Agriculture Reference
In-Depth Information
M
ACROSCALE
I
NTERACTIONS
BETWEEN
E
ARTHWORMS
AND
M
ICROORGANISMS
When CC cropping soils are converted to NT or DD fields, a change in the composition of the soil
microbial communities occurs, with increasing fungal:bacterial ratios (Brussard et al. 1990; Coleman
et al. 1994). Earthworms are also more abundant under NT (DD) than under CC (Chan 2001), and
this has been attributed in part to changes in the composition of the decomposer community. Although
such causal relationships can be inferred readily from observed changes in earthworm community
composition, experimental field evidence supporting such conclusions is scarce. Several experiments
that manipulated earthworm populations under field conditions have cast some light on this relation-
ship. For instance, in small field plots in Ohio, Blair et al. (1997) observed a greater microbial biomass
N in treatments with an experimentally reduced (by electroshocking) earthworm population, and
Hendrix et al. (1998) reported lower microbial biomass N but higher turnover rates in earthworm
addition treatments in Georgia. These results seem to imply that earthworm activity exerts a regulatory
influence on microbial communities, resulting in a lower microbial biomass that is more metabolically
active and has higher turnover rates (Edwards and Bohlen 1996).
There is much evidence indicating the involvement of earthworm activity in the recycling of
C, N, and P in soils (Syers et al. 1979a,b; Hendrix et al. 1987; Bostrom 1988; Parmelee et al. 1998;
Chapter 8
, this volume), which, by implication, suggests that earthworms also modify soil microbial
functions. For instance, as much as 60% of the C losses from earthworms during their life span
can be in the form of mucus secretions (soluble organic C) that act as important microbial stimulants
(Scheu 1991). Carbon losses caused by earthworm respiration are generally not large but can be
as much as 29% of the total heterotrophic respiration in DD systems (Hendrix et al. 1987). In an
extensive study of the role of earthworms in C recycling in a Swedish alfalfa field, Bostrom and
Lofs-Holmin (1988) reported that C flow through a population of
A. caliginosa
(with biomass of
33 kg C ha
−1
) in 1 year totaled 3.8 t C ha
−1
in consumption, 3.8 t C ha
−1
in fecal egestion (casts),
47 kg C ha
−1
lost in respiration, 41 kg C ha
−1
in dead tissues, 10 kg C ha
−1
in cocoon production,
and 40 kg C ha
−1
in tissue production. Mucus production was not assessed in this study.
Earthworm invasion (or newly inoculated) sites, in particular, provide the opportunity to study
the effects of earthworms on elemental cycles without confounding effects of long-term previous
earthworm populations. The results of several studies (Stout 1983; Alban and Berry 1994; Burtelow
et al. 1998) showed that earthworms can induce rapid and large C and N losses, but that these are
unlikely to continue indefinitely. More likely, a new equilibrium will be reached once the earthworm
invasion process has ended and stabilized, in which case the total soil C stocks are lower, but with
certain fractions and particle sizes of OM turning over more rapidly than in the previously uninvaded
soils (Brown et al. 2000).
Nitrogen flows through earthworm populations have been estimated by several authors for
various agroecosystems (Marinissen and de Ruitter 1993); in the alfalfa field studied by Bostrom
and Lofs-Holmin (1988), N flows reached as much as 516 kg N ha
−1
year
−1
. Of this total, 10 kg N
ha
−1
were for tissue production, 2 kg N ha
−1
were in excretions (mostly in plant-assimilable N
forms), and 504 kg N ha
−1
were in casts. Priming effects and indirectly increased N cycling (because
of consumption and activation of microorganisms and invertebrates) are important but difficult to
assess. In the experimental earthworm population reduction/increase experiment in Ohio, the
addition of four earthworm species (mainly
L. terrestris
and
A. tuberculata,
but also
L. rubellus
and
A. trapezoides
; Bohlen et al. 1995) increased amounts of extractable mineral N (NH
4
and NO
3
),
significantly depending on the treatment (Blair et al. 1997). However, the addition of deep-burrow-
ing earthworms (
L. terrestris
) to a nearby field site had no effect on mineral N concentrations but
increased the dissolved organic N, potentially mineralizable N, and microbial biomass N, depending
on the earthworm treatment (Subler et al. 1997). The increased soil leachate volumes (4- to 12-
fold greater) observed at this site, probably because of increased macropore flow (Lachnicht et al.
1997), significantly increased the leaching of dissolved organic N (Subler et al. 1997). Such different
results obtained regarding influences of earthworms on soil microbial biomass and mineral N
