Foliar uptake and release of inorganic nitrogen compounds in Pinus sylvestris L. and Picea abies (L.) Karst.

summary Foliar uptake and release of inorganic nitrogen compounds were studied by immersing current‐year shoots of Scots pine ( Pinus sylvestris L.) and Norway spruce [ Pica abies (L.) Karst] in either NH 4 + ‐ or NO 3 − ‐rain solutions at different N concentrations. The effects of N form, N concentration and tree species on ion influx and efflux were investigated. Spruce shoots absorbed NH 4 + from the external solution. Uptake apparently occurred by diffusion rather than by H + or base cation exchange as commonly accepted, and increased linearly with NH 4 + concentration in the external solution. In contrast, pine shoots released NH 4 + to the external solution. The different reactions of spruce and pine may reflect species differences in physical and chemical properties or differences in tissue N concentration. If the latter is the case, a tree's N status may determine whether the canopy acts as a source or sink for NH 4 + influencing deposition rates to the needle surface. The results show that where NH 4 + concentration on the needle surface exceeds 4 mg 1 −1 , foliar uptake may make a significant contribution to N status. In the absence of NH 4 + ‐base cation exchange, atmospheric inputs of NH 4 + to the canopy appear unlikely to be directly‐responsible for the nutrient deficiencies typical of Dutch forest decline. Neither spruce or pine shoots were able to utilize NO 3 − in the external solution and generally released NO 3 − . Adverse effects resulting from foliar accumulation of wet‐deposited NO 3 − appear unlikely. However, higher NO 3 − concentrations and longer residence times than simulated in this experiment may result in foliar uptake of NO 3 − in the field.