Microbial nitrogen cycling interacts with exogenous nitrogen supply in affecting growth of Sphagnum papillosum.

We assessed whether nitrogen (N) cycling interacts with exogenous N supply in controlling Sphagnum growth, by means of two laboratory experiments on Sphagnum papillosum gametophores. In a first experiment, the plants were cultured under a factorial combination of three factors: presence/absence of dead tissue attached to the living plant parts, presence/absence ofNsupply and twowater levels. In a second experiment, the plants were cultured under a factorial combination of two factors: presence/absence of exogenous N supply and presence/absence of sodium hypochlorite addition. The second experiment was directed to assess the importance of microbial activity. Sphagnum papillosum grew more when capitulum N concentrations were higher. Nitrogen concentration in the capitula was enhanced by high water level and by the presence of dead tissue even when no exogenous N was added. Those conditions were suitable for N to be recycled within Sphagnum tissues, through microbial attack of organic material in the senescing plant tissues and subsequent transport of N containing compounds to the growing capitulum. Nitrogen addition stimulated Sphagnum growth only when environmental conditions hindered microbial activity. The effects of N addition on Sphagnum growth were also influenced by tissue hydration. In fact, dry plants did not take up most of the N added. Under increasing atmospheric N deposition, dry growing seasons are expected to enhance N availability in the rhizosphere, thus favouring vascular plants at the expense of Sphagnum mosses.