Soil nitrogen transformations on a subantarctic island

Abstract

The vascular vegetation of a mire-grassland community on Marion Island (47°S, 38°E) takes up c. 158 mg N m⁻² d⁻¹ in summer. Bryophytes take up c. 36 mg N m⁻² d⁻¹ during their peak growth period. Since inputs of N through precipitation and biological fixation are negligible, mineralization of organic N must have supplied the bulk of this N. From changes in peat inorganic N levels and rates of uptake by the vegetation we estimate mean mineralization rates of 178 mg N m⁻² d⁻¹ in summer and 55 mg N m⁻²d⁻¹ in winter. In situ incubationof peat give a maximum mineralization rate of 48 mg N m⁻² d⁻¹. At this rate the small (700 mg m⁻²) pool of available N in the upper 25 cm of peat would be depleted by the vascular vegetation in about seven days and bryophytes would deplete the available N pool in the top 25 mm in two days. Hence the rate of N mineralization measured by incubation is much too low to account for the fluctuations in concentrations of inorganic N in the peat and the amounts taken up by the vegetation. This may be due to losses through denitrification or to the fact that soil macroinvertebrates were excluded from the incubation.