Agriculture Reference
In-Depth Information
incorporated into the microbial biomass. Nearly 40 to 50% of the earthworm N moved into soil
organic N pools during the first week, but this declined to less than 19% after 16 days. After 16
days, 70% of earthworm N was incorporated into plant biomass. Clearly, the N stored in earthworm
tissues enters the labile soil N pool rapidly, where it is available for uptake by microorganisms and
plants.
E
C
ARTHWORM
OMMUNITIES
Much of the information gathered at lower levels in the ecological hierarchy can be integrated at
the level of the earthworm community. Estimates of earthworm community secondary production
can quantify the direct role of earthworms in the C and N cycle (Parmelee and Crossley 1988).
Secondary production estimates the C and N fluxes through earthworm tissues as the sum of the
production of new earthworm tissue and the losses through earthworm mortality.
Secondary production of earthworms during any given time interval is calculated as the growth
rate times the change in biomass:
P
=
IGR
[(
B
+
B
)/2]
t
, where
P
equals production,
IGR
is the
f
i
f
and
B
i
are the final and initial standing stock biomasses, respec-
tively, in grams AFDW m
−2
observed over a time interval
t
measured in days (Romanovsky and
Polishchuk 1982). The values for all time intervals can then be summed to calculate annual
secondary production.
To obtain an estimate of the C or N flux through earthworm biomass, the annual earthworm
community secondary production estimate is multiplied by the average percentage C or N in
earthworm tissue. Using this method in a no-tillage agroecosystem, we estimated that there was a
40-kg ha
−1
year
−l
flux of N through earthworms (Parmelee and Crossley 1988). When estimates of
N excreted in urine and mucus were included, the total N flux through earthworms increased to
63 kg N ha
−l
year
−l
, indicating that N flux through the earthworm community was significant for
the no-tillage system, and earthworms could process 50% of the N input to soils from plant residues,
accounting for 38% of the N uptake by plants. Furthermore, the N flux through earthworm biomass
exceeded the losses from the system by denitrification and leaching. Although this example illus-
trates the direct importance of earthworms in C and N cycles, it may still be inadequate for a
general conclusion because it relies on laboratory earthworm growth rate data and NeedhamÔs
(1957) possibly inadequate estimates of urine and mucus production.
A more recent study assessing two different methods for estimating earthworm secondary
production and N flux in agroecosystems showed that N flux through the earthworm community
was 18 to 55 kg N ha
−1
year
−1
(Whalen and Parmallee 2000). The lower estimates were for
inorganically fertilized systems, and the higher estimates were for organically fertilized systems.
The higher estimates are within the range of the earlier estimates of Parmelee and Crossley (1988)
and represent up to 30% of crop N uptake. As discussed, reliable estimates of rates of aboveground
earthworm cast production exist, but aboveground casting may constitute only a small proportion
of the total cast production. Studies that quantify belowground casting rates for earthworm com-
munities are needed.
instantaneous growth rate, and
B
E
ARTHWORMS
IN
E
COSYSTEMS
The effects of earthworms on nutrient cycling processes at the individual, population, and commu-
nity levels of the hierarchy are integrated fully at the ecosystem level. Much of the research needed
at the ecosystem level has been discussed in terms of ecosystem budget models (
Figure 9.3
and
Figure 9.4
)
. The main question of further interest concerns how earthworms affect the balance
between processes that lead to conservation or storage of nutrients vs. losses from the system.
Processes that lead to storage of C and N include plant uptake and protection in stable soil aggregates
formed by earthworm casts. Another possible mechanism (as discussed by Lavelle et al. in
Chapter
8
) may involve the types of organic material that earthworms ingest. From our field studies in Ohio,
