1273.4.1 Diversified Crop Rotations
The losses of N in dryland soils through leaching, denitrification and volatilization
can be reduced/replenished by incorporating the legume crops in rotation, as the
legumes can contribute 40–80 kg N ha−^1. For example, under dryland conditions,
the alfalfa (Medicago sativa L.) can fix about 200–250 kg N ha−^1 per year, while pea
can fix 71–91 kg N per growth cycle (Li et al. 1990 , 1992 ). The applied N recovery
from urea in combination with using the twigs and loppings of N fixing tress, and
urea alone was in 1:1 ratio (Sharma et al. 2002 ). However, the success of biological
nitrogen fixation by legumes depends on the correct match between the Rhizobium
strain, the environment and the host legume variety (Beck 1992 ), and these factors
should be considered while including the legume crops in crop rotations in dryland
regions.
In some areas of the world, the root diseases of wheat are controlled through crop
rotation which can also influence the responses of wheat to applied N (Rowland
et al. 1988 ). Crop rotation with legumes on clay soil can reduce the input of applied
N to greater extent in dryland regions (Anderson et al. 1995 ). Inclusion of forage
legumes in cereal-based systems in dryland soils may lead towards the buildup of N
enriched soil organic matter, which enhanced the potentially available N within soil
pool. Interestingly, this increases in soil organic matter due to addition of legumes
in crop rotation caused linear and parallel increase of total mineral, labile and bio-
mass N forms (Ryan et al. 2008d).
During the fallow years, the legumes may help provide soil cover to reduce soil
erosion and nutrient losses, and can fix the atmospheric nitrogen to benefit the fol-
lowing grain crop. In dryland areas, where summer rainfall exceeds 550 mm and the
duration of rainfall is 90 days, mungbean or mashbean ((Vigna mungo (L.) Hepper)
can be grown in the moonsoon season followed by barely or wheat in dry season.
The yield advantage from the legume may be equivalent to 20–30 kg fertilizer
N ha−^1 , for the cool season crops with substantial improvement in fertilizer use
efficiency (Singh and Venkateswarlu 1985 ). In another study, Giri and De ( 1981 )
reported that growing cowpea, (Vigna aconitifolia (Jacq) Marechal), guar
(Cyamopsis tetragonoloba (L.) Taub.) and soybean for 55 days and then using them
as green fodder, followed by growing of barely was equivalent to more than 40 kg
ha−^1 N applied to barely grown after a non-legume fodder crop. In some parts of
Australia, the wheat is not provided by N fertilizer, and the N fixed by legumes is
enough to achieve good wheat yields in some dryland regions (Smith 1983 ).
Mixtures of legumes and non-legumes in dryland regions of the world may be use-
ful to meet the feed requirements of the animals with substantial improvement in
soil fertility. This improvement in soil fertility might be attributed to co-current
transfer of N form the legume to non-legume crop in rhizosphere. Such type of
intercropping also improves the N use efficiency (Eaglesham et al. 1981 ;
Bandyopadhyay and De 1986 ).
Organic inputs from livestock manure, green manures and crop residues may
enhance the fertilizer use efficiency as well as the yield of crops in dryland regions
(Palm et al. 1997 ; Place et al. 2003 ). Few legume species not only fix the atmospheric
Nutrient Management in Dryland Agriculture Systems