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Abstract
Our meta-analysis of 126 nitrogen addition experiments evaluated nitrogen (N) limitation
of net primary production (NPP) in terrestrial ecosystems. We tested the hypothesis
that N limitation is widespread among biomes and influenced by geography and climate.
We used the response ratio (R approximately equal ANPP(N)/ANPP(ctrl)) of aboveground
plant growth in fertilized to control plots and found that most ecosystems are nitrogen
limited with an average 29% growth response to nitrogen (i.e., R = 1.29). The response
ratio was significant within temperate forests (R = 1.19), tropical forests (R = 1.60),
temperate grasslands (R = 1.53), tropical grasslands (R = 1.26), wetlands (R = 1.16),
and tundra (R = 1.35), but not deserts. Eight tropical forest studies had been conducted
on very young volcanic soils in Hawaii, and this subgroup was strongly N limited (R
= 2.13), which resulted in a negative correlation between forest R and latitude. The
degree of N limitation in the remainder of the tropical forest studies (R = 1.20)
was comparable to that of temperate forests, and when the young Hawaiian subgroup
was excluded, forest R did not vary with latitude. Grassland response increased with
latitude, but was independent of temperature and precipitation. These results suggest
that the global N and C cycles interact strongly and that geography can mediate ecosystem
response to N within certain biome types.