Nitrogen fixation is a natural process where atmospheric nitrogen gas, which plants cannot directly use, is converted into a usable form like ammonia. Soybeans, as a type of legume, possess a unique ability to perform this conversion through a partnership with specific soil bacteria. Nitrogen is fundamental for plant growth, serving as a building block for proteins and nucleic acids. This makes nitrogen fixation a particularly important and efficient process in soybean cultivation.
The Symbiotic Partnership: How Soybeans Fix Nitrogen
Soybean plants engage in a mutualistic relationship with soil bacteria, primarily Bradyrhizobium japonicum. These bacteria colonize the roots of the soybean plant, leading to the formation of specialized structures called root nodules. The process of nodule development and nitrogen fixation can begin as early as the V2 growth stage (two trifoliolate leaves) and typically peaks during the reproductive stages of the plant.
Inside these root nodules, the Bradyrhizobium japonicum bacteria convert atmospheric nitrogen gas (N2) into ammonia (NH3), a usable form for the soybean plant. This conversion is carried out by an enzyme complex called nitrogenase, which requires a low-oxygen environment to function effectively. The soybean plant, in return for the nitrogen, provides the bacteria with carbohydrates produced through photosynthesis, supplying the energy needed for the fixation process.
The effectiveness of this symbiotic relationship can be visually assessed by examining the root nodules. Actively fixing nodules have a pink to reddish interior color, which is due to the presence of leghemoglobin, a protein that transports oxygen within the nodule. Young nodules may appear white or gray, while green, brown, or mushy nodules indicate that they are no longer actively fixing nitrogen. This process can contribute 39-182 kg of nitrogen per hectare annually.
Agricultural and Environmental Advantages
Soybean nitrogen fixation offers benefits for agricultural practices by reducing the reliance on synthetic nitrogen fertilizers. This process offers cost savings for farmers by reducing the need for commercial nitrogen. It also decreases the energy consumption associated with the industrial production of these fertilizers.
The environmental advantages are also significant. Reduced application of synthetic nitrogen fertilizers minimizes the runoff of excess nitrogen into waterways, which helps mitigate issues like eutrophication, where nutrient enrichment leads to algal growth and oxygen depletion. Lower production and use of synthetic fertilizers also decrease greenhouse gas emissions. By naturally enriching the soil with nitrogen, soybean cultivation also improves overall soil health and fertility, benefiting subsequent crops in a rotation system. For example, a corn-soybean rotation can increase corn yield by 4.76% to 79.92% compared to continuous corn.
Factors Influencing Fixation Efficiency
Several environmental and management factors influence the efficiency of nitrogen fixation in soybeans. Soil pH plays a considerable role, with most legumes and Rhizobia thriving in a pH range of 5.6 to 7.0. Low soil pH, below 5.0, can lead to aluminum and manganese toxicity, as well as phosphorus deficiency, negatively impacting nodulation and nitrogen fixation.
Adequate soil moisture and appropriate temperatures are also important for the survival and activity of the Rhizobia bacteria and for nodule formation. Excessive soil moisture, leading to water-saturated soils and anaerobic conditions, can be detrimental to both soybean roots and the bacteria, as they require oxygen. Conversely, drought conditions can reduce Rhizobia populations and nodulation, as stomata close, reducing carbon available to the bacteria. Optimal soil temperatures for Rhizobia survival are typically between 40-80°F (4-27°C), with fixation optimal between 27-40°C. Temperatures exceeding 37°C can cause the death of Rhizobia or reduce their efficiency.
Other essential nutrients also impact fixation efficiency. Phosphorus is important for nodule development and nitrogen fixation. Molybdenum, an essential micronutrient, is needed for the formation and function of the nitrogenase enzyme, which catalyzes nitrogen conversion. Other micronutrients like iron, boron, and copper are also required. In fields where soybeans have not been grown recently, or where nodulation has been poor in previous years, inoculating seeds with effective Bradyrhizobium japonicum strains is recommended to ensure a sufficient population of beneficial bacteria.