Agriculture Reference
In-Depth Information
earthworms on them also needs much further study. The importance of earthworms as key organisms
controlling soil food webs also needs further research (Edwards 1999).
The impacts of earthworm activity on soil structure in agroecosystems are receiving increasing
attention. Tomlin et al. (1995) identified several potentially fruitful lines for further research.
Although increased soil porosity and water infiltration are well-known consequences of earthworm
activity, relatively little is known of the mechanisms by which solutes (e.g., agricultural chemicals)
are transported through earthworm burrows. Because earthworm burrow linings may act as both
sources and sinks for solutes, better understanding is needed of their physical, chemical, and
microbial characteristics (e.g., organic chemical nature and exchange properties) (see
Chapter 10
and
Chapter 11
, this volume).
Soil aggregation may be one of the most important soil properties affected by earthworms in
both agricultural and natural ecosystems. Under laboratory conditions, earthworms can enhance
aggregate stability, depending on water content, organic matter quality, time since cast deposition,
and so on (Shipitalo and Protz 1989). However, it is not certain if these results can be extrapolated
into a generalized concept of earthworms improving soil structure under field conditions. The key
to understanding this phenomenon may lie in better knowledge of the nature and dynamics of
organic compounds and their interactions with soil minerals as influenced by earthworms. Both
field and laboratory studies are needed to address this problem adequately, and this should be a
profitable area for future research. The stability of earthworm casts also has implications for the
storage and turnover of nutrients in soil and should receive more attention.
Edwards et al. (1995) and Lee (1995) made a number of recommendations for various kinds
of research specifically on earthworms in agroecosystems. Many ecological questions about earth-
worms remain unanswered, including details on basic life cycles, habitat requirements, and distri-
bution and population patterns in many geographical areas. The basic biology and ecology of
relatively few species are understood, and these are mainly lumbricids. However, Barrios et al.
(1999) summarized the knowledge of the life histories of 59 species of earthworms very effectively.
Projects such as the ÑEarthworms DownunderÒ study in Australia (Baker et al. 1995a) are needed
in agricultural regions worldwide if the resource base available for future earthworm research is
to be assessed. Such efforts will also provide base information for earthworm inoculation studies,
which often are used in laboratory experiments but need to be done much more on a field scale to
assess the potential for earthworms to enhance crop growth and improve soil properties in agroec-
osystems.
A much better understanding of the impacts of various earthworm species and earthworm
community assemblages on soil organic matter dynamics in agroecosystems, especially those under
no tillage or receiving mainly organic inputs, is needed. Earthworms may be a primary mechanism
for the incorporation and decomposition of organic residues in these systems. Also, it could prove
fruitful to include earthworms in simulation models of soil organic matter turnover, which typically
do not explicitly include the effects of the soil biota other than microorganisms and roots (see
Chapter 9
, this volume).
Finally, there is a continuing need for studies into the effects of crop rotations and management
systems (including use of cultivation, pesticides, fertilizers, irrigation, etc.) on earthworm abun-
dance, diversity, and distribution. This is particularly true when new management practices and
cropping systems are tested and management goals include the buildup and maintenance of earth-
worm populations. Much more information along these lines is needed to improve the ability to
manage earthworms in the field as tools to improve soil fertility and productivity.
CONCLUSIONS
The growing number of participants at each successive International Symposium on Earthworm
Ecology attests to the increasing interest in many aspects of earthworm biology and ecology. This
chapter has covered only some of the selected highlights of presentations at the international
