The Eleventh Regional Wheat Workshop For Eastern ... - Cimmyt

Impact ofcropping sequence and fertilizer application on soil parameters - Kamwaga et al.
exhibited higher levels of N03-N than those that had potato one or two years prior to sampling
(i.e., treatments 7, 9, 13 and 14). This indicates that the effect of potato on soil N03-N was
greatest immediately post-harvest and declined over the three years of the experimental period.
Overall, treatments including potato had higher levels of N03-N relative to continuous wheat.
Interestingly, complex sequences (i.e., treatments 13 to 15) that had potato and maize in rotation
with wheat exhibited higher levels ofN03-N compared to other sequences (i.e., treatments 10 to
12) that had maize in rotation with wheat, suggesting a beneficial effect of potato in the rotation
system.
It is not apparent why potato - which is a high consumer of nutrients - would be such a good
break crop for wheat. It could, however, be expected that potato residues, being relatively
succulent, would decompose faster than other crop residues and thus would initially result in
higher soil N03 levels relative to the other crops. The microbially-mediated decomposition
process results in the simultaneous mineralization of soil organic-N and the immobilization of
inorganic-N by the microbial population. Net mineralization occurs when the microbial
population releases more N during decomposition than it can assimilate, while net
immobilization occurs when the microorganisms assimilate more N than is being released from
the decomposing OM. These processes are affected by the C:N ratio of the decomposing material
- mineralization occurs during the decomposition of material with a narrow C:N ratio and vice
versa for immobilization (Jenkinson, 1971). Considering the N content of crop residues, soybean
with 2.2% N would be expected to result in net mineralization on decomposition - relative to a
suggested threshold level of 1.5% N at which immobilization and mineralization are in balance
(Smith et al., 1982). Wheat and maize straw with 0.7 and 1.1 % N content, respectively, would be
expected to immobilize N during decomposition. The N content for rapeseed and potato crop
residues were not found in literature.
It has also been reported that soil disturbance - as occurred during potato harvest - enhances
the decomposition of crop residues due to increased exposure of the residues to microbial attack
(Wall et al., 1994). These authors indicated that soil disturbance tends to aerate the soil, leading
to rapid oxidation of OM. This would be particularly relevant when disturbed soil is wetted by
rain showers soon after harvesting the potato crop. Mean levels of N03-N after rotation with
potato, soybean, maize and rapeseed follow in that order ofmagnitude. The fact that soybean did
not have a bigger effect on soil N03-N may have been due to poor N fixation. Poor nodulation of
soybean was observed during the trial period because the seed had either been improperly
inoculated or the rhizobial inoculant applied had not been effective.
Rapeseed was a poor break crop, resulting in a mean soil N03 level even lower than that of
continuous wheat plots. Maize was apparently a better break crop than rapeseed although this
had not been anticipated - due to the long time required for maize stover to decompose relative
to residues from the other crop species. Among the treatments including rapeseed, soil N03 level
was lowest where rapeseed had been produced in 1994. The same trend was apparent for
treatments based on maize as the break crop. This suggests that these crops were big consumers
of N, and that soil N03 gradually recovered during the second and third succeeding years ­
when the plots were sown to wheat. This therefore indicates that these crops were not beneficial
to the soil with respect to soil N supply. In contrast, the trend for soybean and potato was exactly
the opposite. Soil N03 levels were highest immediately following these crops, but declined over
time during the period of wheat production. This indicates a beneficial effect of these two crops
on soil N levels vis-a-vis continuous wheat production.
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