PLANT-MICROBE COMPETITION FOR SOIL AMINO-ACIDS IN THE ALPINE TUNDRA -EFFECTS OF FREEZE-THAW AND DRY-REWET EVENTS

Amino acids have been shown to be a potentially significant N source f or the alpine sedge, Kobresia myosuroides, We hypothesised that freeze -thaw and dry-rewet events allow this plant species increased access t o amino acids by disrupting microbial cells, which decreases the size of competing microbial populations, but increases soil amino acid conc entrations. To test this hypothesis, we characterized freeze-thaw and dry-rewet events in the field and simulated them in laboratory experim ents on plant-soil microcosms. In one experiment: N-15,C-13-[2]-glycin e was added to microcosms that had previously been subjected to a free ze-thaw or dry-rewet event, and isotopic concentrations in the plant a nd microbial fractions were compared to non-stressed controls. Microbi al biomass and uptake of the labeled glycine were unaffected by the fr eezing and drying treatments, but microbial uptake of N-15 was lower i n the two warmer treatments (dry-rewet and summer control) then in the two colder treatments (freeze-thaw and fall control). Plant uptake of glycine-N-15 was decreased by climatic disturbance, and uptake in pla nts that had been frozen appeared to be dependent on the severity of t he freeze. The fact that intact glycine was absorbed by the plants was confirmed by near equal enrichment of plant tissues in C-13 and N-15. Plants under optimal conditions recovered 3.5% of the added N-15 and microbes recovered 5.0%. The majority of the C-13 and N-15 label remai ned in a non-extractable fraction in the bulk soil. To better understa nd the isolated influences of environmental perturbations on soil amin o acid pools and population sizes of amino-acid utilizing microbes, se parate experiments were performed in which soils, alone, were subjecte d to drying and rewetting or freezing and thawing. Potential respirati on of glycine and glutamate (substrate-induced respiration; SIR) by th e soil microbial communities was unaffected by a single freeze-thaw ev ent. Glycine SIR was decreased slightly (similar to 10%) by the most e xtreme drying treatment, but glutamate SIR was not significantly affec ted. Freezing lowered the concentration of water-extractable amino aci ds while drying increased their concentration. We interpret the surpri sing former result as either a decrease in proteolytic activity in fro zen soils relative to amino acid uptake, or a stimulation in microbial uptake by physical nutrient release from the soil. We conclude that c limatic disturbance does not provide opportunities for increased amino acid uptake by K. myosuroides, but that this plant competes well for amino acid N under non-stressed conditions? especially when soils are warm. We also note that this alpine tundra microbial community's high resistance to freeze-thaw and dry-rewet events is novel and contrasts with studies in other ecosystems.