Mineral Nutrition for Legume-Rhizobia Symbiosis: B, Ca, N, P, S, K, Fe, Mo, Co, and Ni: A Review

Abstract

The intensification and expansion of modern agriculture starting in the middle of the 20th century accounted for a substantial increase in crop yield. However, productivity growth has led to an extraordinary simplification of farming systems and greater reliance on external inputs. The extensive use of pesticides and fertilizers are the cause of frequent health problems and pollution of natural ecosystems. Such evidence has led to debate about the sustainability of current intensive agricultural practices. Organic farming, which aims to produce healthy food and to respect the environment, emerges as an alternative to the negative consequences of conventional farming. In the context of sustainable organic agriculture, the successful use of biological nitrogen fixation without a decrease in productivity will reduce chemical fertilization. For that, it is important to have previous knowledge of mineral nutrient requirements to optimize symbiotic nitrogen fixation and legume crop production. Here, we first review the basic concepts of mineral nutrition, as well as the importance of mineral nutrients specifically for biological nitrogen fixation in the legume-rhizobia symbiosis. Second, a broad summary of the roles of boron and calcium in plants, with special attention to their key functions in nitrogen fixation and legume-rhizobia symbiosis, will be the central topic of this review. Symbiotic nitrogen fixation is an optimal alternative to reducing the application of chemical N-fertilizer, but demand for some nutrients is higher for legume nodule development and function than for non-nodulated legumes, and corrections of nutrient deficiencies are sometimes needed to ensure crop success. Phosphorus is a common limiting nutrient of nodulated legume growth, because of phosphate requirements for nodulation and for the very energy-expensive nitrogen fixation reaction. The enhancement of the association of nodulated-legumes with vesicular-arbuscular mycorhizas, improving phosphorus uptake, is an ecological and cheap way to correct P limitation. Sulphur and potassium are not usually limiting nutrients for nodulated legumes, although a K+ supplement for osmoadaptation has to be considered for growth in saline soils. Similarly, although demand for cobalt or nickel is higher with nodulated than with non-nodulated legumes, the soil limitations of these micronutrients are unclear. Conversely, iron and molybdenum limitations for nodulated legumes are common, even in soil with sufficient Fe and Mo, because of the anaerobic and acidic environment inside the nodule that limits the availability of these micronutrients. Therefore, Fe and Mo fertilization cannot be ruled out in sustainable agriculture based on nodulated legumes. Among mineral nutrients, B and Ca are undoubtedly the nutrients with a major effect on legume symbiosis. Both nodulation and nitrogen fixation depend on B and Ca2+, with calcium more necessary for early symbiotic events and B for nodule maturation. Boron deficiency is very common, and there is a risk of toxicity following B fertilization because it appears at concentrations close to sufficiency; and, in boron-deficient soils, an early small supplement of calcium prevents the effects of B limitation during nodulation. Therefore, a proper B–Ca feeding will greatly correct boron deficiency and improve crop production. Overall, improvement of symbiotic nitrogen fixation in legumes, combined with mycorhizal associations, is a natural fertilizing alternative to conventional chemical fertilizers. Nevertheless, small and controlled application of conventional farming practices has to be considered to correct nutrient limitations, increase crop production, and satisfy the high demand of agriculturally derived food.