Which nutrient is primarily fixed by soil microbes through the nitrogen cycle?

The correct answer is nitrogen, as soil microbes play a crucial role in the nitrogen cycle, which is essential for maintaining soil fertility and ecosystem health. Nitrogen is abundant in the atmosphere, making up about 78% of it, but plants cannot directly utilize atmospheric nitrogen. Instead, it must be converted into a more accessible form through a series of microbial processes.

Microbes, particularly certain bacteria, are responsible for nitrogen fixation, where they convert atmospheric N2 into ammonia (NH3) or related compounds. This process is vital because it transforms nitrogen into forms that plants can absorb and use for growth, such as nitrate (NO3-) and ammonium (NH4+). Other microbial processes in the nitrogen cycle, including nitrification and denitrification, further transform nitrogen compounds within the soil, contributing to the availability of this critical nutrient for plant uptake.

The roles of carbon, oxygen, and phosphorus are important in soil and ecosystem stability, but they do not undergo a similar specialized fixation process by soil microbes like nitrogen does. Carbon, for example, is primarily cycled through the process of photosynthesis and respiration, while oxygen is generally a byproduct of photosynthesis and not fixed in the same sense. Phosphorus, while also essential for plant