Agriculture Reference
In-Depth Information
of the problem likely to increase as a result of acid rain, long-term N fertilization, and legume N
2
fixation. Theoretically, soil acidity is expressed by the concentration of hydrogen and aluminum
ions. However, for crop production, soil acidity is much broader, including deficiency of macro- and
micronutrients and low microbial activity (Graham, 1992; Graham and Vance, 2000). Legumes are
particularly affected, acidity limiting both the survival and persistence of nodule bacteria in the soil
and the process of nodulation itself (Graham and Vance, 2000).
Frame and Newbould (1986) reported that the growth of white clover under symbiotic conditions
was almost nil at pH 4.0 and increased linearly from pH 4.0 to 6.0 and nodulation was reduced
below pH 5.0. For most legumes to grow well and fix atmospheric N, pH values should be around
6.0. When the pH is lower than 5.5, Al toxicity may be a problem for legume growth and dinitrogen
fixation (Frame and Newbould, 1986). In addition, at a lower pH (<5.0), P availability may be a
problem due to the fixation of this element by Al and Fe oxides in acid soils, which may also limit
dinitrogen fixation. Immobilization of Mo can not only cause apparent deficiency of this element in
acid soils, but also can reduce nodulation in legumes. Adverse effects of Mo deficiency in dry beans
grown in South America have been reported by Graham (1981).
7.6.4 s
oIl
f
ertIlItY
Supply of an adequate rate of essential nutrients and in proper proportion is an important factor in
determining the growth and yield of crops. In addition, some nutrients are essential for the develop-
ment of nodules and, consequently, N
2
fixation. N-fixing bacteria have a relatively high requirement
for P, S, Mo, and Fe because these nutrients are either part of the nitrogenase molecule or they are
needed for its synthesis and use (Brady and Weil, 2002). However, higher amounts of NO
3
-N and
NH
4
+
-N in the soil may limit the biological N fixation (Brady and Weil, 2002). In higher amounts,
both forms of N inhibit nodule formation as well as nitrogenase activity (Layzell and Moloney,
1994). Bauer (1981) reported that host plants are able to sever their association with rhizobia quite
dramatically when external N is plentiful. Thornton (1936) observed that exogenous nitrate caused
fewer root hairs and fewer curled root hairs to be produced on alfalfa plants. He suggested that the
reduced numbers of curled root hairs were responsible for the lower number of nodule forms in the
presence of nitrate. Munns (1968a,b,c,d) confirmed and extended Thorn's observations in a classi-
cal series of papers. Munns concluded that the reduction in nodule number could not be attributed
solely or even chiefly to interference by nitrate with any one of the nodulation processes. However,
as indicated previously, nitrate caused both a reduction in the number of infection threads and an
increase in the proportion of abortive infection threads (Munns, 1968c). Moreover, nitrate was able
to delay nodulation by causing a delay in the formation of infected threads (Nutman, 1959).
Nitrate N has long been recognized as an inhibitor of nodulation since it is a highly efficient
metabolic process for the legume (Bhangoo and Albritton, 1976). Peoples and Herridge (1990)
reported that, in a soil with higher concentrations of mineral N, the legume can compensate for
poor N
2
fixation by scavenging N from the soil. Although production in this situation may not be
impaired, the net result of cropping with a legume with deficient nodulation is an exploitation of N
reserves. Soil N fertility is lost and the potential benefit of the legume in a cropping sequence will
not be realized (Peoples and Herridge, 1990).
Nitrate N is reported to be responsible for the decrease in phloem sap supply to nodules. It is
well known that nitrate N alters the pattern of photosynthate partitioning in legume plants (Streeter,
1988), resulting in a decrease in phloem sap supply to nodules within 24-36 h of
NH
3
−
exposure
(Vessey et al., 1988a,b). Minchin et al. (1989) suggested that
NH
3
−
may have an osmotic effect and
inhibit nodule metabolism by increasing the diffusion barrier resistance within the nodule cortex.
Kanayama et al. (1990) have reported that in soybean nodules,
NH
3
−
is converted first into nitrite
and then into nitric oxide, which binds to leghemoglobin to form nitrosylleghemoglobin, a form of
O
2
-binding protein that is unable to facilitate the diffusion of O
2
to the bacteroids in the infected
cells. They have proposed that nitrosylleghemoglobin formation may increase the resistance to O
2
