Repeated freeze-thaw cycles (FTC) in the soil in terrestrial ecosysms with arctic and subarctic climate regulates soil processes and release pulses of nutrients to the soil from lyced cells and disrupted soil aggregates. In this study pulses of nutrients were injected into subarctic soil in situ to mimic a pulse release of nutrients after FTC.

Uptake of nitrogen (N) by mycorrhizal and non-mycorrhizal plants and by soil microorganisms was investigated to reveal acquisition patterns for organic and inorganic nitrogen sources at a subarctic heath. 15N labelled NH4+ or the amino acids glycine and glutamic acid were injected into the soil at 1 cm or 5 cm depth. Samples were analyzed for 15N-enrichment in the soil water extracts of dissolved and microbial fractions and in total soil after 21 days and in leaves of plants species sampled after three, five and 21 days.

Soil microorganisms had very large 15N uptake from all the N sources compared to all plant species, hence, it was dominantly soil microorganisms that benefitted from the pulse release, in parallel suggesting large benefit relative to that of plants, at FTC events. The soil microorganisms incorporated most 15N from the glutamic acid source, intermediate amounts of 15N from the glycine source and least 15N from the NH4+ source, with the precaution that the injected N form may have been mineralized prior to uptake.

In contrast to microorganisms, all ten plant species generally had higher 15N uptake from the NH4+ source than from the amino acid sources regulated by the soil microbial 15N uptake. Non-mycorrhizal plant species had higher 15N uptake than mycorrhizal plant species three days after labelling. Eventually 21 days after labelling, the mycorrhizal plant species had larger uptake of amino acids and even uptake of 15NH4 relative to non-mycorrhizal plant species. In conclusion soil microorganisms are more opportunistic and ready for rapid uptake of pulse released nitrogen, while all the investigated plant species are second to acquire the free nitrogen, with a delayed but eventually larger uptake in the mycorrhizal plant species.