Simultaneous measurement of ammonium, nitrate and proton fluxes along the length of eucalypt roots

Knowledge of the preferred source of N for Eucalyptus nitens will lead to improved fertiliser management practices in plantations. Ion selective microelectrodes were used non-invasively to measure simultaneously net fluxes of NH4+, NO3- and H+ along the tap root of solution-cultured E. nitens. Measurements were conducted in solutions containing 100 mum NH4NO3. The pattern of fluxes was such that there was a large influx of NH4+, a smaller influx of NO3- and large H+ efflux. The ratio of these fluxes was constant, according to the ratio 3:1:-6 (NH4+:NO3-:H+). Within the region 20-60 mm from the root apex of E. nitens seedlings there was spatial and temporal variation in fluxes but flux patterns remained constant. Root hair density did not affect fluxes nor did proximity to lateral roots. Variation was less than that found in previous studies of localised root fluxes using similar high-resolution measurement techniques. It was concluded that small-scale spatial variation in fluxes may have confounded previous studies. There were associations between fluxes of all three ions, the strongest associations being between NH4+ and H+, and NH4+ and NO3-. Overall, these results are consistent with NH4+ being the preferred source N for E. nitens.