This chemical fertilizer is essential for better rhizobial nitrogen fixation

The correct option is phosphorus.

Rhizobia are soil bacteria that fix nitrogen after becoming established inside root nodules of legumes. For this symbiotic relationship to function efficiently and for successful biological nitrogen fixation, several nutrients are required. Among these, phosphorus plays a critical and direct role.
Here is a step-by-step explanation of why phosphorus is essential for better rhizobial nitrogen fixation:

1. Energy Requirements (ATP Production):
The chemical reaction for nitrogen fixation is highly energy-intensive. The nitrogenase enzyme complex reduces atmospheric nitrogen (N₂) to ammonia (NH₃) according to the following general equation:
$$N_2+8H^{+}+8e^{-}+16\text{ATP}\to 2{\text{NH}}_3+H_2+16\text{ADP}+16{\text{P}}_i$$ As shown in the equation, fixing just one molecule of nitrogen requires a minimum of 16 ATP (adenosine triphosphate) molecules. Phosphorus is a fundamental structural component of ATP. Therefore, a deficiency in phosphorus directly limits the availability of ATP, thereby reducing the rate of nitrogen fixation.

2. Nodule Development and Function:
Phosphorus is vital for the initiation, growth, and development of root nodules where Rhizobium bacteria reside. Leguminous plant root nodules are strong sinks for phosphorus, often containing higher concentrations of phosphorus than other plant vegetative parts. Adequate phosphorus nutrition ensures optimal nodule mass, number, and overall activity.

3. Root Growth and Colonization:
Phosphorus promotes healthy root development, increasing the root surface area. A larger root system provides more infection sites for Rhizobium bacteria, facilitating better colonization and establishment of the symbiotic association.