Proefschrift KWR

Balance matters N:P stoichiometry and plant diversity in grassland ecosystems


“Increased N deposition may change species composition in grassland communities by shifting
them to P limitation. Interspecific differences in P uptake traits might be a crucial yet poorly
understood factor in determining the N effects. To test the effects 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 different N supply levels. Furthermore, to test if the N effects on
P uptake was due to N availability alone or altered N:P ratio, we examined several uptake traits
(root-surface phosphatase activity, specific 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. These species had neither
higher SRL, RMR, phosphatase activity per unit root (Paseroot), nor higher total phosphatase
activity (Pasetot: Paseroot times root mass), but higher relative phosphatase activity (Paserel: Pasetot
divided by biomass) than other species. The species common from P-limited grasslands had high
Paserel. P uptake from bound-P was positively correlated with Pasetot for grasses. High N supply
stimulated phosphatase activity but decreased RMR and SRL, resulting in no increase in P uptake
from bound-P. Paseroot was influenced by N:P supply ratio, rather than by only N or P level,
whereas SRL and RMR was not dominantly influenced by N:P ratio. We conclude that increased
N stimulates phosphatase activity via N:P stoichiometry effects, which potentially increases plant
P uptake in a species-specific 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.”

(Citaat: Fujita, Y. – Balance matters N:P stoichiometry and plant diversity in grassland ecosystems, 2010)

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