Plant and Microbial Uptake and Allocation of Organic and Inorganic Nitrogen Related to Plant Growth Forms and Soil Conditions at Two Subarctic Tundra Sites in Sweden

ABSTRACT 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 15N-labeling 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− > glycine, in contrast to the graminoid Carex species which took up added 15N at rates in the order NO3− > 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.