Agriculture Reference
In-Depth Information
fertility and rainfall, the latter ranging from 350 to 1800 mm per annum. African soils
fall into two major categories, the old (dating from the breakup of Gondwana) and the
new, more recent soils. In general the old soils are less fertile than the new, but the latter
may be complicated by the buildup of salts, such as in many regions of Kenya. The
state of knowledge of the ecosystems in different areas varies greatly, so this section is
based on selected references, to illustrate both the diversity of the legume flora and its
contribution to the nitrogen economy of the ecosystem.
The work of Scholes andWalker (1993) in the Nylsvley area of southeast South Africa
illustrates the complexity of the ecosystems, including the confounding effects of the
fauna (including people). They recognise two types of savanna. One, on the less fertile
sites, is dominated by non-nodulating trees, in particular the legume
Burkea africana
,
an evergreen, large-leaved species. However, although savannas are generally thought
of as trees and grasses, there may be a significant number of legumes in the ground
flora. In the case of the
Burkea
savanna, these included ten species with demonstrated
or suspected nitrogen fixing activity (including
Cassia biensis
, which is a synonym for
Chamaecrista biensis
). In the drier, more nutrient rich areas, nodulated
Acacia tortilis
is
dominant, as it is in many parts of Africa (Plate 2.5). The extent and importance of
nodulation in acacias in Africa is very variable. Frequently species that are known to
be able to nodulate do not do so under certain conditions. One recent study in South
Africa (Cramer et al., 2007) used four species of
Acacia
, including
A. tortilis
,grownon
soils with some residual nitrogen, with and without a grass component. In the absence
of grass, the soil nitrogen was sufficient to depress nodulation, but in the presence of
grass, there was good nodulation and nitrogen fixation, as a result of the grass using
the soil nitrogen and 'forcing' the legume to provide its own. A similar situation has
been found in northern temperate grass-legume pastures (Carlsson, 2005), suggesting
that it may be a general phenomenon. In natural ecosystems, legumes seldom grow
in monoculture. Another feature of some African acacias is that their root systems
penetrate great depths, especially in some of the sandier savanna areas, such as parts
of East Africa and Senegal and deserts of the Kalahari andNamibia. Data from leaves of
five species grown in Zimbabwe, where nitrogen fixation was estimated by the natural
abundance
15
Nmethod (Boddey et al., 2000), showed wide variation between species
(data of J.M. Sutherland, cited in Barnes et al., 1997). In particular there was no evidence
of fixation in
A. erioloba
, and no nodules were found on its plants. Indeed there have
been reports that this species cannot nodulate. However, this is not true, but it does
have very long roots that can access mineral nitrogen in ground water and thus may
not always need to nodulate. It is still of use in the overall nitrogen economy of the
system as it makes available nitrogen that other plants cannot access. As mentioned
in Chapter 1, some species of African and South American acacias appear to have lost
the ability to nodulate. Those that can often use a wide variety of rhizobia (Chapter
4), with a wide range of efficiency of nitrogen fixation. The savanna areas where these
species grow generally have very low and erratic rainfall and are nutrient poor. They
do, however, have plenty of light for photosynthesis, and it has been argued that if
water is the major factor limiting growth, nodulation and nitrogen fixation are of lesser
importance (Sprent, 2007). Savannas in other areas, such as parts of SouthAmerica, may
be different. In more humid African savannas, such as in parts of Tanzania, nodulated
acacias may significantly increase soil nitrogen (Cech et al., 2008).
