Ammonium and nitrate uptake rates and rhizosphere pH in non-mycorrhizal roots of Norway spruce [Picea abies (L.) Karst.]

Summary Relationships between root zone temperature, concentrations and uptake rates of NH₄⁺and NO₃^– were studied in non-mycorrhizal roots of 4-year-old Norway spruce under controlled environmental conditions. Additionally, in a forest stand NH₄⁺and NO₃^– uptake rates along the root axis and changes in the rhizosphere pH were measured. In the concentration (C_min) range of 100–150 M uptake rates of NH₄⁺were 3–4 times higher than those of NO₃^– The preference for NH₄⁺uptake was also reflected in the minimum concentration (C_min) values. Supplying NH₄NO₃, the rate of NO₃^– uptake was very low until the NH₄⁺concentrations had fallen below about 100 M. The shift from NH₄⁺to NO₃^– uptake was correlated with a corresponding shift from net H⁺ production to net H⁺ consumption in the external solution. The uptake rates of NH₄⁺were correlated with equimolar net production of H⁺. With NO₃^– nutrition net consumption of H⁺ was approximately twice as high as uptake rates of NO₃^– In the forest stand the NO₃^– concentration in the soil solution was more than 10 times higher than the NH₄⁺concentration (<100 M), and the rhizosphere pH of non-mycorrhizal roots considerably higher than the bulk soil pH. The rhizosphere pH increase was particularly evident in apical root zones where the rates of water and NO₃^– uptake and nitrate reductase activity were also higher. The results are summarized in a model of water and nutrient transport to, and uptake by, non-mycorrhizal roots of Norway spruce in a forest stand. Model calculations indicate that delivery to the roots by mass flow may meet most of the plant demand of nitrogen and calcium, and that non-mycorrhizal root tips have the potential to take up most of the delivered nitrate and calcium.