Legume Inoculants Using Rhizobia Strains Effective to Reduce Nitrous Oxide Emissions

Bedmar E.J.; Delgado M.J.; Mesa S.; Tortosa G.; Rodiño A.P.; Tejada-Hinojoza J.L.; De Ron A.M. · Climate Change and Legumes: Stress Mitigation for Sustainability and Food Security (2023) · DOI: 10.1201/9781003214885-4

Resumen

Although the use of synthetic fertilizers has increased crop yields to feed a growing population, excess nitrogen fertilizer has resulted in nitrate contamination of surface and groundwater, soils, sediments, seas, and oceans. During the N cycle in nature, nitrification and denitrification are the only processes by which the ammonium formed after fixation of atmospheric N2 is transformed into nitrate, which is further reduced again to N2, which is released into the atmosphere. During these processes, nitrous oxide (N2O), a potent greenhouse gas involved in global climatic change, is formed. Legumes are, after cereals, the main source of food for the world population. These plants provide proteins, carbohydrates, minerals, vitamins, oils, fiber, and other compounds of high nutraceutical value and beneficial properties for health. Legumes possess the extraordinary ability to establish atmospheric N2-fixing symbiosis with soil bacteria known as rhizobia. A consequence of the symbiosis is the formation in the roots, sometimes in stems, of specialized structures, the nodules, where the fixation (reduction) of the inert compound N2 to form the biologically active compound ammonium (NH4 +) takes place. This ammonium is subsequently converted into other nitrogenous compounds that are transferred to the rest of the plant for use in the synthesis of amino acids, proteins, etc. This explains the high N content of the leguminous plants, the reason why these plants fertilize the soil where they are grown, and why they are used in crop rotation and in revegetation and reforestation programs. Rhizobia do not nitrify, and most of them lack, or do not express, the complete set of genes responsible for the synthesis or the enzymes involved in denitrification, among them, the nitrous oxide reductase enzyme (Nos) responsible for the reduction of N2O to N2. Since denitrification also occurs within the nodules, nitrate can be converted into N2. In rhizobia lacking the nitrous oxide reductase enzyme, nitrous oxide accumulates and will eventually be emitted into the atmosphere. Thus, it is a paradox that, being nodulated, legumes are a major source of N incorporation into the soil, but they are also producers of the N2O greenhouse gas. Considering the vast extension where these plants are grown, N2O production by nodulated legumes is of major concern. Here, we review the main processes leading to the production of N2O by microorganisms in the N cycle. Special attention is paid to N2O formation in nodules formed by the rhizobial endosymbionts Bradyrhizobium, Ensifer, and Rhizobium of the agriculturally important legume species soybean, alfalfa, and common bean, respectively.

Instituciones: Universidad Nacional San Luis Gonzaga