In order to follow the uptake and allocation of N in different plant functional types and

microbes in two tundra ecosystems differing in nutrient availability, we performed a 15Nlabeling

experiment with three N forms and followed the partitioning of 15N label

among plants, microorganisms and soil organic matter. At both sites the deciduous

dwarf shrub Betula nana and the evergreen Empetrum hermaphroditum absorbed added

15N at rates in the order: NH4

+ . NO3

2 . glycine, in contrast to the graminoid Carex

species which took up added 15N at rates in the orderNO3

2 . NH4

+ . glycine. Carex

transported a high proportion of 15N to aboveground parts, whereas the dwarf shrubs

allocated most 15N to underground storage. Enhanced 13C in Betula nana roots

represents the first field evidence of uptake of intact glycine by this important

circumpolar plant. Plant and microbial uptake of label was complementary as plants

took up more inorganic than organic N, while microbes preferred organic N. Microbes

initially took up a large part of the added label, but over the following four weeks

microbial 15N decreased by 50% and most 15N was recovered in soil organic matter,

while a smaller but slowly increasing proportion was retained in plant biomass.