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 NH ⁺₄ and NO ⁻₃ were studied in non-mycorrhizal roots of 4-year-old Norway spruce under controlled environmental conditions. Additionally, in a forest stand NH ⁺₄ and NO ⁻₃ uptake rates along the root axis and changes in the rhizosphere pH were measured. In the concentration (C_min) range of 100–150 μM uptake rates of NH ⁺₄ were 3–4 times higher than those of NO ⁻₃ The preference for NH ⁺₄ uptake was also reflected in the minimum concentration (C_min) values. Supplying NH₄NO₃, the rate of NO ⁻₃ uptake was very low until the NH ⁺₄ concentrations had fallen below about 100 μM. The shift from NH ⁺₄ to NO ⁻₃ uptake was correlated with a corresponding shift from net H⁺ production to net H⁺ consumption in the external solution. The uptake rates of NH ⁺₄ were correlated with equimolar net production of H⁺. With NO ⁻₃ nutrition net consumption of H⁺ was approximately twice as high as uptake rates of NO ⁻₃ In the forest stand the NO ⁻₃ concentration in the soil solution was more than 10 times higher than the NH ⁺₄ concentration (<100 μM), 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 NO ⁻₃ 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.