Summary: 97Ambio Vol. 31 No. 2, March 2002 © Royal Swedish Academy of Sciences 2002
Human alteration of the N cycle is important in large part be-
cause added N has myriad effects on nontarget ecosystems (1).
Enrichment with N often affects the species composition, pro-
ductivity, dynamics, and diversity of recipient ecosystems (2, 3).
Increased plant growth in response to N additions has been docu-
mented well in a broad range of systems (4, 5). The growth of
many animals, indeed the health of much of humanity, is con-
strained more by protein supply than by energy (6), and decom-
position and other microbial processes can be shaped substan-
tially by the supply of available N.
Our purpose in this paper is to address the question--`why is
N so important to the functioning of ecosystems?' What proc-
esses cause N to be in short supply in many terrestrial ecosys-
tems? It is no mystery why N limits production and yield in in-
tensive agricultural systems--they are designed to maximize the
removal of protein N for human or animal consumption, and they
are disturbed frequently by tillage and harvest, allowing loss of