PRODUCTION AND NUTRIENT DYNAMICS OF PLANT-COMMUNITIES ON A SUB-ANTARCTIC ISLAND .5. NUTRIENT BUDGETS AND TURNOVER TIMES FOR MIRE-GRASSLANDS, FJAELDMARK AND FERNBRAKES

Studies of plant standing crop and of the nutrient concentrations in precipitation, soils and plants have enabled an assessment of the inter- and intra-system nutrient flows for five plant communities at Marion Island (46.degree.54''S, 37.degree.45'' E). These communities, which are representative of those occupying more than 90% of the island''s lowland (below 300 m above sea level) were : a fjaeldmark on a rocky plateau (dominated by the cushion plant Azorella selago), an open fernbrake and closed fernbrake (both dominated by the fern Blechnum penna-marina)and two mire-grasslands (on very wet peats and dominated by graminoid and bryophyte species). Annual net primary production (ANP) at the five communities was high and substantial quantities of nutrients were taken up annually by the vegetation. N (6.5 to 24.8 g m-2 year-1) was the element taken up from the soil in the largest quantities, despite the fact that instantaneous values of available N pools were exceptionally low (0.003 to 0.69 g m-2 to 25 cm depth). Either K (3.5 to 9.9 g m-2 year-1) or Ca (1.7 to 9.7 g m-2 year-1) was taken up in the second largest amount. Net quantities of nutrients translocated into the annual aboveground growth of vascular plants were, except for K and Na, greater than the seasonal mean standing stocks in the aerial biomass. Net translocation estimates ignored leaching losses from the biomass. Nutrient turnover times in the total (living plus dead, above- and belowground) vegetation were between 1 and 4 years, lower than for most Northern Hemisphere tundra communities. The quantities of nutrients in circulation were mostly less than 3% of their total pool (plants plus soil) sizes, except for K (13 to 26%) and, in four of the communities, Mg (6 to 15%). Precipitation inputs of N, K, Ca and Mg were considerably lower than the amounts required in the ANP. No P occurred in the precipitation. Biological fixation of N was much less than the precipitation input. The vascular plant species appear to be less efficient in conserving N through back-translocation from senescing photosynthetic tissue than are most plants of similar life forms from northern hemisphere tundra and tundra-like areas. Only 11 to 30% of the N taken up into the annual aboveground growth was back-translocated before or during senescence. Back-translocations of P (39 to 71%) and K (71 to 96%) were greater . However, all of these estimates ignore leaching losses. Despite the apparently poor ability of the plants to back-translocate N, the total "nutrient costs" of the aboveground ANP at the five sites (14 to 32 mg per g m-2 ANP) were very much in the lower part of the range reported for a wide variety of vegetation types. Nutrient costs of the ANP for the mire-grassland communities were especially low, mainly because of low requirements for Ca and Mg. In view of the small soluble and available pools of some nutrients (especially N and P) and the substantial nutrient requirement for the ANP, it is concluded that net nutrient mineralization in decomposition and nutrient absorption by the vegetation are closely coupled.