Soil microbes convert them into plant-usable compounds
Explanation
The nitrogen cycle is a biogeochemical process that circulates nitrogen through the atmosphere, soil, and living organisms. A critical component of this cycle is the role of decomposers (saprophytes), which recycle nutrients from dead organic matter back into the ecosystem.
Detailed Analysis
When plants and animals die, or when animals excrete nitrogenous wastes (such as urea or uric acid), the nitrogen is locked in complex organic molecules like proteins and nucleic acids. The fate of this nitrogen involves specific biological processes:
- Ammonification: Soil microorganisms, primarily bacteria and fungi, decompose the organic nitrogen found in dead remains and waste products. They break down these complex molecules and release nitrogen in the form of ammonia (`NH_3`) or ammonium ions (`NH_4^+`).
- Nitrification: Subsequently, nitrifying bacteria (such as Nitrosomonas and Nitrobacter) oxidize this ammonia into nitrites (`NO_2^-`) and then into nitrates (`NO_3^-`).
- Result: These inorganic forms (ammonia and nitrates) are soluble in water and can be readily absorbed by plant roots to synthesize new proteins. Thus, the wastes are converted into plant-usable compounds.
Refutation of Incorrect Options
- They sublime into atmospheric nitrogen Incorrect: Nitrogenous wastes do not sublime (turn from solid directly to gas). While denitrification can convert nitrates to nitrogen gas, the immediate fate of organic waste is decomposition into soil nutrients, not sublimation.
- They remain inert in the soil strata Incorrect: Organic matter does not remain inert; it is actively broken down by the soil biotic community.
- They are oxidized into free oxygen Incorrect: The breakdown of nitrogenous waste does not produce free oxygen; in fact, the decomposition process often consumes oxygen.
Key Takeaway
Decomposers play a vital role in the nitrogen cycle by converting organic nitrogen from dead matter and waste into inorganic ammonium and nitrates, replenishing the soil nutrients required for plant growth.