More and more nitrogen keeps pouring into waterways, unleashing algal blooms and creating dead zones. To prevent the problem from worsening, scientists warn, the world must drastically cut back on synthetic fertilizers and double the efficiency of the nitrogen used on farms.
Nitrogen is not the problem (80+% of air is nitrogen). Nitrogen is pretty inert. It is nitrogen based fertilisers, whether they be nitrates or ammonia based. Please get the science right.
It's pretty clear that "nitrogen" means "nitrogen fertilizer" in this context.