Agriculture Reference
In-Depth Information
G RASSLAND M ANAGEMENT
Grassland management can alter herbage production and utilization and the dramatic return of plant
residues to the soil (Andrzejewska and Gyllenberg 1980; Hutchinson and King 1980). The quality
and supply of residues and the responses of soil organisms are likely to be influenced by sward
composition, the extent and type of grazing, the frequency of cutting, and the quantity and type of
fertilizer applied (Curry 1994). Potentially important features of grazing, as opposed to cutting,
include the high proportion of ingested plant material returned to the soil in the form of dung,
heterogeneity associated with selective grazing, uneven distribution of excreta by gregarious domes-
tic animals, and negative impacts of trampling by cattle, particularly at high stocking densities (cf.
Murphy et al. 1995; de Bruyn and Kingston 1997; Byers and Barker 2000). In the absence of
grazing, frequent cutting and removal of herbage could result in much reduced rates of return of
plant residues to the soil.
A survey of the earthworm populations in 68 grassland fields in County Kilkenny, Ireland
(Muldowney et al. 2003), revealed significant positive relationships between intensity of grass-
land utilization as measured by either stocking density or nitrogen fertilizer use and overall
earthworm biomass, but not abundance ( Figure 6.1 ) . The response mainly reflected trends in
populations of anecic earthworm species (mainly L. terrestris ) and seemed to reflect an increase
in body size rather than number. A similar overall trend of increasing earthworm biomass with
increasing intensity of grassland management was reported from Switzerland by Cuendet
(1996). However, no such relationship was detected by Boag et al. (1997) in Scotland, and
Byers and Barker (2000) reported a negative correlation between earthworm populations and
number of dairy cows per hectare in drought-prone soils in Pennsylvania. Preliminary results
from ongoing studies in Ireland do not indicate any consistent relationships between manage-
ment intensity and earthworm biomass in heavy-textured soils prone to poaching. Thus, it
appears that the benefits for earthworms of enhanced food supply from plant residues and dung
at higher fertilizer and stocking levels may be offset by negative effects of trampling in
susceptible soils.
A RABLE C ROPPING
Earthworm populations in arable land are generally lower than those in undisturbed habitats (Chan
2001), but not always. The level of direct mortality associated with cultivations depends on the
severity and frequency of soil disturbance. Ploughing per se does not appear to cause serious
mortality: Cuendet (1983) estimated that 5 to 10% of the earthworm biomass was brought to the
surface by plowing, with about 25% of these earthworms mortally wounded. Rotary cultivation
can reduce numbers by 60 to 70% (Bostrm 1988), and earthworms can be virtually eliminated by
very intensive soil cultivation, such as bed tilling and destoning for potato cropping (Curry et al.
2002). Populations generally recover within 1 year from less-severe forms of cultivation, provided
the disturbance is not repeated. Larger, anecic earthworm species such as L. terrestris and A. longa,
which require a supply of surface litter and have relatively permanent burrows, are the species most
adversely affected by repeated soil disturbance; smaller endogeic species such as A. chlorotica and
A. caliginosa are less affected and can benefit from plowed-in crop residues (Edwards 1983; Lofs-
Holmin 1983). Tropical earthworms have little tolerance for cultivations (Lal, 1987). Earthworms,
especially the larger, deep-burrowing species, are favored by minimum tillage and direct drilling
compared with conventional methods of cultivation (Gerard and Hay 1979; Edwards and Lofty
1982a; Hendrix et al. 1986, 1992; Hutcheon et al. 2001).
Indirect effects of cultivations, which are likely to affect earthworms adversely, include greater
variability in surface soil temperature and moisture regimes in the absence of a permanent vegetation
 
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