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

Abstract

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. © New Phytologist (2004).