Nitrogen in Two Contrasting Antarctic Bryophyte Communities

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Abstract
(1) For 18 months, from October 1978 until February 1980, studies were conducted on the nitrogen inputs and outputs of two contrasting moss-dominated communities on Signy Island, South Orkney Islands [Antarctic]: a semi-ombrogenous dry turf and a soligenous wet carpet. (2) The dry turf was more acidic than the wet carpet (pH 4.3 v. 5.0) and had a lower water content (564 v. 869%, dry wt basis). The turf had lower concentrations per unit dry weight of total nitrogen, phosphorus and potassium (0.79, 0.07 and 0.11%, respectively) than the wet carpet (2.17, 0.53 and 0.16%, respectively). Summer concentrations of extractable inorganic nitrogen were lower in the dry turf (0.22 v. 0.50 mg n (100 g)-1 dry wt peat), but the difference was not significant. Accumulated winter snow overlying the dry turf and wet carpet in October 1979 contained 39 and 42 .mu.g N l-1, respectively. This snow melted and subsequent snow cover, which had accumulated 4 weeks later, contained 113 and 83 .mu.g N l-1, respectively. These higher nitrogen concentrations were probably due to early summer activity by nearby penguins. Melt-water and pools on the surface of the sites in December 1979 contained 230 and 165 .mu.g N l-1 on the dry turf and wet carpet, respectively. (3) Numbers fo sulphate-reducing bacteria (Desulfovibrio and Desulfotomaculum) and clostridia were very low, even in the wet carpet which contained 150 sulphate-reducers and 290 clostridia (100 g)-1 dry wt peat. Thus, although cultures of these bacteria showed acetylene reduction activity, their very low abundance indicates that heterotrophic dinitrogen fixation is unlikely to be important in these communities, especially since no azotobacters were detected. No nitrifying bacteria were found, but substantial numbers of proteolytic and nitrate-respiring bacteria and a small number of denitrifying bacteria occurred. All heterotrophic groups studied were more abundant in the wet carpet than in the dry turf. (4) Cultures of the cyanobacterium Nostoc muscorum from both sites showed high acetylene reduction activity at 15.degree. C. Moss shoots from the dry turf and wet carpet produced ethylene at rates of 0.12 and 0.14 mmol g-1 dry wt h-1, respectively, at 15.degree. C. Epiphytic cyanobactria were probably the main nitrogen-fixing organisms present. (5) Calculated inorganic nitrogen inputs from biological nitrogen fixation and precipitation (including penguin activity) were 45.9 and 64.1 mg m-2 year-1 (dry turf) and 192.4 and 65.1 mg m-2 year-1 (wet carpet).