A cross-over fertilization experiment was carried out in Dutch floating fens to investigate effects on biomass production in the same and the following years. In total 16 fertilizer treatments were applied, combining four treatments in 1999 with four treatments in 2000 (addition of 20 g.m⁻² N, 5 g.m⁻² P, both elements and unfertilized control). The above-ground biomass production of vascular plants was co-limited by N and P in both years. However, in plots that were only fertilized in 1999 the effects of individual nutrients differed between the two years: N-fertilization slightly increased the amount of biomass produced in the same year (1999), whereas P-fertilization did so in the following year (2000). Fertilizer applied in 1999 also influenced the effects of fertilizer applied in 2000. One year after N-fertilization vascular plant growth was still co-limited by N and P, but one year after P-fertilization, vascular plant growth was only limited by N. Bryophyte biomass responded weakly to fertilization. Nutrient concentrations in plant biomass, nutrient standing crops and measurements of N and P availability in the soil indicated that one year after fertilization, the N-fertilizer had mostly ‘disappeared’ from N-fertilized plots, whereas the availability of P remained markedly enhanced in P-fertilized plots. In addition, P-fertilization enhanced the uptake of N by plants the following year. The time-dependence of fertilizer effects was probably caused by (1) higher addition of P than of N relative to the requirements of plants; (2) longer retention of P than of N in the system; (3) positive effect of P-fertilization on the availability of N; (4) contrasting effects of N- and P-fertilization on nutrient losses by plants and/or on their responses to subsequent nutrient addition; (5) changing interactions between vascular plants and mosses (mainly Sphagnum spp.); (6) nutrient export through the repeated harvest of above-ground biomass. To determine which nutrient limits plant growth fertilization experiments should be short, avoiding that indirect effects of a non-limiting nutrient influence results. To indicate how changed nutrient supply will affect an ecosystem longer-term experiments are needed, so that indirect effects have time to develop and be detected.

Nomenclature: Tutin et al. (1964–1993).