Effect of N:K supply ratios on the performance of three grass species from herbaceous wetlands

Shifts from N limitation to P or K limitation in wetlands (e.g. due to mowing and high atmospheric N deposition) are reflected by increased N:P and N:K ratios of plant biomass and changes in species composition. So far, the implications of increased N:K ratios for wetland vegetation have hardly been investigated. We examined how the supply of N and K influences the growth of three wetland grass species (Holcus lanatus, Anthoxanthum odoratum, Deschampsia caespitosa) to determine at what N:K ratios growth is likely to be K limited, how N:K ratios influence biomass allocation and nutrient uptake, and whether the responses to high N:K ratios vary among species. Plants were grown in sand at six N: K supply ratios ranging from 0.17 to 40.5 and combined with two levels of fertility in a factorial design. In 15 weeks of growth, plant biomass increased with increasing N:K supply ratios, indicating that growth was mostly limited by N and not by K across the entire range of N:K ratios. However, there were indications of K deficiency at the highest N:K supply ratio, such as increased leaf mortality and strong reduction of K concentrations during leaf senescence. The response of total plant biomass to nutrient treatments did not differ among the three species. However, other plant traits, such as biomass allocation to roots, leaf senescence and N concentrations, Suggested that D. caespitosa is better adapted to extremely low K availability than the other two species. The short-term responses of the three wetland grass species to N and K supply were similar to those found in previous experiments testing different N:P supply ratios. In both cases, growth depended mainly on N supply, despite high N:K or N:P supply ratios and very low K or P concentrations in plant biomass. In those previous experiments, P supply became more important in the second year. There are indications that the same could also be true for K in a longer-term experiment. Hence, the so-called 'critical' values for N:K ratios of plant biomass in the field might be lower than indicated by our short-term experiment. (C) 2009 Gesellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved.