In soil science and plant nutrition, which of the following forms of nitrogen supplied to plants is most directly associated with the action of biofertilizers?

Introduction / Context:
This question deals with the concept of biofertilizers in agriculture and the chemical forms of nitrogen that plants can readily absorb. Biofertilizers are living microorganisms, such as nitrogen fixing bacteria and cyanobacteria, that enhance nutrient availability in the soil without the environmental drawbacks of excessive chemical fertilizer use. One of their key functions is to convert atmospheric nitrogen or organic nitrogen into forms that plants can actually use, primarily nitrate or ammonium ions. Understanding which nitrogen form is directly linked with the activity of biofertilizers helps you connect microbiology, soil chemistry, and sustainable farming practices.

Given Data / Assumptions:
- The question mentions biofertilizers, which are biological agents rather than synthetic chemicals.
- Four nitrogen related substances are listed as options: urea, ammonia, uric acid, and nitrates.
- You are expected to identify the nitrogen form that is most directly associated with the action of biofertilizers and easily absorbed by plant roots.
- It is assumed that you know that plants usually take up nitrogen as nitrate or ammonium ions from the soil solution.

Concept / Approach:
Biofertilizers like Rhizobium, Azotobacter, and blue green algae work by fixing atmospheric nitrogen into biologically available forms in the soil. They either directly produce ammonium that is further converted by nitrifying bacteria or facilitate processes that lead to increased nitrate availability. Plants predominantly absorb nitrogen as nitrate ions and also as ammonium ions. Among the options, nitrates represent the final oxidized, plant available form that results from microbial activity in the nitrogen cycle. Urea and uric acid are organic or synthetic compounds that need further microbial breakdown, and ammonia is a gaseous or ionic form that is intermediate in the cycle. Therefore, the best match with the phrase associated with the action of biofertilizers and plant uptake is nitrates.

Step-by-Step Solution:
Step 1: Recall that biofertilizers are living microorganisms that improve nutrient status by fixing nitrogen or solubilizing phosphates, rather than being direct chemical fertilizers. Step 2: Remember that plants typically absorb nitrogen from the soil in the form of nitrate ions, NO3 minus, and ammonium ions, NH4 plus. Step 3: Recognize that nitrogen fixing and nitrifying bacteria transform atmospheric nitrogen and organic nitrogen into ammonium and then nitrates, completing key steps in the nitrogen cycle. Step 4: Examine the options. Urea is a synthetic nitrogen fertilizer, uric acid is a nitrogenous waste product in animals, and ammonia is an intermediate that may be further converted. Step 5: Conclude that nitrates are the specific form strongly linked to microbial activity in soil and directly taken up by plants, so Nitrates is the correct answer.

Verification / Alternative check:
You can verify this by looking at diagrams of the nitrogen cycle in environmental science or biology textbooks. These diagrams typically show nitrogen fixing bacteria creating ammonium, which nitrifying bacteria convert into nitrites and then nitrates. Plant roots are shown absorbing mainly nitrate ions as their principal nitrogen source. Urea may be applied as a fertilizer, but it first must be hydrolyzed and further processed by soil microorganisms. Uric acid is not a standard fertilizer in crop fields. This confirms that nitrates are the most appropriate form associated with biofertilizer action and plant uptake.

Why Other Options Are Wrong:
Urea is a commonly used synthetic nitrogen fertilizer manufactured industrially; it is not itself a biofertilizer and must be converted to ammonium and then nitrates by microorganisms before plants can use it. Ammonia is an intermediate in the nitrogen cycle and can be used by plants in some forms, but it is not the main final product emphasized in discussions of biofertilizers and plant uptake. Uric acid is a nitrogenous waste compound excreted by birds and some animals, not a typical soil nitrogen source for crops. Therefore, these options do not directly capture the nitrogen form most clearly linked to biofertilizer activity in soils.

Common Pitfalls:
A frequent mistake is to select urea simply because it is a very familiar fertilizer name, without noticing that the question explicitly mentions biofertilizers, which are biological in nature. Some students may also choose ammonia because they know it contains nitrogen, but they forget that plants commonly take up nitrogen as nitrates in agricultural soils. Confusing the broader category of nitrogen fertilizers with the specific role and products of biofertilizers can also lead to errors. To avoid these pitfalls, always connect biofertilizers with microbial transformations of nitrogen and focus on the plant available forms highlighted in the nitrogen cycle.

Final Answer:
The nitrogen form most directly associated with the action of biofertilizers and readily taken up by plants is Nitrates.