Tapio Salo, Riikka Keskinen, Maria del Rosario Dominguez Carrasco, Johanna Suomi, Kari Ylivainio

Substituting fossil-based mineral fertilizers with bio-based products – impacts on potentially toxic elements in soil and crops

Introduction

Substituting fossil-based mineral fertilizers with bio-based products – impacts on potentially toxic elements in soil and crops. Investigating bio-based fertilizers' impact on toxic elements (As, Cd, Ni, Pb) in soil and crops. Study reveals low PTEs in BBFs but highlights Cd sensitivity. Recommends strict EU limits.

Abstract

Similar to conventional mineral fertilizers, circular bio-based fertilizer (BBF) products may contain potentially toxic elements (PTEs). In this study, conducted as part of the LEX4BIO project, the contents of arsenic (As), cadmium (Cd), nickel (Ni) and lead (Pb) were surveyed in 37 phosphorus (P) BBFs and 7 nitrogen (N) BBFs representing different product function and component material categories. In addition, mass balances of these elements were calculated over 100-year scenarios of P fertilization with a product that was low or high in the assessed elements, a common mineral phosphate fertilizer (MPF), and a hypothetical fertilizer containing the maximum amount of these PTEs allowed by the EU regulations. The examination was performed using case-specific conditions for Finland, Denmark, France, Spain, Hungary, and Germany, in which countries the products had been tested. Although in some individual products the current maximum EU limits were exceeded for Cd and Ni, the contents of the targeted PTEs were overall low in the studied products. The dominant trends in the contents and fluxes of As, Ni, and Pb were decreasing in all fertilizer-use scenarios with observed BBF concentrations, but the maximum allowed rates would instead commonly lead to prominent increases. The highest sensitivity to variation in fertilizer input rates, along with some substantial increases in the environmental contents and fluxes was observed for Cd. However, with low-Cd BBFs, favorable development is also achievable for Cd fluxes. It is recommended that PTE levels remain well below the maximum EU limits.

Review

This study rigorously investigates the potential for bio-based fertilizers (BBFs) to introduce potentially toxic elements (PTEs) into agricultural systems, a critical concern given the push towards circular economy solutions in agriculture. By surveying arsenic (As), cadmium (Cd), nickel (Ni), and lead (Pb) levels in a diverse set of 37 phosphorus and 7 nitrogen BBFs, the research establishes a robust baseline for understanding the elemental composition of these emerging products. The comprehensive approach, which includes 100-year mass balance calculations under various fertilization scenarios across six European countries (Finland, Denmark, France, Spain, Hungary, and Germany), provides invaluable long-term perspectives on the environmental implications of substituting conventional mineral fertilizers. The inclusion of scenarios comparing low- and high-PTE BBFs, common mineral phosphate fertilizers (MPFs), and hypothetical fertilizers at maximum EU-allowed limits further strengthens the analytical framework. The findings offer a nuanced picture of the environmental safety of BBFs. While some individual products were found to exceed current EU maximum limits for Cd and Ni, the overall PTE content in the studied BBFs was generally low. Crucially, the long-term mass balance projections indicate a favorable trend for As, Ni, and Pb, showing decreasing contents and fluxes across all scenarios utilizing observed BBF concentrations. This contrasts sharply with hypothetical scenarios where fertilizers contain PTEs at the maximum allowed EU rates, which would commonly lead to significant increases. Cadmium emerges as the element requiring the most careful management, exhibiting the highest sensitivity to variations in fertilizer input rates and showing potential for substantial increases in environmental contents and fluxes. However, the study reassuringly demonstrates that even for Cd, favorable environmental development is achievable through the use of low-Cd BBFs. This research makes a significant contribution to the scientific understanding required for a sustainable transition in fertilizer practices. By providing data-driven insights into the long-term impacts of BBFs on soil and crop PTE levels, it directly informs policy-making and product development within the circular economy framework. The study's recommendation that PTE levels in BBFs remain well below current maximum EU limits is a critical takeaway, underscoring the need for vigilant quality control and product design to mitigate potential risks. Overall, the work provides an essential evidence base for promoting the safe and effective adoption of bio-based fertilizers, ensuring that the benefits of circular nutrient management are realized without compromising environmental or food safety.

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