Agriculture Reference
In-Depth Information
In general, potentially nodulated legumes aremore likely to do so in seasonally flooded
soils than in non-flooded soils (Moreira et al., 1992). The levels of major rivers such as
the Amazon and the Orinoco fluctuate by 10 or more metres. In the Orinoco, apparently
active nodules can be found on trees at all times (Sprent, 2001). In the Amazon, where
the floodwater is rich in nutrients, different species may be active at different seasons.
In a detailed study of one area, Kreibich et al. (2006) concluded that the aquatic legume
Neptunia oleracea
fixed significant amounts of nitrogen inwhat they termed the 'aquatic'
phase, whereas various other legumes, mainly trees, were more important during the
'terrestrial' phase. However, their data do not exclude the possibility that nodules on
the tree species were active also in the aquatic phase. Unravelling the contribution of
legumes to the nitrogen cycle in such areas is made more complicated by high rates of
denitrification in the wet season.
2.5 Temperate regions
Legumes in northern temperate areas are almost exclusively papilionoid, although a
few nodulated (some herbaceous species of
Chamaecrista
) and non-nodulated (species
of
Gleditsia
) caesalpinioids have strayed into North America. In Australia, there are
many species of
Acacia
, sub-genus
Phyllodineae
, that occur in the cooler, temperate
areas, including rain forest (see above). These, and all the papilionoids of temperate
regions, can and usually do nodulate. This section will cover Mediterranean areas,
temperate and boreal areas, and high-altitude legumes.
2.5.1 Mediterranean ecosystems
Mediterranean ecosystems are found between 30
◦
and 40
◦
north and south and are
characterised by mild wet winters and warm dry summers. Agricultural legumes have
been grown in some for millennia, for example the Fertile Crescent in the Middle
East, where the cool season grain legumes (peas, faba beans, lentils and chickpeas)
are still central to human and animal nutrition. However, there are four other regions
of the world where similar conditions prevail, and these are listed in Table 2.4. Area
data are from Myers et al. (2000), where all are listed as biodiversity hot spots. IUCN
give slightly lower figures for some areas, but either way most of them are very
small compared with the ecosystems discussed above. All are on the Western edge
of continents where the weather is moderated by cold ocean currents. The native
Tab l e 2 . 4
Mediterranean ecosystems. Original extent
Geographical region
Approximate area/10
6
km
2
Mediterranean Basin
2.36
Southern California and Baja
0.32
Southwestern Australia
0.31
Central Chile
0.30
Cape of South Africa
0.74
