H+/OH- Excretion and Nutrient Uptake in Upper and Lower Parts of Lupin (Lupinus angustifolius L.) Root Systems

The cultivatioo of narrow·leafed lupins (Lupinus angustifolius L) increases rates of subsoil acidification, and this is thought to be partly related to their pattern of nutrient uptake and H+ jOHexcretion. The main hypothesis of this study was that H+ and OHexcretion is not distributed evenly over the entire length of the root system but is limited to zones where excess cation or anion uptake occur. Seedlings of nodulated lupins were grown in solution culture using vertically split pots that allowed the upper and lower zones of the root system to be supplied with varying concentrations of K+ and NO;. Net H+/OHexcretion was equated to the addition of NaOHjHCI required to maintain a constant pH in the nutrient solution during a 4·d treatment period and nutrient uptake was measured by depletion from solution in each zone of the split pots. The excess of cation over anion uptake was positively correlated with H+ excretion in each rooting zone. In zones where K+ was supplied at 1200 jtM, cation uptake was dominated by K+ and up to twice as much H+ was excreted than in zones where K+ was absent. In zones where NO; was supplied at 750 I'M, the anion/cation uptake was balanced, however H+ excretion continued to occur in the zone. When NO; was supplied at 5000 pM, anion uptake exceeded cation uptake but there was no OHexcretion. Organic acid anions may be excreted by lupins to maintain their internal electroneutrality when anion uptake exceeds cation uptake. Rhizosphere pH would not increase unless the pKa of the excreted organte anions was greater than the external pH.