Leaching the good stuff: nitrogen and phosphorus in real and experimental urban agricultural settings – Nature

Executive Summary

This report analyzes the environmental impact of urban agriculture, specifically focusing on nitrogen and phosphorus leaching and its implications for water quality. Based on empirical data from three coordinated studies in the United States and Sweden, the findings indicate that while urban agriculture provides multiple services aligned with the Sustainable Development Goals (SDGs), it poses a significant risk to water quality if not managed carefully. Annual nutrient input rates were found to be poor predictors of nutrient leaching, with legacy effects from previous soil management being a more critical factor, particularly for phosphorus. This highlights a critical trade-off between achieving SDG 2 (Zero Hunger) and SDG 12 (Responsible Consumption and Production) through urban food production and organic waste recycling, and protecting SDG 6 (Clean Water and Sanitation) and SDG 14 (Life Below Water). The report concludes that long-term monitoring and careful nutrient management are essential to mitigate leaching risks and ensure urban agriculture contributes positively to creating sustainable cities (SDG 11).

Introduction: Urban Agriculture and Sustainable Development Goals

Urban agriculture is a vital component of multifunctional urban ecological infrastructure, contributing to several Sustainable Development Goals. It directly supports:

• SDG 2 (Zero Hunger): By enhancing local food production and food security.
• SDG 3 (Good Health and Well-being): Through recreation and improved mental health.
• SDG 11 (Sustainable Cities and Communities): By creating green spaces, mitigating urban heat islands, and improving stormwater attenuation.
• SDG 12 (Responsible Consumption and Production): Through the recycling of organic waste into compost, promoting a circular economy.

However, the multifunctionality of urban agriculture presents potential trade-offs. Inefficient resource management can lead to negative environmental impacts, challenging the achievement of other critical SDGs. The over-application of recycled organic inputs, such as compost, often leads to the accumulation of nitrogen (N) and phosphorus (P) in soils. This nutrient surplus creates a significant risk of leaching into groundwater and surface water bodies, directly undermining:

• SDG 6 (Clean Water and Sanitation): By contributing to water pollution.
• SDG 14 (Life Below Water): By causing eutrophication in aquatic ecosystems.

This report presents empirical data to understand the dynamics of nutrient leaching from urban agricultural systems, aiming to provide insights for management practices that balance its benefits with its environmental risks, thereby supporting a holistic approach to achieving the SDGs.

Research Methodology

Data was collected from three distinct research projects in Minneapolis-St. Paul, USA, and Linköping, Sweden, using a standardized protocol to measure nutrient leaching. These studies represent a continuum from controlled to observational settings:

1. Controlled Experiment (USA): A 7-year study in a university research garden with uniform initial soil conditions, testing different compost types and input levels.
2. Semi-Observational Study (USA): A three-season experiment applying compost inputs to plots across four existing urban farms and community gardens, where background conditions varied.
3. Observational Study (Sweden): A three-season observation of existing garden plots across four allotment areas, where gardener practices were documented but not controlled.

In all studies, leachate was collected weekly from zero-tension lysimeters installed 30 cm below the soil surface to determine nitrate and phosphate concentrations. This comparative approach allows for an assessment of whether findings from controlled experiments are applicable to real-world conditions and how different contexts influence nutrient loss.

Key Findings and Analysis

Disconnect Between Annual Nutrient Inputs and Leaching

A primary finding across all three studies was that annual nutrient input rates were not strong predictors of nutrient leaching during the same growing season. While plots receiving nutrient inputs generally leached more than control plots, there was no systematic correlation between the amount of input and the amount of leachate. This disconnect suggests that achieving SDG 12 (Responsible Consumption and Production) through compost application requires a more nuanced approach than simply tracking annual inputs. The lack of a direct relationship points to the influence of other complex factors, such as soil storage capacity and legacy nutrients, which can create a delayed pollution problem, threatening SDG 6 (Clean Water and Sanitation) even when current practices appear sustainable.

The Role of Legacy Phosphorus in Water Quality Degradation

The research highlights that historical land management and cumulative nutrient inputs play a significant role in current leaching patterns, especially for phosphorus. A significant positive correlation was found between plant-available soil phosphorus and phosphate concentrations in leachate. In many of the studied plots, soil phosphorus levels were already above thresholds recommended by national agricultural guidelines. This indicates that many urban gardens have become legacy sources of phosphorus pollution. This historical accumulation poses a long-term threat to water quality, making it difficult to achieve the targets of SDG 6 and SDG 14 (Life Below Water) without addressing the existing soil nutrient surplus. The findings suggest that past efforts to improve soil fertility may have inadvertently created future environmental liabilities.

Temporal Dynamics of Nutrient Loss

Long-term data from the 7-year controlled study revealed that the effects of different nutrient inputs on leaching become more apparent over time. For example, nitrate leaching from compost-amended plots was significantly higher than in control plots during years 2-5 of the experiment, likely due to the slow mineralization of organic nitrogen added in previous years. Similarly, differences in phosphate leaching between high- and low-input treatments became consistent after the first year. Furthermore, monitoring in Sweden revealed that over 50% of annual nutrient leaching occurred during the non-growing season. This underscores the necessity of long-term, year-round monitoring to develop effective management strategies that protect water quality and support the goal of SDG 11 (Sustainable Cities and Communities).

Recommendations for Sustainable Urban Agriculture

To maximize the contribution of urban agriculture to the SDGs while minimizing its negative impacts, a shift towards more precise and informed nutrient management is required. Based on the findings, the following actions are recommended:

• Adopt Balanced Nutrient Application: To prevent the over-application of phosphorus, which is common with compost-based fertilization, practitioners should apply organic amendments to meet crop P requirements and supplement with inorganic nitrogen as needed. This approach helps achieve SDG 2 (Zero Hunger) without compromising SDG 6.
• Promote Integrated Management Practices: The adoption of practices such as drip irrigation, cover cropping, and agroforestry can alter water and nutrient dynamics, reducing the risk of leaching. This aligns with the principles of sustainable agriculture under SDG 2 and SDG 12.
• Implement Long-Term Monitoring and Support: Regular soil testing and access to advisory services are crucial for urban growers to make informed decisions. Secure land tenure is also essential to encourage long-term investments in sustainable infrastructure and practices, contributing to resilient and sustainable communities under SDG 11.

Conclusion: Balancing Multifunctionality for Sustainable Cities

Urban agriculture is a valuable asset for building sustainable, healthy, and food-secure cities. However, this report demonstrates a significant trade-off between its benefits and the potential for water quality degradation through nutrient leaching. The key challenge lies in managing the nutrient cycle effectively. Legacy pollution from past practices and the slow release of nutrients from organic matter mean that simply managing annual inputs is insufficient. For urban agriculture to be a net-positive contributor to the Sustainable Development Goals, particularly SDG 11 (Sustainable Cities and Communities), it must be guided by evidence-based, long-term management strategies. By balancing nutrient inputs, addressing legacy soil conditions, and supporting growers with knowledge and resources, cities can harness the full potential of urban agriculture while safeguarding vital water resources for future generations, thereby advancing a truly integrated and sustainable urban development agenda.

Analysis of Sustainable Development Goals in the Article

1. Which SDGs are addressed or connected to the issues highlighted in the article?

1. SDG 2: Zero Hunger

   The article discusses urban agriculture as a practice for food production within cities. It mentions that “Food production and recreation are notable benefits across these croplands,” directly linking the practice to agricultural productivity, which is a core component of achieving food security and sustainable agriculture under SDG 2.

2. SDG 6: Clean Water and Sanitation

   This is a central theme of the article. The entire study is designed to understand the “adverse impact on water quality” from urban agriculture. It empirically measures “nitrogen and phosphorus leaching” and explicitly states that “nutrient losses from urban agriculture could pose a risk to water quality.” This directly addresses the goal of ensuring the availability and sustainable management of water.

3. SDG 11: Sustainable Cities and Communities

   The research is set entirely within an urban context, examining urban agriculture as a form of “Multifunctional urban ecological infrastructure (UEI)” which is a “key component of sustainable cities.” The study’s focus on managing the environmental trade-offs of urban practices to minimize water quality impairment is directly related to making cities more sustainable and reducing their adverse environmental impact.

4. SDG 12: Responsible Consumption and Production

   The article heavily focuses on resource management and efficiency. It discusses how “careful management is needed to balance” the benefits of “organic waste recycling” against leaching risks. It highlights inefficient resource use, where “application rates of these recycled inputs often exceed the annual amount of nitrogen (N) and phosphorus (P) harvested in crops,” pointing to unsustainable production patterns in urban agriculture.

5. SDG 15: Life on Land

   The article addresses terrestrial ecosystems, specifically urban soils. It examines how agricultural practices affect soil health, noting that “Repeated over-application leads to accumulation, especially of P in the soil.” The discussion of “Legacy effects from previous soil management” and the measurement of soil nutrient content relate to the sustainable management of land and soil to prevent degradation.

2. What specific targets under those SDGs can be identified based on the article’s content?

• Target 2.4: Sustainable food production and resilient agricultural practices

  The article explores how to achieve sustainable urban agriculture by investigating nutrient management practices. It aims to provide data to “make recommendations” for “careful nutrient management” that can maintain yields while minimizing environmental harm, thus contributing to more sustainable and resilient food production systems.

• Target 6.3: Improve water quality by reducing pollution

  This target is directly addressed. The study’s primary objective is to quantify nutrient leaching (a form of pollution) from urban gardens to understand and mitigate their impact on water quality. The research measures nitrate and phosphate in leachate to assess the risk of water quality impairment.

• Target 11.6: Reduce the adverse per capita environmental impact of cities

  The article investigates a specific adverse environmental impact of an urban activity—nutrient pollution from urban agriculture. By studying the drivers of nutrient leaching, the research aims to inform management practices that would reduce this negative environmental footprint of cities.

• Target 12.2: By 2030, achieve the sustainable management and efficient use of natural resources.

  The study points out that in urban agriculture, “resource use (e.g., nutrient inputs) was more inefficient” in some cases. It analyzes the relationship between nutrient inputs (a natural resource) and losses, aiming to find ways to use these resources more efficiently and sustainably.

• Target 12.5: By 2030, substantially reduce waste generation through prevention, reduction, recycling and reuse.

  The article highlights that “urban agriculture can support nutrient circularity through organic waste recycling” by using compost. It notes that “Compost products were the largest contributor to annual inputs,” demonstrating a direct link to waste recycling. However, it also examines the environmental challenges associated with this practice, contributing to a better understanding of how to optimize it.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

Yes, the article mentions and uses several specific, quantifiable indicators:

• Concentrations of nitrate (NO₃^–-N) and phosphate (PO₄^3--P) in leachate

  This is a primary indicator used throughout the study to measure the level of water pollution, directly relevant to Target 6.3. The article states, “The teams collected leachate weekly… filtering samples, and determining nitrate (NO₃^–-N) and phosphate (PO₄^3--P) concentrations.”

• Nutrient leaching flux (kg per hectare per growing season)

  This indicator measures the total amount of nutrients lost from the soil over a specific area and time. It provides a comprehensive measure of the environmental impact (Target 11.6) and the inefficiency of nutrient use (Target 12.2).

• Annual nutrient input rates (kg N ha⁻¹ and kg P ha⁻¹)

  The study quantifies the amount of nitrogen and phosphorus applied to gardens annually. This is a direct indicator of resource use and management practices, relevant for measuring progress towards sustainable agriculture (Target 2.4) and efficient resource use (Target 12.2).

• Plant-available soil phosphorus (P) levels

  The article measures soil P content to understand “legacy effects” and the risk of future leaching. It notes a “significant correlation between plant-available soil P and higher PO₄^3—P leachate concentrations,” making it a key predictive indicator for water quality impairment (Target 6.3) and soil health (SDG 15).

• Use of recycled organic inputs (Compost)

  The article identifies the type and amount of inputs used, noting that “Compost products were the largest contributor to annual inputs.” This serves as an indicator for the adoption of waste recycling practices (Target 12.5).

4. Table of SDGs, Targets, and Indicators

Source: nature.com