Final answer:
In aquatic environments, nitrogen fixation is the process that contributes significantly to non-symbiotic nitrogen fixation.
Step-by-step explanation:
In aquatic environments, the process that contributes significantly to non-symbiotic nitrogen fixation is b) nitrogen fixation. This is because nitrogen fixation refers to the conversion of nitrogen gas (N₂) into more reactive forms like ammonia (NH₃), which can be utilized by plants. Notably, nitrogen fixation can occur through the action of free-living aquatic microorganisms that fix nitrogen independently from plants. Such non-symbiotic nitrogen-fixing bacteria are crucial for the input of usable nitrogen into aquatic ecosystems.
Addressing the related multiple-choice questions, the process that produces an inorganic compound that plants can easily use is b) nitrogen fixation. The specific process wherein nitrogen gas (N₂) is converted into ammonia (NH₃) is e) nitrogen fixation. Finally, anammox is a process that contributes to denitrification but does not play a role in non-symbiotic nitrogen fixation.