EFFECT OF FORMS OF NITROGEN SUPPLY ON MOBILIZATION OF PHOSPHORUS FROMA PHOSPHATE ROCK AND ACIDIFICATION IN THE RHIZOSPHERE OF TEA

Nitrogen (N) is the main fertiliser input to tea plantations because o f the large removal of this element with regular harvests of young sho ots in the field. The form of N supply is known to influence the uptak e of other plant nutrients, notably phosphorus (P), through its effect on soil pH in the rhizosphere. A glasshouse study was conducted to te st the effect of N form (NH4+, NO3-, or both) on the transformation of soil P in the rhizosphere and its availability to tea (Camellia sinen sis L.) plants fertilised with sparingly soluble Eppawala phosphate ro ck (EPR). Four-month-old tea (TRI 2025) plants were grown in rhizosphe re study containers containing an Ultisol from Sri Lanka (pH 4.5 in wa ter) amended with EPR and KCl at 200 mu g P or K/g soil, and mixed wit h (NH4)(2)SO4 (100% NH4+-N), NH4NO3 (50% NH4+-N and 50% NO3--N), and C a(NO3)(2) (100% NO3--N) at the rate of 200 mu g N/g soil, with a contr ol (no N fertiliser), as treatments. Rhizosphere pH decreased compared with the bulk soil when N was supplied as NH4+ or NH4++NO3- forms, an d increased when N was supplied as NO3-. The cation-anion balance esti mations in the plants showed that the plants had taken up more NO3- th an NH4+ even in (NH4)(2)SO4 treated soil, suggesting high nitrificatio n rates, especially in the rhizosphere, in spite of using a nitrificat ion inhibitor. More EPR dissolved in the rhizosphere compared with tha t in the bulk soil, regardless of the N form applied. The (NH4)(2)SO4 treatment had the highest dissolution rate of EPR in the rhizosphere, whereas Ca(NO3)(2) treatment had the lowest, reflecting the degree of acidification in the rhizosphere. Resin-P and NaOK-P-i (inorganic P) c oncentrations were lower and NaOH-P-o (organic P) concentration was hi gher in the rhizosphere than in the bulk soil. Plant and possible micr obial uptake of P is the main reason for the decrease in resin-P and N aOH-P-i. The increase in NaOK-P-o concentration in the rhizosphere is believed to be due to transformation of P-i to P-o by the high microbi al activity in the rhizosphere. The (NH4)(2)SO4 treatment caused the h ighest depletion of resin-P but lowest depletion of NaOH-P-i, probably due to the fixation of P by the soils at the low pH in the rhizospher e. The study revealed that the use of the NH4+ form of fertiliser can increase acidification in tea rhizosphere compared with bulk soil and this can enhance the effectiveness of PR fertiliser utilisation by tea plants.