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rectly by crop canopy production and consequent competition with the crop.
Although soil conditions were not monitored in this experiment, the differ-
ent fertilizer sources may have resulted in contrasting soil pH levels and dif-
ferences in the availability of non-N nutrients. Additional experiments
focused on soil chemical conditions would be useful for understanding
mechanisms through which fertility sources may affect weed community
composition.
Competition between legumes and non-legumes
Many legume crops are able to satisfy a sizable proportion of their N
requirements using atmospheric N
2
fixed by symbiotic bacteria on their roots.
Most non-leguminous species lack access to this N source and are limited
instead to the use of soil N. Because of this physiological difference, legumes
may compete strongly with non-legumes and yield well under conditions of
low soil N availability. In experiments in which soil N was manipulated by
adding ammonium nitrate, the grass species
Lolium rigidum
overtopped,
shaded, and suppressed the growth of subterranean clover under high soil N
conditions; in contrast, clover grew vigorously and dominated the mixed
species canopy under low soil N conditions (Stern & Donald, 1962). Similarly,
pea grown with
Brassica hirta
produced 185% more biomass when ammonium
sulfate fertilizer was not applied than when it was (Figure 5.5a), whereas
B.
hirta
in mixture with pea produced 69% less biomass without ammonium
sulfate than with it (Figure 5.5b).These patterns reflect direct effects of soil N
conditions on crop and weed growth,as well as indirect effects that soil N con-
ditions triggered through their influence on interspecific competition.Acom-
parison of biomass values from pure stands and mixtures shows, for example,
that competition from pea reduced the growth of
B.hirta
54% under low soil N
conditions, but had no significant effect on the weed's growth under high soil
N conditions (Figure 5.5b).
Such data suggest that minimizing soil inorganic N levels may be one
approach for enhancing the performance of certain legume crops growing in
association with nitrophilous weeds and for placing additional stress upon
the weeds. The approach would not be effective for varieties or species of
legumes that fix little atmospheric N and depend heavily on soil inorganic N
to produce adequate yields, e.g., short-season cultivars of common bean,
Phaseolus vulgaris
(Laing, Jones & Davis, 1984). To determine which legumes
and weeds might be most amenable to management through manipulation of
soil N levels would require inoculation of the legumes with appropriate
strains of bacterial symbionts, and measurement of crop and weed growth
under contrasting soil N conditions.Much in the same way that Tilman (1988,
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