Figure 4.7 Effects of tillage on germination, dormancy, and death of weed seeds.
Tillage can directly induce inappropriate germination by scarification of hard seeds and by providing a light flash. However, inappropriate germination due to scarification of hard-seeded species like Abutilon theophrasti and Convolvulus arvensis during tillage is probably a small source of mortality since (i) usually only a fraction of seeds in the seed bank are hard, (ii) most of these probably escape scarification during any given tillage operation, and (iii) some of the germinating seeds emerge as seedlings. If a seed of a light-sensitive species receives a brief flash of light prior to reburial beyond the depth from which emergence can occur, the seed may be stimulated to germinate inappropriately. This is more likely with tillage methods like moldboard plowing or rototilling that move large amounts of soil vertically. The effect probably kills a larger proportion of small-seeded than large-seeded species, since most small-seeded species must be close to the surface for successful emergence, and most large-seeded species lack a light requirement (see Chapter 2). The role of both scarification and light flash in tillage-induced seed mortality requires experimental investigation.
Tillage probably also promotes the inappropriate germination of weed seeds by the same mechanisms that it stimulates appropriate germination, namely, by increasing temperatures and the amplitude of temperature fluctuations, modifying the soil atmosphere, and changing the chemistry of the soil solution, particularly nitrate concentration. However, stimulation of inappropriate germination by tillage may be less than stimulation of appropriate germination, since the magnitude of all of these effects declines with increasing depth, and seeds that germinate close to the surface often produce emerged seedlings. The magnitude of inappropriate germination due to changes in the soil environment induced by tillage needs to be examined.
Seed mortality often varies with depth in the soil (reviewed by Mohler, 1993), and as tillage moves seeds, it exposes them to changes in mortality risk. However, in most studies to date, the seeds were confined in packets or containers that excluded most seed predators. Even more important, seedling emergence could not be observed. Since germination is generally greater for seeds closer to the surface due to light, warmer soil, and fluctuating temperatures (see Chapter 2), the greater apparent mortality often observed near the surface may have been due to death of individuals that would have emerged had they not been restrained. For a few studies (Stoller & Wax, 1973; Dawson & Bruns, 1975; Froud-Williams, Chancellor & Drennan, 1983; Moss, 1985b), Mohler (1993) computed seed mortality by subtracting the number of emergents from the number of seeds that disappeared. These data indicated that survival of seeds not producing seedlings decreased with depth as often as it increased (Mohler, 1993). However, several additional studies on Avena fatua and Helianthus annuus have shown increased survival when seeds were incorporated into the soil (Banting, 1966; Wilson, 1972; Wilson & Cussans, 1972, 1975; Robinson, 1978), and recent work on several broadleaf species (Mohler, 1999) showed consistent increase in seed survival with depth. In general, biological activity increases toward the soil surface, and action of seed predators and pathogens should be greatest there. In addition, desiccation and exhaustion of dormant seeds is more likely near the surface than in the cooler, moister soil below. Nevertheless, the pattern of seed mortality with depth apparently varies among species, and probably with soil and cultural conditions as well.
Plowing and other tillage that raises deeply buried seeds will bring many sufficiently near the surface to prompt germination. However, only some of these successfully emerge, and the tillage operation has essentially killed those that fail. This effect is highly dependent on the germination biology of the species present. Moreover, it cannot be studied simply by placing seeds at different depths, since seeds in the soil are likely to have different dormancy status than seeds from the laboratory shelf or refrigerator.
In summary, tillage decreases the weed seed bank. Frequent tillage decreases the seed bank more rapidly than infrequent tillage. Tillage that produces more vertical displacement of seeds probably creates more true seed mortality, though shallow tillage may decrease the seed bank more rapidly through seedling emergence. Except for a few studies that have distinguished death by germination or disappearance from loss of viability (Sanchez del Arco, Torner & Fernandez Quintanilla, 1995), few data are available on the causes of seed mortality in field conditions.
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