FOLIAR UPTAKE AND RELEASE OF INORGANIC NITROGEN-COMPOUNDS IN PINUS-SYLVESTRIS L AND PICEA-ABIES (L) KARST

Foliar uptake and release of inorganic nitrogen compounds were studied by immersing current-year shoots of Scots pine (Pinus sylvestris L.) and Norway spruce [Picea 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 l-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.