Cycling and retention of nitrogen in European beech (Fagus sylvatica L.) ecosystems under elevated fructification frequency

Abstract

Abstract. Atmospheric deposition of nitrogen (N) has exceeded its demand for plant increment in forest ecosystems in Germany. High N inputs increased plant growth, the internal N cycling within the ecosystem, the retention of N in soils and plant compartments, and the N output by seepage water. But the processes involved are not fully understood, especially the role of fructification which has increased in its frequency. A field experiment using 15N labelled leaf litter exchange was carried out over a 5.5 years' period at seven long-term monitoring sites with European beech (Fagus sylvatica L.) ecosystems to study the impact of current mast frequency on N cycling. Mean annual leaf litterfall contained 35 kg N ha−1, but about one half of that was recovered in the soil 5.5 years after the establishment of the leaf litter 15N exchange experiment. Retention of leaf litter N in the soil was more closely related to the production of total litterfall than to the leaf litterfall indicating the role of fructification of beech trees in the amount of leaf N retained in the soil. In these forests fructification occurred commonly in intervals of 5 to 10 years, which has now changed to every two to three years as observed during this study period. Seed cupules contributed 51 % to the additional litterfall in mast years which caused a high nutrient demand during their decomposition due to their very high carbon (C) to N and C to phosphorus (P) ratios. Higher mast frequency increased the mass of mean annual litterfall by about 0.5 Mg ha−1 and of litterfall N by 8.7 kg ha−1. Mean net primary production (NPP) increased by about 4 %. Mean total N retention in soils calculated by input and output fluxes was unrelated to total litterfall indicating that mast events were not the primary factor controlling total N retention in soils. Despite reduced N deposition since the 1990s about 5.7 kg N ha−1 out of 20.7 kg N ha−1 deposited annually between 1994 and 2008 were retained in soils notably at acid sites with high N / P and C / P ratios in the organic layers and mineral soils. Ongoing N retention increased the N / P ratios in acid soils with moder type humus forms and reduced the availability of P for plant growth and litter decomposition. Trees retained twice as much N compared to soils by biomass increment particularly in less acid stands where the mineral soils had low C / N ratios.