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. Atmospheric deposition of nitrogen (N) has exceeded itsdemand for plant increment in forest ecosystems in Germany. High N inputsincreased plant growth, the internal N cycling within the ecosystem, theretention of N in soil and plant compartments, and the N output by seepagewater. But the processes involved are not fully understood, notably theeffect of fructification in European beech (Fagus sylvatica L.) on N fluxes. The frequencyof fructification has increased together with air temperature and Ndeposition, but its impact on N fluxes and the sequestration of carbon (C) and N insoils have been hardly studied. A field experiment using 15N-labeled leaflitter exchange was carried out over a 5.5-year period at seven long-termEuropean beech (Fagus sylvatica L.) monitoring sites to study the impact of current mastfrequency 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 yearsafter the establishment of the leaf litter 15N exchange experiment. Inthese forests, fructification occurred commonly at intervals of 5 to 10 years, which has now changed to every 2 years as observed during thisstudy period. Seed cupules contributed 51 % to the additional litterfallin mast years, which creates a high nutrient demand during their decompositiondue to the very high ratios of C to N and C to phosphorus (P).Retention of leaf litter 15N in the soil was more closely related tothe production of total litterfall than to the leaf litterfall, indicatingthe role of seed cupules in the amount of leaf N retained in the soil.Higher mast frequency increased the mass of mean annual litterfall by about0.5 Mg ha−1 and of litterfall N by 8.7 kg ha−1. Mean net primaryproduction (NPP) increased by about 4 %. Mean total N retention in soilscalculated by input and output fluxes was unrelated to total litterfall,indicating that mast events were not the primary factor controlling total Nretention in soils. Despite reduced N deposition since the 1990s, about 5.7 out of 20.7 kg N ha−1 deposited annually between 1994and 2008 was retained in soils, notably at acid sites with high N/P and C/Pratios in the organic layers and mineral soils, indicating P limitation forlitter decomposition. Trees retained twice as much N compared to soils bybiomass increment, particularly in less acidic stands where the mineral soilshad low C/N ratios. These results have major implications for ourunderstanding of the C and N cycling and N retention in forest ecosystems.In particular the role of mast products in N retention needs more research inthe future.

biogeosciences

Cycling and retention of nitrogen in European beech (&amp;lt;i&amp;gt;Fagus sylvatica&amp;lt;/i&amp;gt; L.) ecosystems under elevated fructification frequency