By Agrositio. 04/03/2022 | 17:03
Nitrogen availability and plant growth are closely related and their conjunction results in the production of all crops, including legumes that incorporate or fix this nutrient from the air. The fixed amount of nitrogen is variable and responds to the joint characteristics of various environmental factors such as temperatures, water balance, and soil components that affect both the host plants (legumes) and the microorganisms in symbiosis (rhizobia).
Under conditions of high productivity, lower growth limitations, the highest nitrogen fixation contributions are achieved in soybean crops (Racca et al., 2001, Collino et al. 2015). This is how, from crop management decisions, this behavior is reflected in the fact that the strategies to improve crop performance are the same as those required to increase its potential for biological nitrogen fixation (Santachiara et al. 2019).
The proliferation of microorganisms, the infection of root hairs and the growth of plants vary according to the concentration of various elements in the soil solution. Nutrient availability is one of the soil components that intervenes integrally in the biological nitrogen fixation process. For this reason, the administration of nutrients, in order to avoid limitations in the growth of legumes, participates directly in nitrogenous nutrition and results in the production of these species. When fertilizing with phosphorus, in places limited in this element, it improves both the formation of nodules and their growth or size, mainly those located on the main root. Adequate sulfur availability also contributes positively to nodule formation (Santachiara et al., 2019). This is shown by the compilation of comparative evaluations of fertilization with phosphorus or sulfur in the Pampas region, where on average between slightly more than 2 to 3 nodules are described on the main root of plants fertilized with these nutrients (Fig.1).
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Fig. 1: Difference in soybean plant nodulation between phosphorus and sulfur fertilization treatments in the Pampas region (Average of studies reported by Daz-Zorita et al. 2019).
Phosphorus regulates biological nitrogen fixation directly by restricting nodule development and function (Olivera et al. 2004), and in extreme cases may inhibit their formation (Hellsten and Huss Danell, 2000). And indirectly, by limiting plant growth, it regulates the availability of energy to lead to fixation processes (Divito and Sadras, 2014). Given limitations in the availability of phosphorus, in addition to the smaller number and size of nodules (Date, 2000; Cassman et al. 1980), the permeability of the nodules is reduced and in turn the nitrogenase activity is lower (Drevon et al. Harwig, 1997; Tang et al. 2001; Hellsteen and Huss Danell, 2000) reducing nitrogen fixation (Sa and Israel, 1998). In contrast, with sufficient phosphorus inputs, the nodules are more abundant and larger, as is the nitrogen fixation activity (Valverde et al., 2002; Gentili et al., 2002).
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Observations abound showing that both nodulation and grain production in soybean respond positively to increases in phosphorus availability (Colacelli et al., 2000; Daz-Zorita et al., 1999; Medina et al., 2000; Rubione et al., 2000). et al., 2002). Under similar crop growth conditions, the critical levels at which the greatest nodulation is reached are often higher than those to be reached to maximize grain production (Fig.2).
The functional articulation between the components of the biological nitrogen fixation system in pastures with legumes is clearly described in the following sentence by the teacher-researcher in forage production at FA UNLPAm Ing. Oscar Hernndez: “legumes fix nitrogen because they grow” . It is a short sentence that summarizes both the roles and the integration between the actors in this symbiosis. The virtuous circuit of nutrition in legumes requires anticipating their active growth. Therefore, to achieve effective nitrogen nutrition, a functional pillar of biological processes, a favorable growth context is required. Today, in much of Argentina with legume crops, this favorable condition is achieved by implementing effective fertilization and inoculation strategies. Adequate plant nutrition supports the biological fixation of nitrogen, so the contribution of limiting elements to plant growth such as phosphorus, sulfur and some micronutrients results in synergistic responses in the production of legumes.
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