N : P ratios in terrestrial plants: variation and functional significance


Author for correspondence: Sabine Güsewell Tel: +00411 632 4307 Fax: +00411 632 1215 Email: sabine.guesewell@env.ethz.ch



  •  Summary

  • Introduction
  • II Variability of N : P ratios in response to nutrient  supply
  • III Critical N : P ratios as indicators of nutrient  limitation
  • IV Interspecific variation in N : P ratios
  • Vegetation properties in relation to N : P ratios
  • VI Implications of N : P ratios for human impacts  on ecosystems
  • VII Conclusions
  •  Acknowledgements

  •  References


Nitrogen (N) and phosphorus (P) availability limit plant growth in most terrestrial ecosystems. This review examines how variation in the relative availability of N and P, as reflected by N : P ratios of plant biomass, influences vegetation composition and functioning. Plastic responses of plants to N and P supply cause up to 50-fold variation in biomass N : P ratios, associated with differences in root allocation, nutrient uptake, biomass turnover and reproductive output. Optimal N : P ratios – those of plants whose growth is equally limited by N and P – depend on species, growth rate, plant age and plant parts. At vegetation level, N : P ratios <10 and >20 often (not always) correspond to N- and P-limited biomass production, as shown by short-term fertilization experiments; however long-term effects of fertilization or effects on individual species can be different. N : P ratios are on average higher in graminoids than in forbs, and in stress-tolerant species compared with ruderals; they correlate negatively with the maximal relative growth rates of species and with their N-indicator values. At vegetation level, N : P ratios often correlate negatively with biomass production; high N : P ratios promote graminoids and stress tolerators relative to other species, whereas relationships with species richness are not consistent. N : P ratios are influenced by global change, increased atmospheric N deposition, and conservation managment.