Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 239

Table 5.2. Mean daily maximum and minimum soil temperature and amplitude of
temperature fluctuation at 5 cm depth under hairy vetch mulch

Soil temperature b (°C)

Year Mulch biomass a (g m 2 ) Maximum Minimum Amplitude

1989 0 30.2 20.6 9.6

462 27.6 21.4 6.2

924 26.9 21.1 5.8

1990 0 26.3 15.5 10.9

319 23.7 17.0 6.7

638 23.6 16.3 7.3

1991 0 34.0 18.8 15.3

375 27.9 19.4 8.5

Notes:
a Biomass levels correspond to 0%,100%,and 200% ofthe biomass produced by the winter cover
crop in 1989 and 1990,and 0% and 100% in 1991.
b Mean of7 days in 1989,22 days in 1990,and 28 days in 1991.
Source: Teasdale & Mohler (1993),Teasdale & Daugherty (1993).

peratures or tends to cause modest night-time warming (Bristow,1988; Fortin
& Pierce, 1991; Teasdale & Mohler, 1993; Creamer et al ., 1996 a ), its principal
effect on temperature conditions is to decrease the amplitude of diurnal fluc-
tuations. For example, Teasdale & Mohler (1993) and Teasdale & Daughtry
(1993) observed mean maximum-minimum differences at 5 cm depth of
6-8 C without mulch (Table 5.2).

Creamer et al .(1996 a ) reported similar differences at 10 cm depth,with a mean
amplitude of 5

° C with mulch and fluctuations of 10-15 °

° C beneath mulch and 10 ° C for bare soil (computed from their

Figure 3).
For comparison, Totterdell & Roberts (1980) found that although a 5

C
temperature fluctuation promoted germination of Rumex crispus and R. obtusi-
folius , 10-15

°

C was required for 100% germination. Similarly, Benech-Arnold
et al . (1988) reported only 9% germination of after-ripened Sorghum halapense
seeds exposed to 4

°

° C fluctuations for 20 cycles but 35% germination for those

exposed to 10 C fluctuations. Thus, although the magnitude of temperature

fluctuations under mulch is sufficient to break dormancy of some seeds of
species sensitive to temperature fluctuations, the decrease in amplitude by
mulch can lower the percentage germination substantially.
Mulch decreases convection, which decreases the gradient in partial pres-
sure of water vapor between the soil and the general atmosphere. Together
with lower soil temperatures, this reduces evaporation from the soil surface
and keeps soil moist for a longer period (Griffith, Mannering & Box, 1986;
Teasdale & Mohler, 1993). These factors may facilitate the germination of

°