INCREASED UPTAKE OF NATIVE SOIL-NITROGEN BY ROOTS OF LOLIUM-MULTIFLORUM LAM AFTER NITROGEN-FERTILIZATION IS EXPLAINED BY A STIMULATION OF THE UPTAKE PROCESS ITSELF

Plants fertilized with N usually take up more of the native N soil compared to unfertilized plants. This so-called priming effect has been frequently described but as yet no explanation involving changes in nitrate uptake rates by roots has been proposed. An experimental procedure was designed for the measurement of nitrate uptake rates by excised roots of plants grown in field conditions. Analysis of Lolium multiflorum Lam. shoots from fertilized (60 kg N-15-KNO3 ha(-1)), and from control plots (no N applied), showed higher uptake of native soil N (+ 18 kg N ha(-1)) in fertilized plants during the first two weeks. This was largely independent of biomass production and resulted in higher values of apparent fertilizer efficiency than real fertilizer efficiency as measured by N-15 labelling. A large increase in rates of nitrate uptake by excised roots occurred during the first 14 days following N fertilization. Uptake rates following fertilizer application increased by 62 and 51% in relation to control plants, respectively, 7 and 14 days after N application. This increase was partially explained by higher nitrate concentrations in the soil solution, but mainly by a stimulation of the uptake process itself. Both kinetic and quantitative arguments suggested that the priming effect following N fertilization could be linked to a stimulation of nitrate uptake by roots.