Elevated atmospheric CO2 and increased nitrogen deposition: effects on C and N metabolism and growth of the peat moss Sphagnum recurvum P. Beauv. var. mucronatum (Russ.) Warnst

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Abstract
Summary: Sphagnum bogs play an important role when considering the impacts of global change on global carbon and nitrogen cycles. Sphagnum recurvum P. Beauv. var. mucronatum (Russ.) was grown at 360 (ambient) and 700 μL L−1 (elevated) atmospheric [CO2] in combination with different nitrogen deposition rates (6, 15, 23 g N m−2 y−1), in a short‐ and long‐term growth chamber experiment.After 6 months, elevated atmospheric [CO2] in combination with the lowest nitrogen deposition rate, increased plant dry mass by 17%. In combination with a high nitrogen deposition rate, biomass production was not significantly stimulated.At the start of the experiment, photosynthesis was stimulated by elevated atmospheric [CO2], but it was downregulated to control levels after three days of exposure. Elevated [CO2] substantially reduced dark respiration, which resulted in a continuous increase in soluble sugar content in capitula. Differences in growth response among different nitrogen and CO2 treatments could not be related to measured carbon exchange rates, which was mainly due to interference of microbial respiration.Doubling atmospheric [CO2] reduced total nitrogen content in capitula but not in stems at all nitrogen deposition rates. Reduction in total nitrogen content coincided with a decrease in amino acids, but soluble protein levels remained unaffected. Thus, elevated [CO2] induced a substantial shift in the partitioning of nitrogen compounds in capitula. Soluble sugar concentration was negatively correlated with total nitrogen content, which implies that the reduction in amino acid content in capitula, exposed to elevated [CO2], might be caused by the accumulation of soluble sugars.Growth was not stimulated by increased nitrogen deposition. High nitrogen deposition, resulting in a capitulum nitrogen content in excess of 15 mg g−1 dw, was detrimental to photosynthesis, reduced water content and induced necrosis. We propose a capitulum nitrogen content of 15 mg g−1 dw as a possible bioindicator for the detection of nitrogen pollution stress in oligo‐mesotrophic peat bog ecosystems.At the lowest nitrogen deposition level, nitrogen recovery was higher than 100%, which indicates substantial dry deposition and/or gaseous nitrogen fixation by bacteria, associated with Sphagnum. Increasing nitrogen deposition rates decreased nitrogen recovery percentages, which indicates reduced efficiency of nitrogen fixation.