Aluminium and nitrogen on pH variation and cation-exchange capacity in Brachiaria decumbens

Abstract

Brachiaria decumbens was grown in nutrient solution with nitrogen applied either as NO₃; or as NH₄. Al was added (to four - week old plants) at 0, 0.75, 3, and 6 ppm, pH was read daily, and adjusted to 4.2 each 48 hours. Plants under NO₃ were able to raise the solution pH up to 5 at Al levels up to 1.5 ppm. Above this level, plants under NO₃ showed higher accumulation of Al and a marked reduction in root cation - exchange capacity (CEC). Changes in pH NO₃ - fed plants were related to the size of root system but not to imbalances in (N+ P) - (Ca + K) uptake. Plants under NH₄ showed an uniform trend in pH change, and no effects of Al levels on root and shoot Al contents were detected. Effects of Al levels on root CEC were much less accentuated in plants under NH₄ than under NO₃. The mechanisms through which N-carriers can affect the responses of Brachiaria plants to Al stress is discussed.