Nitrogen cycle

Nitrogen is an essential plant macronutrient, constituting a crucial component of both structural (e.g. lignin) and metabolic substances (e.g. enzymes). Plants' requirements for N are higher than for any other macronutrient; for example, the average concentration of N in plants is four times that of the next most common element, potassium. In many forest ecosystems N is, or should be, a factor limiting growth. However, this situation has changed in the past 150 years with advent of N inputs from air pollution and agriculture. Before industrialisation, new N inputs into ecosystems were derived from atmospheric deposition and biological fixation, but the relative importance of these sources has changed rapidly.

Before industrialisation, biological nitrogen fixation was the major source of N into ecosystems. In forests N is fixed biologically by a number of organisms, including trees, mosses and soil blue-green algae. Atmospheric inputs were mainly from N fixation and from natural phenomena such as lightning, resulting in N inputs of 1–2 kg ha−1 yr−1. Today, even remote forest ecosystems may have elevated inputs of N. Total inorganic N deposition in or close to the centres of large industrial activity range from about 3 to 30 kg N ha−1 yr−1 in North America, 5–75 kg N ha−1 yr−1 in Europe and 8–40 kg N ha−1 yr−1 in China, far exceeding the natural deposition rate.