Alternatives to Mineral Nitrogen Fertilizers in Agriculture: State of the Art, Challenges and Future Prospects

Despite being one of the most abundant elements in the biosphere, nitrogen remains a limiting factor in agricultural systems [1]. The use of nitrogen fertilizers produced via the Haber–Bosch process has significantly increased crop yields and global food production over the past century. However, the prolonged and excessive application of mineral fertilizers has disrupted the natural nitrogen cycle in soils [2]. Each year, large quantities of reactive nitrogen compounds—including nitrogen oxides (NO_x), ammonia (NH₃), nitrous oxide (N₂O), and nitrate (NO₃⁻)—are released into water bodies, soils, and the atmosphere [3,4]. This has led to increased environmental degradation, such as water eutrophication and greenhouse gas (GHG) emissions [5,6]. To develop a more environmentally sustainable agriculture, it is essential to investigate and implement alternatives that reduce reliance on mineral nitrogen fertilizers without compromising crop productivity [7].

In this Special Issue, eleven research teams have published studies addressing the minimization of environmental impacts associated with nitrogen fertilization and the optimization of nutrient management strategies. Hagedorn et al. (Contribution 1) evaluated drainage water management (DWM) as a controlled drainage strategy to reduce nitrogen losses and GHG emissions in a corn–soybean rotation system over three years. They found that while DWM effectively reduced NO₃⁻ export, it had no statistically significant effect on N₂O or CH₄ emissions, suggesting its potential as a low-risk mitigation strategy. Rebello et al. (Contribution 2) conducted a systematic review on nitrogen fertilization in tea (Camellia sinensis L.) cultivation. They assessed the impact of excessive nitrogen inputs on soil health and evaluated alternative practices such as organic and mixed fertilizers, controlled-release formulations, nitrification inhibitors, and biochar amendments. While many of these alternatives reduced nitrogen loading, some trade-offs were observed, including reduced yields or increased N₂O emissions. The authors emphasized the importance of site-specific nutrient management tailored to local conditions. Sanyal and Graham (Contribution 3) investigated the use of the Haney Soil Test to predict optimal nitrogen requirements for winter wheat under semi-arid conditions. Their results highlighted the role of in-season precipitation in determining nitrogen needs and underscored the utility of soil respiration and soil health metrics in refining N recommendations. However, they also identified the need for further research to improve estimates of nitrogen mineralization. Varga et al. (Contribution 4) reviewed nitrogen use efficiency in sugar beet production, where increasing root yield does not necessarily correspond to higher sucrose concentration. They stressed the importance of optimizing nitrogen availability to achieve both high yield and desirable root quality. In viticulture, González-Lázaro et al. (Contribution 5) assessed the foliar application of methyl jasmonate and urea on grape and wine composition. Their findings revealed season-dependent effects on polysaccharide and monosaccharide profiles, highlighting the need for further research into the underlying mechanisms. Using life cycle assessment (LCA), Litskas (Contribution 6) compared the environmental impacts of various nitrogen fertilizers. Results showed that organo-mineral fertilizers (compost enriched with synthetic N) had the highest environmental footprint, while cattle manure emerged as the most environmentally favorable option in terms of water use, fossil energy consumption, and global warming potential. Massey and Davis (Contribution 7) explored the use of nitrogen-fixing cyanobacteria as a sustainable alternative to synthetic fertilizers. Their review covered current applications and challenges, including economic, infrastructural, and geopolitical barriers to widespread adoption. Tortosa et al. (Contribution 8) examined the synergistic effects of Bradyrhizobium diazoefficiens inoculation and “alperujo” compost on soybean productivity. Their results demonstrated improved plant biomass, nitrogen content, and symbiotic efficiency, supporting the integration of compost to enhance legume-based fertilization strategies. Da Costa-Cezar et al. (Contribution 9) investigated the use of potassium iodate as a urease inhibitor in urea-based fertilizers. Their findings showed reduced ammonia volatilization and improved nitrogen uptake in maize, although effectiveness varied with soil type and temperature. Ibiapina de Jesus et al. (Contribution 10) evaluated nitrogen mineralization and availability in two different soils amended with three commercial organic fertilizers. Their study emphasized the influence of soil properties and temperature on nitrogen release dynamics. Lastly, Pérez-Álvarez et al. (Contribution 11) assessed the effects of algal extracts, Rhizobium sp., and Trichoderma asperellum on nitrogen assimilation and yield in hybrid maize. Among the biofertilizers tested, Rhizobium sp. had the most significant impact on crop performance, confirming its potential role in sustainable nitrogen management.

The studies presented in this Special Issue provide a comprehensive overview of current strategies aimed at reducing the environmental footprint of nitrogen fertilization in agriculture. Collectively, they demonstrate that while alternatives to mineral nitrogen fertilizers—such as organic amendments, biological nitrogen fixation, and improved management tools—hold significant potential, their effectiveness is often context-dependent and influenced by soil, climate, and crop type. Advancing sustainable nitrogen management will require integrative approaches supported by robust agronomic research, site-specific recommendations, and interdisciplinary collaboration. Future efforts must focus on optimizing these alternatives to ensure environmental protection without compromising agricultural productivity or food security.

I sincerely thank the contributing authors for advancing knowledge toward sustainable nitrogen use and more environmentally responsible agricultural systems.