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 NH4+- or NO3−-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 NH4+ from the external solution. Uptake apparently occurred by diffusion rather than by H+ or base cation exchange as commonly accepted, and increased linearly with NH4+ concentration in the external solution. In contrast, pine shoots released NH4+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 NH4+ influencing deposition rates to the needle surface. The results show that where NH4+ concentration on the needle surface exceeds 4 mg 1−1, foliar uptake may make a significant contribution to N status. In the absence of NH4+-base cation exchange, atmospheric inputs of NH4+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 NO3− in the external solution and generally released NO3−. Adverse effects resulting from foliar accumulation of wet-deposited NO3− appear unlikely. However, higher NO3− concentrations and longer residence times than simulated in this experiment may result in foliar uptake of NO3− in the field.