Soil samples and black walnut (Juglansnigra L.) measurements were taken in plots of a 14-year-old plantation combining two walnut spacings with four interplanted nitrogen-fixing species and a control, lnterplantings of walnut with black locust (Robiniapseudoacacia L.) and autumn-olive (Elaeagnusumbellata Thunb.) had the highest total soil nitrogen concentrations in the top 30 cm of soil, followed by European alder (Alnusglutinosa L. [Gaertn.]), lespedeza clover (Lespedezastriata [Thunb.] Hook & Arn.), and control plots. Soil nitrogen concentrations were greatest in the surficial 6 cm of soil. At the closer, 3.7- by 4.9-m spacing in autumn-olive plots where walnut growth and competition were greatest, and at both spacings in plots with girdled black locust, nitrogen concentrations in the soil were especially high. This is consistent with speculation that recent stress on autumn-olive from walnut competition and stress on locust from past stem girdling resulted in increased inputs of nitrogen into soil. Walnut size was greatest by far in autumn-olive plots, where the spreading form of the shrub afforded soil shading and weed control. Walnut size was generally greater in mixed plots at the wider 3.7 m by 9.8 m spacing, where walnut competition with itself was least. However, walnut basal area was not strongly correlated with mean total soil nitrogen concentration of the top 30 cm of soil in a plot (r2 = 0.32 at closer walnut spacings), suggesting that the net accretion of nitrogen in soil accounted only in part for measured differences in walnut growth in mixed plantings with nitrogen-fixing plants.
Comparison of nitrogenase and nitrate reductase activities in two nitrogen-fixing tree species: black alder (Alnus glutinosa) and black locust (Robinia pseudoacacia)
