The methods used to make nitrogen accessible to most organisms, particularly plants, include:
B. Bacterial fixation - This refers to the process by which certain bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃) or related compounds that can be used by living organisms.
D. Lightning strike - Lightning can indeed convert nitrogen in the atmosphere into nitrates (NO₃-) through the intense heat and energy of the strike, making it accessible to plants.
So, the correct options are B and D.
A. Ammonification is a process that recycles nitrogen in the ecosystem, but it does not directly make atmospheric nitrogen accessible.
C. Denitrification is the process by which nitrates are converted back into nitrogen gas, removing it from the accessible forms in the ecosystem.