Nitrogen pollution in the air is devastating for many sensitive wild plants, which is why so much of the countryside is becoming a vast carpet of nettles, hogweed, hemlock and other rampant vegetation that feasts on nitrogen. In many places, these are running out of control.
Much has been written about the damage to human health from nitrogen oxides given off by traffic, but the damage to sensitive plants has gone largely unnoticed. Excessive nitrogen also comes from ammonia from fertilisers and manures, with much of the countryside awash with nitrates running off farmland.
— source theguardian.com