Increased n affects uptake of eight grassland species: the role of root surface phosphatase activity

Abstract

Increased N deposition may change species composition in grassland communities by shifting them to P limitation. Interspecifi c diff erences in P uptake traits might be a crucial yet poorly understood factor in determining the N eff ects. To test the eff ects of increased N supply (relative to P), we conducted two greenhouse fertilization experiments with eight species from two functional groups (grasses, herbs), including those common in P and N limited grasslands. We investigated plant growth and P uptake from two P sources, orthophosphate and not-readily available P (bound-P), under diff erent N supply levels. Furthermore, to test if the N eff ects on P uptake was due to N availability alone or altered N:P ratio, we examined several uptake traits (root-surface phosphatase activity, specifi c root length (SRL), root mass ratio (RMR)) under varying N:P supply ratios. Only a few species ( M. caerulea , A. capillaris , S. pratensis ) could take up a similar amount of P from bound-P to that from orthophosphate. Th ese species had neither higher SRL, RMR, phosphatase activity per unit root (Pase root ), nor higher total phosphatase activity (Pase tot : Pase root times root mass), but higher relative phosphatase activity (Pase rel : Pase tot divided by biomass) than other species. Th e species common from P-limited grasslands had high Pase rel . P uptake from bound-P was positively correlated with Pase tot for grasses. High N supply stimulated phosphatase activity but decreased RMR and SRL, resulting in no increase in P uptake from bound-P. Pase root was infl uenced by N:P supply ratio, rather than by only N or P level, whereas SRL and RMR was not dominantly infl uenced by N:P ratio. We conclude that increased N stimulates phosphatase activity via N:P stoichiometry eff ects, which potentially increases plant P uptake in a species-specifi c way. N deposition, therefore, may alter plant community structure not only by enhancing productivity, but also by favouring species with traits that enable them to persist better under P limited conditions.