Bogs are globally important sinks of atmospheric carbon (C) due to the accumulation of partially decomposed litter that forms peat. Because bogs receive their nutrients from the atmosphere, the world-wide increase of nitrogen (N) deposition is expected to affect litter decomposition and, ultimately, the rate of C accumulation. However, the mechanism of such biogeochemical alteration remains unclear and quantification of the effect of N addition on litter accumulation has yet to be done. Here we show that seven years of N addition to a bog decreased the C:N ratio, increased the bacterial biomass and stimulated the activity of hydrolytic and oxidative enzymes in surface peat. Furthermore, N addition modified nutrient limitation of microbes during litter decomposition so that phosphorus became a primary limiting nutrient. Alteration of N release from decomposing litter affected bog water chemistry and the competitive balance between peat-forming mosses and vascular plants. We estimate that deposition of about 4 g N m-2 yr-1 will cause a mean annual reduction of fresh litter C accumulation of about 40 g m-2 primarily as a consequence of decreased litter production from peat-forming mosses. Our findings show that N deposition interacts with both above and below ground components of biodiversity to threaten the ability of bogs to act as C sinks, which may offset the positive effects of N on C accumulation seen in other ecosystems

Increasing rates of atmospheric nitrogen (N) deposition may reduce growth and accelerate decomposition of Sphagnum mosses in bogs. Sphagnum growth and rates of Sphagnum litter decomposition may also vary because of climate change as both processes are controlled by climatic factors. The initial purpose of this study was to assess if growth and litter decomposition of hummock and lawn Sphagnum species varied with increasing N input in a factorial mid-term (2002)(2003)(2004)(2005) experiment of N and phosphorus (P) addition, in a bog on the southern Alps of Italy. However, as the experimental period was characterized by an exceptional heat wave in summer 2003, we also explored the interacting effects of fertilization and strongly varying climate on growth and decomposition rates of Sphagnum. The heat wave implied strong dehydration of the upper Sphagnum layer even if precipitation in summer 2003 did not differ appreciably from the overall mean. Sphagnum production was somewhat depressed by high levels (3 g m À2 yr À1 ) of N addition without concomitant addition of P presumably because of nutrient imbalance in the tissues, but production rates were much lower than the overall means in 2003, when no effect of nutrient addition could be observed. Adding N at high level also increased the potential decay of Sphagnum litter. Higher CO 2 emission from N-fertilized litter was due to amelioration of litter chemistry showing lower C/N quotients in the N-fertilized treatments. Rates of CO 2 emission from incubated litter also were more strongly affected by water content than by nutrient status, with practically no CO 2 emission detected when litter was dry. We conclude that higher rates of atmospheric N availability input may depress Sphagnum growth because of P, and presumably potassium, (co-)limitation. Higher N availability is also expected to promote potential decay of Sphagnum litter by ameliorating litter chemistry. However, both effects are less pronounced if the growing Sphagnum apex and the underlying senescing tissues dry out.

Summary• Nitrogen and phosphorus were added experimentally in a bog in the southern Alps. It was hypothesized that alleviating nutrient limitation will increase vascular plant cover. As a consequence, more carbon will be fixed through higher rates of net ecosystem CO 2 exchange (NEE).• The vascular cover did increase at the expense of Sphagnum mosses. However, such vegetation changes were largely independent of the treatment and were probably triggered by an exceptional heatwave in summer 2003.• Contrary to the tested hypothesis, NEE was unaffected by the nutrient treatments but was strongly influenced by temperature and water-table depth. In particular, ecosystem respiration in the hot summer of 2003 increased dramatically, presumably owing to enhanced heterotrophic respiration in an increased oxic peat layer.• At the end of the experiment, the Sphagnum cover decreased significantly in the nitrogen-fertilized treatment at hummock microhabitats. In the long term, this will imply a proportionally greater accumulation of vascular litter, more easily decomposable than the recalcitrant Sphagnum litter. As a result, rates of carbon fixation may decrease because of stimulated respiration.

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Summary1 A 3-year experiment involving nutrient addition and removal of one of two coexisting dwarf shrub species was conducted in two community types in a subalpine heathland on the northern Apennines (Italy). Vaccinium uliginosum occurred at all sites but was associated with the deciduous Vaccinium myrtillus at more sheltered nutrient-rich sites (HV community), and with the evergreen Empetrum hermaphroditum where the habitat was poorer and more exposed (EV community). Length of current-year shoots and fruit production of each species were determined in each of the 3 years, and standing crop at the beginning and end of the experiment.2 The length of current-year shoots of both deciduous species, but not of the evergreen, varied considerably between years, presumably due to varying temperatures at the beginning of the three growing seasons. Fruit production also varied. 3 Fertilization promoted an increase in the length of V. uliginosum shoots at the HV community but not at the EV community. The removal of V. uliginosum enhanced shoot elongation in V. myrtillus (HV community) but reduced shoot elongation in E. hermaphroditum (EV community). Neighbour removal did not aect shoot length of V. uliginosum at either community. There were few treatment eects on fruiting of these clonal species. 4 The standing crop in untreated stands did not change during the experimental period. Changes in shoot length resulting from environmental manipulations were not accompanied by consistent variation in the standing crop of any species. Standing crop increased only for V. myrtillus after removing V. uliginosum at the HV community (same direction as shoot length). The standing crop of E. hermaphroditum did not change after removing V. uliginosum at the EV community, although shoot length was signi®cantly reduced. The standing crop of V. uliginosum was unaected by neighbour removal and was decreased by nutrient addition at both communities. 5 None of the shrubs appeared able to utilize more abundant resources to increase above-ground biomass. However, increased shoot length in nutrient-rich habitats gave V. uliginosum a superior ability to capture light compared with V. myrtillus, the latter being more successful when the coexisting competitively superior species was removed. This would support Grime's theory, indicating that competition becomes more important as soil resource levels increase. Positive interactions appeared to play a role in the more extreme habitat, where E. hermaphroditum normally bene®ted from the shelter of the V. uliginosum canopy but was able to adapt its architecture when exposed.