Final answer:
Soil nitrification is the biological process of converting ammonium (NH4+) into nitrate (NO3-) in soils, involving two types of nitrifying bacteria that convert NH4+ to NO2- and then NO2- to NO3-.
Step-by-step explanation:
Soil nitrification refers to the conversion of NH4+ (ammonium) to NO3- (nitrate) in soils. This biological process consists of two steps primarily conducted by distinct groups of microorganisms. First, nitrifying bacteria such as Nitrosomonas convert ammonium into nitrite (NO2-). Following this, another group of nitrifying bacteria, such as Nitrobacter, transform nitrite into nitrate.
Nitrogen fixation, on the other hand, is a separate process in which nitrogen-fixing bacteria convert atmospheric nitrogen gas (N2) into organic compounds like ammonia (NH3). The process by which nitrogen gas is returned to the atmosphere, primarily as a result of bacterial action, is known as denitrification, where nitrates are converted back into nitrogen gas (N2).
The correct option for the question '“Soil nitrification refers to which process?' is D: conversion of NH4+ to NO3-.