Rapid 15N uptake and metabolism in fine roots of Norway spruce

Short-term N uptake by Norway spruce roots was investigated after tracer application both in the field and in the laboratory. In the forest the influence of wood ash or liquid fertiliser treatments on the uptake was studied. A rapid uptake of the 15NH415NO3 tracer into the fine roots was observed. Within 1 day of application about 50% of the maximum δ15N value observed was measured and within 1 week as much as 70–90%. The 15N enrichment in fine roots was not affected by wood ash and liquid fertiliser treatments applied in the previous year. δ15Ν increased continuously until 2 months after 15N application and decreased to 60% of its maximum value within 1 year. Nine months after the tracer application, an analysis of δ15N distribution throughout the root system was conducted. The δ15N values were highest in roots of the topsoil compared to roots at deeper soil layers, and higher in fine roots than in larger roots. The 15N was not translocated within the root system. The laboratory experiment, using Norway spruce seedlings, aimed at describing short-term 15NO3 uptake and assimilation into free amino acids with 2 mM or 20 mM nitrate application. The 15N was detected within 4 h to 1 day in the amino acids Glu, Gln, Asp of roots, while in Asn, Ala and Ser and in the shoots, the 15N was retrieved at the earliest after 1 day. The enrichment factor in the amino acids increased to a maximum within 3–7 days, depending on the nitrate concentration applied.