In what way do lightning strikes play an important role in a nutrient cycle?

converting phosphorus into a form that can enter the atmosphere
converting phosphorus into phosphates that can dissolve in water
splitting atmospheric nitrogen allowing it to combine with oxygen
converting ammonia into atmospheric nitrogen that can be returned to the atmosphere

The right option is; splitting atmospheric nitrogen allowing it to combine with oxygen

Lightning is the flash of light caused by the discharge of atmospheric electrical charge. In nitrogen cycle, lightning play an important role by indirectly adding nitrogen to the soil. It transforms atmospheric nitrogen into a form that can be used by plants (a process known as nitrogen fixation). The strong bonds of atmospheric nitrogen molecule are broken (split) by the bolts of lightning that transmits electrical energy. After the molecules are separated, the nitrogen atoms quickly combine with oxygen in the atmosphere to form nitrogen dioxide which also dissolves in water (within the atmosphere) to form nitric acid. The nitric acid also form nitrates which is being released to the ground in rain drops. The nitrates then spread into the soil in a form that can be used by plants.

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.