Juglone (5-hydroxy-1,4 napthoquinone) and soil nitrogen transformation interactions under a walnut plantation in southern Ontario, Canada

Juglone (5-hydroxy-1,4-naphthoquinone), a chemical substance produced by bl ack walnut (Juglans nigra L.), inhibits the growth and existence of some be neficial soil microorganisms, especially Frankia spp. isolate ArI3 and Rhiz obium japonicum. However, no studies to date have reported on the effect of juglone on soil ammonification and nitrification. A field study and labora tory incubation study to investigate this were conducted. In the field, in situ soil ammonification and nitrification were measured within and outside of a 60-year-old black walnut plantation and a eight-year-old poplar (Popu lus spp. clone DN 177) plantation. In the lab, soil (Sandy Fox Loam), colle cted in the absence of black walnut trees, was incubated for periods of one to six weeks in the presence of varying concentrations of juglone. In the field, peak summer mean nitrate accumulation rates in soils within the blac k walnut and poplar plantation were 163 and 95 mu g 100 g(-1) dry soil day( -1) respectively and in soils outside the plantations, 104 (black walnut) a nd 78 (poplar) mu g 100 g(-1) dry soil day(-)1 respectively. No accumulatio n of ammonium at the end of the incubation period was noted. Therefore, no inhibition effect of juglone on nitrification in the field was observed, an d laboratory incubation results confirmed the results of the field study. R esults from these studies should address concerns about nitrification inhib ition under walnut based intercropping systems.