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

TLDR

Nitrogen and phosphorus availability limit plant growth in most terrestrial ecosystems. The review examines how variation in the relative availability of N and P, as reflected by plant biomass N:P ratios, influences vegetation composition and functioning. The authors synthesize studies measuring plant biomass N:P ratios across species, ecosystems, and experimental manipulations to assess their functional significance. Plant biomass N:P ratios vary up to 50‑fold and are linked to root allocation, nutrient uptake, turnover, and reproduction; optimal ratios depend on species traits, and ratios below 10 or above 20 often signal N or P limitation, with higher ratios favoring graminoids and stress‑tolerant species and being affected by global change and management. The paper includes sections on variability, critical ratios, interspecific variation, vegetation properties, implications, conclusions, acknowledgements, and references.

Abstract

Summary 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. Contents Summary 243 I Introduction 244 II Variability of N : P ratios in response to nutrient supply 244 III Critical N : P ratios as indicators of nutrient limitation 248 IV Interspecific variation in N : P ratios 252 V Vegetation properties in relation to N : P ratios 255 VI Implications of N : P ratios for human impacts on ecosystems 258 VII Conclusions 259 Acknowledgements 259 References 260