Productivity of 3 different 2-year crop rotations, namely continuous wheat,
wheat-chickpea, and wheat-fallow, was measured over 4 consecutive seasons
beginning in 1991-92 at the ICARDA station, Tel Hadya, Syria. Nitrogen (N)
fertiliser (30 kg N/ha at sowing) was broadcast every other year in the
continuous wheat only. 15N-labelled fertiliser was used
to quantify the amount of nitrogen supplied to the crops through current and
past applications of fertiliser and by N2 fixation. The
remaining N in the crop was assumed to come from the soil.
In any single season, wheat yields were unaffected by rotation or N level.
However, 2-year biomass production was significantly greater (32%, on
average) in the continuously cropped plots than in the wheat-fallow rotation.
On average, <10% of the N in the wheat crop came from fertiliser in
the season of application, and <1·2 kg N/ha of the residual
fertiliser was recovered by a subsequent wheat crop. Chickpea fixed 16-48 kg
N/ha, depending on the season, but a negative soil N budget was still
likely because the amount of N removed in the grain was usually greater than
the amount of atmospheric N2 fixed. Uptake of soil N was
similar in the cereal phase of all 3 rotations (38 kg N/ha, on average),
but over the whole rotation at least 33% more soil N was removed from
continuously cropped plots than from the wheat-fallow rotation, suggesting
that the latter is a more sustainable system.
A laboratory study showed that although wheat and chickpea residues enhanced
the gross rate of N mineralisation by c. 50%, net
rates of N mineralisation were usually negative. Given the high C/N ratio
of the residue, immobilisation, rather than loss processes, is the likely
cause of the decline in the mineral N content of the soil. Consequently,
decomposition of crop residues in the field may in the short term reduce
rather than increase the availability of N for crop growth.