Nitrogen is fixed, or combined, in nature as nitric oxide by lightning and ultraviolet rays, but more significant amounts of nitrogen are fixed as ammonia, nitrites, and nitrates by soil microorganisms. More than 90 percent of all nitrogen fixation is effected by them.(source #7)

Groundwater is water that exists underground in saturated zones beneath the land surface. The upper surface of the saturated zone is called the water table. (source #15)

Nitrification, a process carried out by nitrifying bacteria, transforms soil ammonia into nitrates (NO3−), which plants can incorporate into their own tissues. (source #7)

Pertains to all water present above the substrate or soil surface including water contained in an ocean, river, stream, lake, pond, lagoon, or impoundment reservoir. (source #8)