European alder, black walnut

European alder has been observed to decline at age 8 years in plantings when mixed with black walnut. It has been surmised that black walnut allelopathy was the most likely cause for the black alder decline (25). 

To gain more understanding of the European alder decline and because of our concern about the future growth of black walnut planted with nitrogen-fixing species, a study was initiated to measure soil juglone concentration and to estimate the number of Nitrobacter and Nltrosomonas bacteria in a black walnut plantation containing plots of black walnut alone and in mixture with European alder and autumn-olive. 

The mean juglone concentration in soil beneath plots of autumn-olive/ black walnut was significantly lower than in soil beneath European alder/black walnut and black walnut-alone plots (Table I). Juglone concentrations also differed with sampled depth (Table I). Significantly higher concentrations were present at the 0-8 cm depth than at lower depths. 

Soil beneath European alder/black walnut plots had higher nitrate nitrogen levels than soils beneath autumn-ollve/black walnut or walnut-alone plots (Table III). Mean total nitrogen did not differ significantly between treatments. 

Mean organic matter was greatest in the European alder/black walnut treatment, followed by the walnut-alone and autumn-olive/black walnut treatments (Table III). The mean pH was lowest in the autumn-olive/black walnut treatment, followed by the European alder/black walnut and walnut-alone treatments. 

Treatment means for pH and organic matter varied significantly between treatments, except for the pH between European alder/black walnut and walnut-alone. According to Alexander (32), nitrification is almost negligible at pH 5.0. Nitrification in soil beneath red alder (Alnus rubra Bong.) was reported at a pH as low as 3.5 (33). Obviously nitrification has not been eliminated in soil beneath treatment plots in our study. 

The almost bare to sparsely covered understory under autumn-olive in autumn-olive/black walnut plots has been attributed to reduced light intensity caused by the dense shade. The understory conditions rapidly approach those of a hardwood "forest" compared to the open European alder/black walnut and walnut-alone plots (38). Thus, the autumn-olive shorten the time it would normally take old-field conditions to disappear (37). The overall height of autumn-olive and the understory plot conditions have not changed in the last 4 or 5 years, while growth of the black walnut has continued at an improved rate (39). 

In summary, there is little reason to be concerned about allelopathy in mixed plantations where walnut is grown for timber as the harvested crop. Allelopathy does appear to be a factor to consider before planting European alder as a nurse crop with black walnut. Black walnut has had no apparent effect on the autumn-olive but is probably responsible for the decline and mortality of the European alder. These results present interesting possibilities for future research on the plant/soil/microbial relationships related to the metabolism of aromatics. 

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