History of N-fixing
Microbial products have enormous potential to provide alternative, more sustainable approaches for agricultural production while protecting the environment. However, what has been proven in research is not fully translating to consistent, beneficial results in the field – which is causing some to question their effectiveness.
Symbiotic biological N-fixing products for legumes (referred to as inoculants) date back 120 years (“Nitragin”; Nobbe and Hiltner 1896). The biological N-fixing process occurs spontaneously due to the indigenous rhizobia population in the soil; however, some regions do not have naturalized populations and farmers commonly apply inoculants. Soybean inoculation is more common in South America than in the United States. In Brazil and Argentina, nearly all soybeans are inoculated with elite N-fixing Bradyrhizobium strains. In the United States, only about 15% of farmers use soybean inoculants. Factors that have contributed to less use of soybean inoculants in the United States include planting soybean varieties with limited response to inoculation and limited yield increases in soils with soybean history, due to the established symbiotic bacteria populations in the soil (Graham et al, 2004).
Recent Developments
Nitrogen fixation in cereal crops (specifically corn) has been actively researched for decades. Several new non-symbiotic N-fixing products (Table 1) were recently introduced to the US corn market. Together these new microbial inoculants are impacting several million acres, only a small fraction of the ~90 million acres of corn planted in the United States and ~500 million acres planted worldwide each year.
Why are biological N-fixing products so successful in Brazil?
Several key factors have contributed to the widespread use in Brazil:
1. Intense research: Brazil has a long history of intense research with soybean inoculants, leading to the identification of four elite Bradyrhizobium strains most effective with soybean cultivars and soil conditions experienced in Brazil (Dobbereiner and Duque, 1980; Peres and Vidor, 1980; Peres et al 1993). Success with soybean inoculation led to demand for microbial inoculants for additional crops, with focused development of elite Azospirillum strains for use in cereal crops (Santos et al 2021).
2. Commercial standards: From the beginning, Brazil created legislation defining the commercial standards with regards to concentration (minimally 1×109 CFU/mL), purity, shelf-life guarantee (greater than 6 months), and the absence of biological contaminants in the products.
3. Strict quality control: Brazil created government programs to perform official quality control analyses of commercial N-fixing products (de Souza et al 2019).
Regulation of commercial biofertilizer products varies widely worldwide. Brazil, France, and Canada have mandatory product registration, with samples analyzed randomly every year. Commercial standards for seed treatment require at least 105 to 106 CFU per seed (Hermann and Lesueur 2013). Presently, no product regulation exists in the United States; however, a regulatory framework is being developed under the Plant Biostimulants Act of 2023.