Agriculture Reference
In-Depth Information
In reduced soil conditions, such as flooded rice,
NH
4
+
may predominate in the absorption process.
The topic of
NH
4
+
versus
NO
3
−
nutrition of plants has been extensively reviewed (Hayes and Goh,
1978; Hageman, 1984; Mengel et al., 2001; Fageria and Baligar, 2005b). It has been proven that
most annual crops grow best when supplied mixtures of
NH
4
+
and
NO
3
−
under controlled conditions
(Fageria and Baligar, 2005b).
When NH4
+
uptake exceeds
NO
3
−
uptake, soil solution pH decreases and when
NO
3
−
uptake
exceeds
NH
4
+
uptake, soil solution pH increases. Furthermore, nearly 70% of the cations or anions
taken up by the plants are ammonium or nitrate (Van Beusichem et al., 1988). The
NH /NO
4
+ −
ion
uptake can change the rhizosphere pH up to two units higher or lower compared with the bulk
soil (Mengel et al., 2001). This change in pH may influence the uptake of other essential nutrients
from soil solution by plants. In consequence of the lowering of rhizosphere pH by ammonium, an
increased uptake has been reported of phosphate (Riley and Barber, 1971) and also of micronutri-
ents (Schug, 1985) as compared with nitrate treatment.
Legumes are exceptions in that they acidify the rhizosphere even after the supply of nitrate
(Marschner and Romheld, 1983). In legumes, the uptake rate of cations is greater than that of anions,
since they acquire nitrogen from the atmosphere through nitrogen fixation rather than by nitrate
uptake (Aguilars and Van Diest, 1981; Marschner and Romheld, 1983; Jarvis and Hatch, 1985).
However, some workers have reported that the acidification of the legume rhizosphere is associated
with specific properties of legumes, which normally acidify the rhizosphere even under nitrate-fed
conditions (Hinsinger, 1998; Marschner, 1995). The pH also changes with the excretion of organic
acids by roots and by microorganism activities in the rhizosphere. Further, the CO
2
produced by
roots and microorganism respiration can dissolve in soil solution and may form carbonic acid and
lower the pH. Soil buffering capacity (clay and organic matter content) and initial pH are the main
parameters that determine changes in soil pH.
In addition, the absorption of
NH
4
+
occurs faster than the absorption of
NO
3
−
(Gaudin and Dupuy,
1999). In addition, the assimilation of
NO
3
−
required energy equivalents up to 20 adenosine tri-
phosphate (ATP) mol
−1
NO
3
−
, whereas
NH
4
+
assimilation required only 5 APT mol
−1
NH
4
+
(Salsac
et al., 1987). Similarly, Bloom et al. (1992, 2003) reported that root absorption and assimilation of
1 mol
NH
4
+
requires or consumes 0.31 mol O
2
, whereas 1 mol of
NO
3
−
consumes 1.5 mol O
2
. This
means that
NO
3
−
uptake consumes about five times more energy compared to
NH
4
+
ion uptake. While
NH
4
+
can be assimilated directly into amino acids,
NO
3
−
must first be reduced to
NO
2
−
and then
NH
4
+
via nitrate reductase and nitrite reductase, a process that implies an additional energetic cost
(Hopkins, 1999). Potential energy savings for yield could be obtained if plants were supplied only
NH
4
+
(Huffman, 1989). This concept has not been consistently observed, nor is it easy to conduct
experiments on this given the nature of the N-cycle dynamic (Raun and Johnson, 1999; Fageria
et al., 2006).
The effectiveness of the two N forms on the growth and N uptake varies with the type of cultivar
and
NH /NO
4
3
+ −
ratio. For example, supplying N in solution entirely as
NH
4
+
or
NO
3
−
has been shown to
inhibit plant growth when compared to plant growth in solution containing 25% or 50% of either N
form (Nittler and Kenny, 1976). Gashaw and Mugwira (1981) reported that triticale, wheat, and rye
produced higher dry matter with combinations of 25/75; 50/50, and 75/25
NH
3
+
− −
ratios
than with either N source alone. Warncke and Barber (1973) found that there were no differences
between the relative rates of
NH
3
+
and
NO
3
−
absorption by corn but increasing each N form reduced
the uptake of the other N form. It has been shown that corn, wheat, and oats prefer 50% of N as
NO
3
−
for maximum growth (Diest, 1976). Barber et al. (1992) reported that, overall, manipulation of soil
NH /NO
4
N/NO
N
4
3
+ −
ratios had few effects on corn development or yield under field conditions.
Plants grown on
NO
3
−
maintain a relative homeostasis of their N concentration and internal
NO
3
−
over a wide range of external concentrations, which suggests an efficient mechanism to control
NO
3
−
uptake (Glass et al., 2002). In contrast,
NH
4
−
uptake seems poorly regulated (Britto and Kronzucker,
2002). The imbalance between
NH
4
+
uptake and assimilation rates depends on a variety of fac-
tors, including plant species and carbohydrate availability (Schjoerring et al., 2002). Although the
3
