Seasonal patterns of juglone in soil beneathJuglans nigra (black walnut) and influence ofJ. nigra on understory vegetation.

The allelopathic nature ofJ. nigra L. was investigated in several planted mixed hardwood stands located near Syracuse, New York. Concentrations of chloroform-extracted juglone from soil collected beneathJ. nigra was determined by thin-layer chromatography (TLC) and high-pressure liquid chromatography (HPLC). Soil juglone concentrations were corrected based on recovery of synthetic juglone added to soil. Soil juglone levels were high in the spring, decreased during the summer, and were high again in the fall. The quantification of juglone from soil by HPLC was found to be more accurate than by TLC. Regression analysis indicated that individual tree variation in soil juglone levels could not be explained by differences in soil moisture, pH, organic matter content, and texture. The results of juglone recovery experiments suggest that chloroform-extractable juglone does not persist in soil. Juglone degradation by microorganisms could only explain a portion of the juglone decline. Ordinations revealed that the herbaceous and woody vegetation beneathJ. nigra, in comparison to vegetation beneathAcer saccharum andQuercus rubra, is distinct in only one of the four stands studied. This vegetational difference did not appear to be a consequence of any strong allelopathic influences ofJ. nigra (Scheffe's method of contrast, chi-square analysis). The allelopathic nature of juglone under these field conditions is questionable.