Summary. Continuous cereal cropping in southern
Queensland and northern New South Wales has depleted native soil nitrogen
fertility to a level where corrective strategies are required to sustain grain
yields and high protein content. The objective of this study was to examine
the performance of chickpea in chickpea–wheat rotations in terms of
yields, water use and N2 fixation. The effects of sowing
time and tillage practice have been studied.
Chickpea grain yields varied from 356 kg/ha in 1995 to 2361 kg/ha in
1988; these were significantly correlated with the total rainfall received
during the preceding fallow period and crop growth. Almost 48% of total
plant production and 30% of total plant nitrogen were below-ground as
root biomass.
Mean values of water-use efficiency for grain, above-ground dry matter, and
total dry matter were 5.9, 14.2 and 29.2 kg/ha.mm, respectively. The
water-use efficiency for grain was positively correlated with the total
rainfall for the preceding fallow and crop growth period although cultural
practices modified water-use efficiency. The potential
N2 fixation was estimated to be 0.6 kg
nitrogen/ha.mm from 1992 total dry matter nitrogen yields assuming all of
the nitrogen contained in chickpea was derived from the atmosphere.
Sowing time had a much larger effect on grain yield and
N2 fixation by chickpea than tillage practice
(conventional tillage and zero tillage) although zero tillage generally
increased grain yields. The late May–early June sowing time was found to
be the best for chickpea grain yield and N2 fixation
since it optimised solar energy use and water use, and minimised frost damage.
Nitrogen fixation by chickpea was low, less than 40% nitrogen was
derived from atmosphere, representing less than 20 kg nitrogen/ha.year.
The potential for N2 fixation was not attained during
this period due to below-average rainfall and high soil
NO3-N accumulation because of poor utilisation by the
preceding wheat crop. Increased soil NO3-N due to
residual from fertiliser N applied to the preceding wheat crop further reduced
N2 fixation. A simple soil nitrogen balance indicated
that at least 60% of crop nitrogen must be obtained from
N2 fixation to avoid continued soil nitrogen loss. This
did not occur in most years.
The generally negative soil nitrogen balance needs to be reversed if chickpea
is to be useful in sustainable cropping systems although it is an attractive
cash crop. Sowing time and zero tillage practice, possibly combined with more
appropriate cultivars, to enhance chickpea biomass, along with low initial
soil NO3-N levels, would provide maximum
N2 fixation.
Seasonal variations in photosynthesis, intrinsic water-use efficiency and stable isotope composition of poplar leaves in a short-rotation plantation