Soil nitrogen mineralization in plantations of Juglans nigra interplanted with actinorhizal Elaeagnus umbellata or Alnus glutinosa

Nitrogen mineralization rates were estimated in 19-year-old interplantings of black walnut (Juglans nigra L.) with dinitrogen fixing autumn-olive (Elaeagnus umbellata Thunb.) or black alder (Alnus glutinosa L. Gaertn.) and in pure walnut plantings at two locations in Illinois USA. N mineralization rates were measured repeatedly over a one year period using in situ incubations of soil cores in oxygen-permeable polyethylene bags at 0-10 and 10-20 cm soil depths, and also by burying mixed-bed ion-exchange resin in soil. Mineralization rates were highest in summer and in plots containing actinorhizal Elaeagnus and Alnus in contrast with pure walnut plots. Elaeagnus plots at one location yielded 236 kg of mineral N ha^-1 yr^-1 in the upper 20 cm of soil, a value higher than previously reported for temperate decidous forest soils in North America. The highest mean plot values for N mineralization in soil at a location were 185 kg ha^-1 yr^-1 for Alnus interplantings and 90 kg ha^-1 yr^-1 for pure walnut plots. Plots which had high N mineralization rates also had the largest walnut trees. Despite low pH (4.1 and 6.5) and low extractable P concentrations (1.4 and 0.7 mg kg^-1 dry mass) at the two locations, nitrification occurred in all plots throughout the growing season. NO₃^--N was the major form of mineralized N in soil in the actinorhizal interplantings, with NH₄⁺-N being the major form of mineral N in control plots. Walnut size was highly correlated with soil nitrogen mineralization, particularly soil NO₃^--N production in a plot.