"Extra nitrogen" refers to an additional amount of a gas called nitrogen. It is used to describe a situation where there is more nitrogen than is typically present or required. Full definition
The bat, in turn, helps the plant by providing extra nitrogen through its feces. (sciencemag.org)
They then estimated how much extra nitrogen and phosphorus would actually be available from natural sources and found that there would not be enough, revising the models accordingly. (sciencemag.org)
Since the growth of phytoplankton is limited by available nitrogen, and carbon dioxide is central to the growth process, Jones believes that supplying extra nitrogen to phytoplankton would increase the uptake of carbon. (newscientist.com)