Critical foliar N:P ratios as indices of nutrient limitation in terrestrial ecosystems

Background/Question/Methods Nutrient availability commonly limits net primary productivity.  In marine ecosystems, the C:N:P ratio of planktonic communities has been found to be quite constant at the global scale and variations from this ratio are widely used to diagnose nutrient limitation.  A series of factorial fertilization studies in European wetlands found that N- limited sites were characterized by mass based N:P ratios of ≤ 14, while ratios of  ≥16 indicated P-limited sites.  These values have since been used to indicate the nature of nutrient limitation in terrestrial ecosystems ranging from temperate grasslands to tropical rainforests.  I used data from factorial fertilization studies in the literature to test the effectiveness of these critical ratios as indices of nutrient limitation at the community scale across a number of terrestrial biomes.  

Results/Conclusions .  The final dataset included results from 25 different studies representing 42 individual sites. Nitrogen to phosphorus ratios on a mass basis varied between 2.3 and 29 and increased from N-limited sites to P-limited ones, though the differences were not statistically significant (13.2 ± 1.6, 12.5 ± 1.7 and 15.3 ± 2.3 for N-limited, co-limited and P-limited respectively, P = 0.76, ANOVA). Using mass-based critical ratios of 14 and 16 resulted in the misidentification of the limiting nutrient over half the time. These results suggest that N:P ratios in terrestrial ecosystems reflect the influences of regional patterns in soil development as well as plant physiological strategies at multiple timescales.