Elser, J.J. et al. Ecology Letters. (2007).

"Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine, and terrestrial ecosystems".

Reviewed: 12/02/11

Past research has demonstrated the importance of nitrogen and phosphorus as limiting nutrients across a wide variety of habitats. The relative merits of each one as the primary nutrient of limitation however has been debated across systems. Does the biological demand for N and P depend on the system of study, namely terrestrial, freshwater and marine, or is the biological machinery necessary for photosynthesis and autotroph-ism similarly affected by nutrient additions or limitations across the globe? A meta-analysis comprised of a little more than 1000 separate experiments was the basis for this study. Experiments were drawn from a wide variety of habitat types across each system, and from multiple latitudes. Above- and below-ground experiments were also used for the terrestrial systems. The results of each study was converted into a log-transformed response ratio that looked at the effect of N, P, or N+P addition on some metric of community production or biomass. Only studies that manipulated both N and P within the experimental design were included in the meta-analysis.
Additions of nitrogen and phosphorus were found to induce a significant, non-zero, positive response across all system types. The community response to P additions, averaged across all habitat subtypes, did not significantly differ across the systems, though the response to N alone, as well as to N+P, did differ. Marine systems received a particularly large boost from additions of N relative to P. A synergistic effect was noted from the dual addition of N and P across all systems, whereby the addition of both limiting nutrients induced a larger response than the sum of each nutrient effect alone. The reason for this is unclear and may depend on the habitat of study. Say if for example bacterial autotrophs are better able to cope with reductions in N as long as they are not experiencing a simultaneous decrease in P.
Terrestrial and Freshwater systems were both shown to be equally limited by N and P, though marine was not as previously stated. Within systems however, the equivalence of N and P as limiting nutrients was not always true at the habitat subtype level. Grasslands were shown to be equally limited by both nutrients, while forests responded more readily to phosphorus additions.
The similarities in nutrient limitation across systems highlights the need for researchers to consider both major nutrients in their assessments of ecosystem dynamics, while the variation seen within habitat leads to the notion that every system of study should be assessed independently if possible in order for adequate conclusions to be drawn. The source, cycling, and use of nutrients within each system was not assessed in this article. Though these three dynamics may be of more relative importance within a system depending on the biological demand for a certain quantity, and quality, of nutrients. There are global scale differences in how nutrients are turned over within systems and where they come from. The merits of this study comes from its ability to draw large scale conclusions about community response to key nutrient additions, however a similar broad scale look at the internal dynamics of these nutrients within communities would also be beneficial to future assessments, particularly as they relate to global change.