Application of fungistatics in soil reduces N uptake by an arctic ericoid shrub (Vaccinium vitis-idaea)

In arctic tundra soil N is highly limiting, N mineralization is slow and organic N greatly exceeds inorganic N. We studied the effects of fungistatics (azoxystrobin [Quadris (R)] or propiconazole [Tilt (R)]) on the fungi isolated from ericaceous plant roots in vitro. In addition to testing the phytotoxicity of the two fungistatics we also tested their effects on growth and nitrogen uptake of an ericaceous plant (Vaccinium uliginosum) in a closed Petri plate system without root-associated fungi. Finally, to evaluate the fungistatic effects in an in vivo experiment we applied fungistatics and nitrogen isotopes to intact tundra soil cores from Toolik Lake, Alaska, and examined the ammonium-N and glycine-N use by Vaccinium vitisidaea with and without fungistatics. The experiments on fungal pure cultures showed that Tilt was more effective in reducing fungal colony growth in vitro than Quadris (R), which was highly variable among the fungal strains. Laboratory experiments aiming to test the fungistatic effects on plant performance in vitro showed that. neither Quadris (R) nor Tilt (R) affected V uliginosum growth or N uptake. In this experiment V ufiginosum assimilated more than an order of magnitude more ammonium-N than glycine-N. The intact tundra core experiment provided contrasting results. After 10 wk of fungistatic application in the growth chamber V vitis-idaea leaf %N was 10% lower and the amount of leaf N-15 acquired was reduced from labeled ammonium (33%) and glycine (40%) during the 4 d isotope treatment. In contrast to the in vitro experiment leaf N-15 assimilation from glycine was three times higher than from (NH4)-N-15 in the treatments that received no-fungistatics. We conclude that the function of the fungal communities is essential to the acquisition of N from organic sources and speculate that N acquisition from inorganic sources is mainly inhibited by competition with complex soil microbial communities.