Agriculture Reference
In-Depth Information
in earthworm casts and burrows, that are critical to a mechanistic understanding of earthworm
effects but are difficult to extrapolate to the ecosystem scale. In addition to the challenge of
integrating information and analysis across spatial scales, there is the challenge of extrapolating
the short-term effects of earthworms through time, which, as Darwin (1881) noted more than a
century ago, can lead to cumulative changes at the landscape scale.
Further advances in the understanding of the role of earthworms in nutrient cycling processes
will depend on experimental studies, modeling approaches, and conceptual advances that integrate
across spatial and temporal scales. The greatest challenge to making these advances will continue
to be integrating across the ecological hierarchy and understanding the effects of earthworms on
nutrient cycling at the scale of whole ecosystems and landscapes through time.
OVERVIEW
In this chapter, we focus on research that integrates soil biology, chemistry, and physics and that
demonstrates how earthworms affect multiple soil nutrient cycling processes, and we highlight
potentially fruitful areas for new research. Although the majority of papers presented at the Fifth
International Symposium on Earthworm Ecology in 1994 (Edwards 1998) were conducted in
agricultural soils and systems, there has been a substantial increase since that time in research from
a variety of natural ecosystems, including tropical forests and grasslands (Fragoso et al. 1999a,b;
Lachnict et al. 2002; Liu and Zou 2002; Jimnez et al. 2003; Sanchez-De Leon et al. 2003),
temperate grasslands (Callaham and Blair 1999; Callaham et al. 2001), and temperate forests (Scheu
and Schaefer 1998; Lachnicht and Hendrix 2001; Bohlen et al. 2004a). Thus, many of the previously
noted gaps in the understanding of the effects of earthworms on nutrient cycling processes in natural
systems (Parmelee et al. 1998) have been filled. Many studies continue to focus on exotic earthworm
and peregrine species, especially in regions where these species dominate the earthworm fauna.
Some more recent research has begun to examine the influences of native species of earthworms
on nutrient cycling properties (e.g., Callaham and Hendrix 1998; Callaham et al. 2001; Lachnicht
and Hendrix 2001). Native communities of earthworms often inhabit ecosystems that have been
less disturbed by humans. These communities offer unique opportunities for research that may
elucidate differences in the effects of native and exotic species on nutrient cycling. Such research
is also badly needed from the standpoint of conservation of the native earthworm fauna. Although
much research still focuses on the effects of lumbricids on nutrient cycling, there has been a
substantial increase in research on the effect of earthworms on soil fertility and nutrient cycling in
the tropics and the importance of other earthworm families, such as the Megascolecidae and
Glossoscolecidae (Lavelle et al. 1999).
Lavelle et al. (see
Chapter 8
, this volume) highlight the importance of spatial and temporal
scales in addressing the roles of earthworms in nutrient cycling processes. The majority of research
has relied on laboratory microcosms or small, manipulated field enclosures that typically span short
periods of days to months and, occasionally, to years. The problem of expanding from small spatial
and short temporal scales to long-term effects at the field or landscape level is a major barrier to
an improved understanding of how earthworms affect nutrient cycles (Blair et al. 1995b).
One fruitful approach for investigating the longer-term effects of earthworms on nutrient cycling
involves the direct introduction or invasion of earthworms into new habitats (see
Chapter 5
, this
volume). Examining changes in nutrient cycling or storage through time following such earthworm
introductions provides a rigorous basis for understanding the integrated effects of earthworms on
nutrient cycling.
Lavelle et al. (see Chapter 8, this volume) stress the importance of earthworm feeding behaviors,
spatial distribution patterns, and the change in earthworm communities during ecological succes-
sion, all of which may contribute to the long-term effects of earthworms on nutrient cycling
processes. They stress the need to determine the quality and quantity of soil organic matter ingested
by earthworms because earthworms may alter the ratio of labile to recalcitrant organic matter and
