Mechanisms to overcome ecosystem nitrogen and phosphorus limitation

Abstract

Nutrients limit plant growth in many ecosystems, and many species develop adaptations to either obtain more nutrients or to avoid losses. Much past ecological work has focused either on temperate nitrogen-limited systems or has addressed infertile soils as a single entity, rather than addressing the nature of the limitation. Although low nitrogen (N) and phosphorus (P) soils are found worldwide, the consequences of N and P limitation for plants may vary due to fundamental differences in the soil mobility, availability, uptake, and cellular uses of N and P. Using a well-studied chronosequence in the Hawaiian Islands, which contains both low N and P soils but which is dominated by similar plant communities, foliar N and P plant responses to fertilization were examined across a diverse group of species and life forms. Here it is shown that in response to N fertilization, foliar N concentrations do not significantly increase. In contrast, P fertilization leads to highly elevated foliar P concentrations and lower N:P values. This accumulation of P is evident across all species, occurs regardless of site fertility, results in increased concentrations of P in both inorganic and organic forms, and may be highly advantageous due to the patchy nature of soil P availability. Results indicate that greater variation in foliar P than foliar N after fertilization transcends soil nutrient availability and may be widespread among ecosystems. Such P accumulation means that N and P concentrations may not be in close association after pulses of soil P. It also provides a mechanism whereby ecosystem P limitation may be overcome with external inputs more easily than N limitation and points for the need to think about N and P limitation not just as infertile stressful conditions for plants, but as two fundamentally different phenomena.