The importance of N:P ratio

Soil fertility in terrestrial ecosystems has received increased attention from ecologists since it is now widely recognized that nutrient availability drives ecosystem functioning and processes (Wardle and Zackrisson, 2005). N and P are believed to be the most limiting nutrients in many terrestrial ecosystems particularly forests. Availability of N and P vary considerably during soil development as P is lost through leaching and fixation while N accumulates through biological N fixation (Walker and Syers, 1976; Crews et al., 1995). Thus, young soils have the tendency to be N limited while old soils are P limited. Ecosystem studies have confirmed this relationship of N and P indicated by the N:P ratio. It has been found that the leaf N:P ratio can detect nutrient limitation for wetland terrestrial ecosystem. An N:P ratio >16 indicates P limitation which is in clear agreement with the Redfield ratio (Redfield, 1958) for marine ecosystems. An N:P ratio <14 indicates N limitation and between 14 and 16 means either N or P is limiting (Koerselman and Meuleman (1996). It has also been reported that P limitation relative to N is widespread in terrestrial ecosystems (Elser et al., 2000a and 2000b) and that it is the cause of biomass decline in forest ecosystems in strongly weathered soils (Wardle et al., 2004a). Kitayama (2005) argued, however, that despite P limitation, tropical rain forests in Southeast Asia are still able to maintain high biomass as a result of high species diversity.

Elemental stoichiometry or the ratio of key elements such as carbon (C), nitrogen (N), and phosphorus (P) in organisms is useful in analyzing how the organisms influence or is being influenced by the ecosystem in which they are found (Elser and Dobberfuhl, 1996). While the elemental stoichiometry (Redfield ratio) of 106 C: 16 N: 1P is well-established for marine ecosystems it is just starting to be applied to terrestrial ecosystems. Thus, Elser and Urabe (1999) suggested that scientists working in other ecosystems (e.g. forest) might profitably apply stoichiometric approaches to food web dynamics and nutrient cycling. This is particularly valid for terrestrial systems since autotroph biomass N:P in terrestrial and freshwater systems has been found to be closely similar (Elser et al., 2000b)

References

Crews T.E et al. 1995. Changes in soil phosphorus fractions and ecosystem dynamics across a long chronosequence in Hawaii. Ecology 76: 1407-1424; Elser, J.J., Dobberfuhl, D.R., 1996. Organism size, life history, and N:P stoichiometry. Bioscience 46, 674-685; Elser, J.J. et al. 2000a. Biological stoichiometry from genes to ecosystems. Ecology Letters 3, 540-550; Elser, J.J, et al.2000b. Nutritional constraints in terrestrial and freshwater food webs. Nature 408, 578-580; Kitayama, K., 2005. Comment on ecosystem properties and forest decline in contrasting long-term chronosequences. Science 29, 633b; Koerselman, W., Meuleman, A.F.M., 1996. The vegetation N:P ratio: a new tool to detect the nature of nutrient limitation. Journal of Applied Ecology 33, 1441-1450;Redfield, A.C., 1958. The biological control of chemical factors in the environment. Am. Sci. 46, 205-221; Wardle, D.A., Zackrisson, O., 2005. Effects of species and functional group loss on island ecosystem properties. Nature 435:806-810;Warlde, D.A et al.2004a. Ecosystem properties and forest decline in contrasting long-term chronosequences. Science 305:509-513; Wardle, D.A. et al. 2004b. Ecological linkages between aboveground and belowground biota. Science 304: 1629-1633.

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