Agriculture Reference
In-Depth Information
we observed that earthworms can selectively ingest organic material that has a low C:N ratio
(Bohlen et al. 1997; Ketterings et al. 1997). Consequently, the remaining organic material has a
higher C:N ratio, with the potential for slower rates of decay and greater immobilization of nutrients.
Loss pathways include a more rapid turnover of microbial biomass and greater production of CO
2
,
denitrification, leaching, and runoff.
Studies of the effects of earthworm invasions or introductions into new habitats provide further
insights into the effects of earthworms on the net storage or losses of nutrients at the ecosystem
scale. In these situations, whole system changes in storage and distribution of nutrients can be
compared before and after earthworm invasions or between invaded and uncolonized sites. Studies
in temperate forests indicated that earthworm invasions can lead to a decrease in soil C storage
(Alban and Berry 1994; Bohlen et al. 2004a,b). Similarly, introduction of earthworms to pastures
in New Zealand led to losses of 300 to 1000 kg C ha year
−1
(Stout and Goh 1980).
Less information is available on the effects of earthworm invasions on the losses or retention
of N. However, earthworm invasion of northern forests in the United States apparently did not alter
total soil N significantly, indicating that any N that was mobilized by earthworm invasion was
retained within these N-limited systems (Bohlen et al. 2004a). Furthermore, earthworms altered
the distribution and abundance of fine roots and increased total soil microbial biomass, thereby
changing the potential patterns of uptake and turnover of nutrients in the plant and microbial
community (Fisk et al. 2004; Groffman et al. 2004). Invasion or intentional introductions of
earthworms into new environments will continue to provide insights on the effects of earthworms
on nutrient cycling at the ecosystem scale (see
Chapter 5
, this volume).
E
ARTHWORMS
IN
L
ANDSCAPES
The effects of earthworms on nutrient cycling processes at the landscape level have yet to be
explored fully, but a much better understanding of the spatial dynamics of earthworm populations
is emerging. Lavelle et al. (see
Chapter 8
, this volume) present an intriguing concept of earthworm
patch dynamics in which patches of different species and different size classes of earthworms move
through the soil, thereby altering soil structure and nutrient cycling processes and contributing to
soil heterogeneity.
Poier and Richter (1992) investigated the spatial distribution of earthworms and identified patch
sizes of 20 to 50 m and found correlations between earthworm populations and soil carbon and
aggregate densities. Hendrix et al. (1992) examined the populations and distribution of earthworms
in relation to landscape factors in the southeastern United States. They concluded that earthworm
populations were related to soil textural properties, quantity and quality of plant inputs, and standing
stocks of soil organic matter. Callaham and Blair (1999) reported that the distribution and relative
abundance of native and exotic earthworms in temperate grassland was influenced by land use
practices, including burning, mowing, and nutrient additions. Similarly, Bohlen et al. (1995) found
that the earthworm community structure in seven small agricultural watersheds was influenced by
cropping patterns, geographic location, and tillage. Examination of the presence and absence of
earthworms in a northern temperate forest revealed a spatial pattern in which invasive earthworms
were present along the edges of the forest near previous agricultural land use but were encountered
less commonly in the forest interior, which gave a picture of invasion dynamics at the landscape
scale (Bohlen et al. 2004a).
More such studies are needed, and the next step should be to link spatial distribution patterns
to the effects of earthworms on nutrient cycling properties and feedbacks between soil physical
and hydrological properties and earthworm populations. The ultimate goal of landscape-level
research should be to identify the major source vs. sink relationships for changes in soil C and N
pools. For example, the influence of earthworms on soil erosion and surface runoff may also
influence the losses or retention of nutrients at the landscape level (Sharpley et al. 1979). As
