency of fertilizer use in cropping systems globally and provides critical information for farmers and policymakers to improve N management for higher productivity and less pollution.

Enhancing nitrogen (N) and phosphorus (P) use efficiency (NUE and PUE) is essential for advancing sustainable agriculture and reducing dependency on non-renewable fertilizers. However, the long-term dynamics of NUE and PUE across major crops remain poorly understood at a global scale. Here, we compile a comprehensive global database encompassing 3360 observations across 205 countries to analyze trends in NUE and PUE for major crops from 1961 to 2018. Today, PUE and NUE are still suboptimal, particularly in developing regions, emphasizing the need for context-specific strategies to improve nutrient use efficiency. Global mapping shows that NUE and PUE are highly context-dependent, with variations observed by crop type and region. For instance, rice achieves optimal NUE and PUE in tropical zones, while wheat performs best in temperate climates. Notably, maize continues to exhibit significant nutrient inefficiencies, especially in China and the United States, with considerable N and P surpluses. Taken together, this global analysis provides spatially explicit insights to guide region-specific efforts toward improving nutrient use efficiency, supporting sustainable agricultural practices and reducing global fertilizer dependence.

Measuring the nitrogen isotope compositions (δ¹⁵N) of nitric oxide (NO) from different sources helps to quantify the relative contributions of atmospheric NO_x. Soil is one of the most important sources of atmospheric NO_x, but only limited measurements on the δ¹⁵N of soil‐emitted NO exist, hampering our ability to partition sources to air pollution. Here we conducted soil incubations to measure the δ¹⁵N‐NO under defined aerobic or anaerobic conditions, favoring either nitrification or denitrification. Soils were collected from seven sites spanning three ecosystems in northern China (two agricultural, two forest, and three grassland sites). We found that the δ¹⁵N‐NO and their associated N isotope fractionations were significantly different between anaerobic and aerobic conditions in seven soils. Under aerobic condition, the δ¹⁵N‐NO ranged from −62‰ to −50‰ (averaged −56 ± 4‰), being significantly more negative (by 23‰) than those under anaerobic condition (−45‰ to −23‰, averaged −33 ± 7‰). The apparent N isotope fractionation for NO production under aerobic condition (¹⁵ε_aerobic = 61 ± 3‰) was significantly higher (by 26‰) than under anaerobic condition (¹⁵ε_anaerobic = 35 ± 6‰), with a small variability among ecosystem types. Our study demonstrates that the δ¹⁵N‐NO from different soils are very different from fuel combustions (mainly from 0‰ to +20‰), supporting that measuring ¹⁵N is a useful tool to partition the contributions of soil NO to atmospheric NO_x. Our results also imply δ¹⁵N‐NO produced by nitrification and denitrification distinctly different, as these two processes are dominant processes producing NO under aerobic and anaerobic conditions, respectively.