Nutrient limitation and botanical diversity in wetlands:: Can fertilisation raise species richness?

The 'resource balance hypothesis' proposes that the species richness of grassland vegetation is potentially highest when the N:P ratio of plant tissues is 10-15 (co-limitation), so that species richness could be raised by fertilisation with N or P at sites with lower or higher N: P ratios, respectively. Here we use data from field surveys in Swiss, Dutch and American fens or wet grasslands to analyse what changes in N: P ratios might produce noticeable changes in species richness. Plant species numbers, above-ground biomass, tissue N and P concentrations and soil pH were recorded in plots of 0.06-4m(2). In each data set, plots with intermediate tissue N: P ratios (6-20) were on average most species-rich, but N: P ratios explained only 5-37% of the variation in species richness. Moreover, these effects were partially confounded with those of vegetation biomass and/or soil pH. The unique effects of N: P ratios (excluding those shared with biomass and pH) explained 11-17% of variation in species richness. The relationship between species richness and N: P ratios was asymmetric: plots with high N: P ratios were more species-poor than those with low N: P ratios. This was paralleled by a smaller species pool size at high N:P ratios (estimated from species numbers in multiple records), suggesting that fewer species are adapted to P-limited conditions than to N-limited conditions. According to these data, species richness in wetlands may possibly be raised by P-fertilisation when the initial N: P ratio of the vegetation is well above 20, but this option is not recommended for nature conservation as it might promote common species at the expense of rare ones.