Quantifying Nutrient Budgets for Sustainable Nutrient Management

Abstract

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.

Understanding and reconciling different approaches for quantifying nitrogen use efficiency in cropping systems

“…In principle, NUEs are de ned and quanti ed to assess how much of the N input to a system is used effectively by the product and harvested from the system 5 . In practice, three major approaches developed under different experimental and operational settings can lead to divergent results and possibly differing interpretations, even for the same experimental site and for the same growing season (Fig.…”

“…To measure the e ciency of N use in crop production and determine the impacts of N inputs on the environment, an indicator of nitrogen use e ciency (NUE) is widely and increasingly used by agronomists, biogeochemists, policymakers, and other stakeholders at various temporal and spatial scales 4 . Unfortunately, while the use of NUE is growing, interpretation and synthesis of results is impeded by widely differing de nitions and calculation methods, hindering comparisons among studies and experience-sharing among researchers and regions 5 . Understanding the differences and connections among existing approaches for quantifying NUE is particularly important and timely due to the growing policy interest in evaluating the agricultural sustainability 6 .…”

Nutrient Budgeting of Primary Nutrients and Their Use Efficiency in India

“…While defining the nutrient budget in soil-plant system for assessing the efficiency of crop nutrient management, we come across two major challenges, viz., (i) the efficiency assessment is based on the assumption that the system has achieved a steady state but long-term observations are often required to ensure the quasi steady state, especially for cropping systems with multiyear rotation practices, perennial crops, or large nutrient reserves in soil; (ii) whether to include crop residue as part of the productive output of the system can lead to significant difference in NUE assessment, e.g., the N use efficiency for the soil-plant system can increase from 43% to over 58% (depending on the harvest index) if crop residue is included as one of the productive outputs. However, the consideration of crop residue should depend on whether the residue is harvested (i.e., removed from the site) and how it is used, for which information is often limited [41]. the amount of fertilizer applied in their fields, since it is one of the major costs for their production.…”

“…The International Fertilizer Industry Association (IFA) also reports fertilizer production and consumption data on a national scale based on surveys with fertilizer companies [43,44]. While most countries have reported total synthetic N fertilizer consumption from the two data sources in approximate agreement, a few countries (e.g., China) show up to 30% underestimation by the Food and Agriculture Organization of the United Nations (FAO) data source, leading to different global fertilizer consumption estimates from the two data sources in recent decades [41]. Only a few efforts have downscaled the fertilizer application rate considering the heterogeneity among regions and crop types [45].…”

“…However, using nutrient budgets to inform sustainable nutrient management faces multiple major challenges, e.g., challenges to define and assess "sustainable" nutrient management with nutrient budgets, connect actions by stakeholders with their impacts on the environment, effectively communicate nutrient budgets to engage actions etc. [41]. NUE focuses on cereal systems because those consume the bulk of the world's fertilizer, but the principles discussed are similar in all agricultural crops.…”