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Abstract
The objective of the experiment was to identify the most favorable nitrogen (N) source ratio of ammonium (NH4 +) and nitrate (NO3 −) for aeroponically-grown Douglas-fir when pH was maintained at pH 4.0. Seedlings were grown in controlled environments with solutions containing 0:100, 20:80, 40:60, 60:40, 80:20 or 100:0 NH4 +:NO3 − ratios. Nutrient additions in the aeroponic culture units were controlled by solution conductivity set points. Seedling growth and nutrient allocation was observed for 45 days. Different NH4 +:NO3 − ratios resulted in significant differences in the rate of N addition, growth, morphology, and nutrient allocation. Seedlings grown in solutions containing 60 or 80% NO3 − were characterized by a combination of high growth and photosynthetic rates, high and stable internal plant N concentrations, and sufficient levels of other essential nutrients. High proportions of NH4 + in solution resulted in low rates of N addition, stunted lateral root growth, and may have been toxic.
ACKNOWLEDGMENTS
The authors wish to thank J. Vallentgoed, T. Bown, and A. Harris (Pacific Forestry Centre) for their help in analyzing samples and maintaining the experiments. We wish to thank R. Metcalfe and S. Robbins (University of Victoria) for their help in processing samples. This project was funded by the Forest Science Program—Forest Investment Account Project Y062240.