et al., 2003). When NH4 +–N is the dominant form of nitrogen
taken up plants, H+ ions are excreted by the root system in order to
maintain internal electro-neutrality resulting in a net acidification
within the rhizosphere. In contrast, the absorption of NO3 − causes
the accumulation of organic anions in the plant and OH− efflux
from root system resulting in an increase in soil pH in the
rhizosphere (Dijkshoorn, 1962; Kirkby and Knight, 1977; Jarvis
and Robson, 1983a,b). This process has implications for the
vertical soil pH gradient as Black (1992) found that an alkaline
subsoil (NpH 7.0) was likely to be due to the uptake of leached
NO3 − that was countered by the excretion of alkali at the same
depth. Similarly, Noble and Hurney (2000) suggested that
increased soil pH at 100 cm under long-term sugarcane was
associated with differential uptake of leached NO3 −. Further,
Armour et al. (2005) observed significant increases in soil pH to a
depth of 80 cm after 4 years of NO3-based fertilizer applications to
an irrigated crop of Digitaria melanjiana, whilst equivalent rates
of N applied as NH4 resulted in acidification.