Microorganisms that increase the nitrogen content of soils through conversion of atmospheric N2 to ammonia are performing which process?

Introduction:
Soil fertility depends on biological processes that move nitrogen between atmospheric, inorganic, and organic pools. This item asks you to select the process that directly adds new reactive nitrogen to the biosphere from atmospheric N2.

Given Data / Assumptions:

• Atmospheric N2 is inert and unavailable to plants without conversion.
• Some microbes convert N2 to NH3, increasing bioavailable nitrogen.
• Other nitrogen transformations recycle existing fixed nitrogen but do not add new nitrogen.

Concept / Approach:
Nitrogen fixation reduces atmospheric N2 to ammonia (NH3), which is then assimilated into organic forms or further transformed in soils. Denitrification removes fixed nitrogen by returning it to gaseous forms (N2, N2O), while nitrification converts NH3 to NO2- and NO3-, enhancing mobility but not total nitrogen content.

Step-by-Step Solution:
Identify the direction relative to atmospheric N2: only fixation adds new reactive nitrogen. Exclude denitrification because it depletes fixed nitrogen to gas. Exclude nitrification because it transforms, not creates, fixed nitrogen. Select nitrogen fixation as the process that increases soil nitrogen content.

Verification / Alternative check:
Symbiotic Rhizobium-legume systems and free-living diazotrophs (Azotobacter, Clostridium) demonstrate measurable increases in soil nitrogen via fixation.

Why Other Options Are Wrong:

• Denitrification: Converts nitrate to N2/N2O, reducing available nitrogen.
• Nitrification: Oxidizes ammonia to nitrite/nitrate; no net nitrogen gain.
• All of these: Incorrect because only fixation adds new nitrogen from the atmosphere.

Common Pitfalls:
Confusing enhanced plant uptake after nitrification with an increase in total nitrogen; supply increases only through fixation.

Final Answer:
nitrogen fixation is the process that increases soil nitrogen content.