Fig. 6. Potassium chloride-extractable and microbial biomass ¹⁵N values measured before the isotopic tracer addition (18 Feb. 2004), during snowmelt (4 Mar. 2004), and in midsummer (12 July 2004). Different lowercase letters indicate significant differences in KCl-extractable ¹⁵N and microbial biomass ¹⁵N among the three sampling dates ( = 0.05). Error bars indicate the standard error of the group means.