This third article in the fertilizer series looks at the innovation and potential solutions for the negative impacts of synthetic nitrogen fertilizers through the lens of the Right Source in the “Four R” best practices. The first article “Fertile Ground for Change,” covered the environmental and social impacts of nitrogen pollution, especially on water quality. The second article, “The Four “Rs” of Addressing the Fertilizer Problem,” reviewed the scale of the nitrogen problem, gaps in nitrogen use efficiency, and the barriers to implementing the current “4R” practices of Right Source, Right Rate, Right Time, Right Place.
Growing Solutions
The readiest solutions to the nitrogen problem begin with on-farm practices that improve soil health and reduce nitrogen losses to the environment. Given the rising costs of synthetic fertilizer, reducing usage without impacting yield hold the key for both environmental goals and farm profitability. Yet, fully implementing these solutions is complex. Enhanced efficiency fertilizers (EEFs) and biological inputs offer a means to close the nitrogen use efficiency gap by focusing on the Right Source.
Right Source
The “R” of Right Source requires matching fertilizer to the needs of the crop and soil as well as site specific factors that reduce the risk of nutrient loss (improving nitrogen use efficiency). Site specific factors include soil and climate, crop and stage of plant growth, precipitation and timing of rainfall or irrigation with fertilizer application, and management practices including crop rotation, cover crop use, and tillage.
There are different fertilizer innovations for the type of product including biofertilizers and enhanced efficiency fertilizers (EEFs).
Biofertilizers
Biofertilizers, excluding compost and manure or sewage sludge products, include alternatives to synthetic N sources that are derived from living organisms or their residues. In general, biofertilizers do not fully replace synthetic N. Instead, these naturally derived inputs increase nutrient uptake by crops, make nutrients like phosphorous in soil more plant available, increase plant growth, and improve disease resistance (Ammar et al., 2023). Biofertilizers can be applied as a foliar treatment, soil amendment, or seed treatment. On average, these treatments improve yield between 10-40% (Ammar et al., 2023). Biofertilizers are sourced from organisms including bacteria, algae, fungi, and plants.
Bacteria
Beneficial bacteria include nitrogen-fixing Azotobacter and Cyanobacteria which contribute to nutrient cycling and phosphorus bioavailability, improve soil health, control disease, and increase plant growth and production. Nitrobacter reduces methane emissions from rice production. Technologies that enable large-scale, continuous, and fast manufacturing of these bacteria contribute to more cost-effective inputs. Furthermore, improved production of microbial fertilizers and inputs has helped scale their use, including technologies such as a carrier substrate [ES1] that increases microbe survival in soil and their viability and effectiveness (Ammar et al., 2023).
Algae
Both microalgae, including nitrogen-fixing green and blue-green algae, and macroalgae like seaweed are used as biofertilizers. Seaweed fertilizers are high in organic matter and nutrients, offering the potential to displace N fertilizer use. These inputs are often formulated as foliar applications that stimulate plant growth. Red algae improve seed germination and fungal and pest resistance in corn (Safinaz & Ragaa, 2013). Seaweed is also effective as a soil amendment to reduce salinity. High salinity soils result from irrigation water or excessive synthetic fertilizer use.
Fungi
Different species of fungi benefit crops through fixing nitrogen, making nutrients like phosphorous, potassium, and zinc more available, or supporting disease resistance. Anyone familiar with soil health has likely heard of arbuscular mycorrhizal fungi (AMF), which helps build soil aggregates that improve carbon sequestration and water holding capacity (Ammar et al., 2023).
AMF and other fungi-based inputs are often formulated as soil inoculants. While AMF can partner with beneficial bacteria, this helpful fungus is not effective in fields with high concentrations of phosphate and N or uses of fungicides and pesticides. AMF works best in fields with moderate applications of organic fertilizer. Practices such as intensive tillage, and crop rotation with nonmycorrhizal crops negatively affects the diversity and abundance of AMF (Kuila & Ghosh, 2022).
Plants or Green Manure
Using plant matter as fertilizer, known as green manure, is a centuries-old practice. This is the same principle as including a legume crop in rotation or using a cover crop. Other approaches include intercropping of a nitrogen-fixing plant with the cash crop, which increased nitrogen use efficiency by 47% to 120% in some studies, accompanied by a 25% to 25% reduction in N fertilizer use (Gu et al., 2021). Green manure practices, especially cover crops, have added benefits of reducing soil loss and runoff, and improving soil health, or reducing weed growth and suppressing soil-borne diseases (UNH, 2024).
Challenges
Currently, biofertilizers face different policy hurdles globally. For example, the EU regulations permit only four types of organisms in bacterial and fungal inputs, Azotobacter spp., Rhizobium spp., and Azospirillum spp., and mycorrhizal fungi. However, this list is expected to grow (Santos et al., 2024). Biofertilizers also require formulation technologies [ES2] that keep inoculants and products viable during storage, transport and field application, or better match fungal inoculants to field conditions (Fadiji et al., 2024; Kuila & Ghosh, 2022).
Opportunities
Despite remaining policy hurdles and the need to scale emerging technology improvements, the biofertilizer market for the US alone in 2024 was approximately USD 3.55 billion. The US market is projected to grow to USD 4.47 billion by 2026 (Santos et al., 2024). More conservative global projections project a value of USD 4.77 billion by the end of 2032, according to Global Market Insights Inc. with an 8.5% CAGR during 2024-2032 (Hopkins, 2024).
Enhanced Efficiency Fertilizers
EEFs use additives, coatings, or chemical formulations to reduce N losses from volatilization, denitrification, leaching, and immobilization. These products include nitrification inhibitors (NIs) that suppress Nitrosomonus bacteria in the soil, slowing or stopping the conversion of ammonium to nitrite (Hergert et al., n.d.). NIs break down over time, depending on moisture and temperature, allowing nitrification to occur. Urease inhibitors (UIs) block ammonia volatilization. This allows time for precipitation or irrigation to move urea into the soil. As with NIs, urease inhibitors break down over time after application.
EEFs also include chemical formulations and coatings such as polymers to decrease the solubility or conversion of the fertilizer to N forms that are usable in the N cycle. In general, EEFs offer several benefits:
• Beneficial in fields with high potential for N loss.
• Better nitrogen use efficiency for surface applications with heavy crop residue cover or high soil pH.
• EEFs can increase yield from2-6%. However, results vary by crop, with grain crops showing less yield benefit.
• UIs reduce ammonia volatilization by up to half.
• NIs reduce nitrous oxide emissions (N2O) 24-48% depending on application and conditions.
• May enable reduced nitrogen use by up to 15-20%.
(Fan et al., 2022; Howard, 2022)
Challenges
Despite the benefits, synthetic EEFs have significant downsides including increased costs. In addition, some synthetic NI products are ecotoxic for terrestrial and aquatic species or alter soil biology. NI use also increases ammonia volatilization. (Fan et al., 2022; GEA, 2022)
Opportunities:
Some plant root systems secrete their own biological nitrification inhibitor (BNI). There is opportunity to develop BNI inputs, breed high BNI-yielding crops, or intercrop BNI-producing crops with conventional crops to enhance nitrogen utilization efficiency (Wang et al., 2021; Dillon, 2024; Kuppe & Postma, 2024). BNIs offer the same benefits as synthetic chemicals without the risk of ecotoxicity.
Conclusion
While scalability, technology, and policy barriers to adoption of biofertilizers exist, these barriers are being addressed with rapid gains in the technology to deliver microbial and fungal crop inputs. Improved insights will also help align the right biofertilizer treatment to specific field and crop needs. Policies to support adoption are improving across the global market. As a result, biofertilizers will play a significant role in the future of sustainable agriculture and reducing reliance on synthetic N fertilizer.
The final article in the series will focus on the other three “Rs” of Right Rate, Right Time, and Right Place and the growth of precision agriculture technologies.
References
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