NITROGEN LIMITATION ON LAND AND IN THE SEA - HOW CAN IT OCCUR

The widespread occurrence of nitrogen limitation to net primary production in terrestrial and marine ecosystems is something of a puzzle; it would seem that nitrogen fixers should have a substantial competitive advantage wherever nitrogen is limiting, and that their activity in turn should reverse limitation. Nevertheless, there is substantial evidence that nitrogen limits net primary production much of the time in most terrestrial biomes and many marine ecosystems. We examine both how the biogeochemistry of the nitrogen cycle could cause limitation to develop, and how nitrogen limitation could persist as a consequence of processes that prevent or reduce nitrogen fixation. Biogeochemical mechansism that favor nitrogen limitation include: the substantial mobility of nitrogen across ecosystem boundaries, which favors nitrogen limitation in the "source" ecosystem - especially where denitrification is important in sediments and soils, or in terrestrial ecosystems where fires is frequent; differences in the biochemistry of nitrogen as opposed to phosphorus (with detrital N mostly carbon-bonded and detrital P mostly ester-bonded), which favor the development of nitrogen limitation where decomposition is slow, and allow the development of a positive feedback from nitrogen limitation to producers, to reduced decomposition of their detritus, and on to reduced nitrogen availability; and other more specialized, but perhaps no less important, processes. A number of mechanisms could keep nitrogen fixation from reversing nitrogen limitation. These include: energetic constraints on the colonization or activity of nitrogen fixers; limitation of nitrogen fixers or fixation by another nutrient (phosphorus, molybdenum, or iron) - which would then represent the ultimate factor limiting net primary production; other physical and ecological mechanisms. The possible importance of these and other processes is discussed for a wide range of terrestrial, freshwater, and marine ecosystems.