Biomass N:P ratios as indicators of nutrient limitation for plant populations in wetlands

The conservation or restoration of seminatural vegetation often involves measures that influence the availability of nutrients and consequently the plant species composition., The ability to predict effects of modified nutrient availability on species composition would therefore help to choose appropriate management strategies. The aim of this study was to test whether short-term effects of nitrogen or phosphorus enrichment can be predicted from nutrient ratios in plant biomass. At 11 species-rich sites in,Dutch fens and dune slacks, small plots Were fertilized with N, P, N + P, or not fertilized (control). The aboveground biomass, N and P concentrations, and N:P ratios, were compared between fertilized and control plots for all sufficiently abundant plant populations in the summers preceding and following fertilization. Of 121 populations, only 45 had their biomass enhanced significantly by fertilization. Populations enhanced by P fertilization had on average higher biomass N:P ratios than those enhanced by N,. due to higher N and lower P concentrations. However, N:P ratios did not differ among populations enhanced by N, N + P, or neither nutrient. High biomass N:P ratios (>20) therefore indicated that a plant population would probably respond to P-fertilization, but low or intermediate N:P ratios (<20) did not indicate whether the population would respond to N or N + P fertilization. The predictive value of N:P ratios was not higher if only species from one site or only species with the same growth form were considered. It is concluded that N:P ratios are not suitable to predict how changed nutrient supply will affect plant species composition, especially in the long term. However, results suggest that biomass N:P ratios do reflect the relative availability of N and P to plants and may indicate the degree of N or P deficiency experienced by a plant population even more reliably than fertilization experiments. N:P ratios can be used to investigate how the relative availability of N and P influences various ecological processes and how it is affected by human. impacts or management.