Agriculture Reference
In-Depth Information
[23] state that there are three principal determinants of community assembly:
1) dispersal constraints, 2) environmental constraints, and 3) internal dynam-
ics. Weed responses to these constraints is complex, as weeds can be affected
at numerous stages in their life cycle.
Potential tillage effects on weed community development
Numerous constraints (filters) affecting weeds exist in agricultural systems.
Timing and intensity of tillage is potentially one of the more important 'fil-
ters' affecting assembly trajectory of weed communities [20]. Tillage affects
weed growth and productivity in several ways. Tillage uproots, dismembers
and buries growing weeds and dormant perennating organs; thereby killing
many plants [24]. However, infrequent tillage can increase the productivity
of some perennials, such as quackgrass [ Elytrigia repens (L.) Nevski], by
fragmenting rhizomes and thus releasing rhizome buds from apical domi-
nance. Additionally, tillage implements can spread vegetative fragments and
seeds throughout a field, or from field to field, and thus contribute to weed
dispersal.
Tillage also moves weed seeds vertically and horizontally and changes the
soil environment in ways that can promote germination and establishment. In
the absence of tillage, seeds infiltrate into soil via cracks, frost action and soil
fauna activity. However, this type of infiltration is limited and slow. Thus, most
weed seeds remain at or near the soil surface in zero tillage systems [25, 26].
Seeds on the soil surface, compared to buried seeds, are more susceptible to
environmental extremes such as desiccation and freeze-thaw cycles [27] and
predation by rodents, soil fauna, and pathogens [28-30].
Tillage promotes germination of many agricultural weeds, provided that soil
disturbance comes at a time of year when seeds are not innately dormant [24,
31, 32]. Agricultural weeds are thought to have adapted to respond to cues
associated with soil disturbance because their small seedlings make them poor
competitors early in life. Recently-tilled land is often warmer, has greater diur-
nal temperature fluctuations, higher nitrate concentration, and increased aera-
tion [33]. Emergence of some weeds may be greater in loose (tilled) soil than
in compacted (untilled) soil [34]. Additionally, tillage exposes buried seeds to
light that facilitates germination of many species [35].
Crop residues on the soil surface increase with zero tillage. Crop residues
can suppress weed establishment by altering environmental conditions related
to germination, physically impeding seedling growth, and through allelopath-
ic interactions [36, 37]. Zero tillage also tends to increase the number and/or
diversity of soil microbes [38], soil fauna [39], earthworms [40] and mycor-
rhizal associations of crops and weeds [41, 42]. These organisms can directly
or indirectly affect weed seed survival and germination as well as weed growth
and productivity.
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