Agriculture Reference
In-Depth Information
TABLE 6.4
Food Requirement and Supply for the Earthworm
Population in a Winter Cereal Field
g DM m
−
2
g N m
−
2a
Litter (stubble, cattle slurry)
484
6.1
Roots and rhizodeposition
a
300
5
Requirement for maintenance and production
340Ï1050
4.6Ï7.1
a
Estimated; see text.
Although population densities and biomass fluctuated considerably over the period of study,
3.7 times the mean biomass, depending on the method of calculation. The estimates for nitrogen
turnover, via excretion and mucus production, were derived from
N laboratory studies conducted
15
with juvenile
.
The main sources of organic matter input to the soil were postharvest crop residues, autumn-
applied cattle slurry, and roots. Crop residues and slurry inputs were estimated, on the basis of six
50
L. terrestris
; the mean
nitrogen content of this material was 1.25%. Root inputs can be substantial but are difficult to
quantify. No direct measurements were available in the present example, but tentative estimates
can be made based on the results of studies that traced the fate of
Z
50 cm samples collected in September 1990, as 484.2 °199.5 (SE) g DM m
−
2
CO
-labeled photosynthate in
14
2
growing cereal plants.
Keith and Oades (1986) and Jensen (1994) calculated that the total amount of carbon translo-
cated belowground during the growing season in wheat and barley was equivalent to about one
third of the carbon contained in harvested grain and straw. Harvested dry matter at Lyons amounted
to about 1500 g m
, suggesting that the total input of organic matter to the soil via the roots could
be on the order of 500 g DM m
−
2
. Assuming that 40% of this is lost through root respiration (Jensen
−
2
1994), the amount available for root development and rhizodeposition (turnover of fine roots and
carbohydrate exudates) could be about 300 g DM m
. Rhizodeposition could account for about
−
2
half of this, with the remainder located in macroroots.
Studies on nitrogen turnover in spring barley in Sweden indicated that the quantity of nitrogen
allocated to roots was equivalent to about 25% of that removed at harvest (Andrn et al. 1990); on
this basis, the total nitrogen input to the soil from roots at Lyons could be about 5 g m
. Thus,
the total nitrogen input from aboveground litter and roots could be on the order of 800 g DM and
11 g N m
−
2
annually, with most of this becoming available to decomposer organisms in the autumn.
Other potential food sources for the earthworm populations are reingested casts, microbial biomass,
and soil organic matter, which could include significant quantities of nutrients recycled from dead
earthworm tissues.
Assuming an average earthworm tissue production rate of 200 mg fresh mass per gram dry
−
2
year
;
the estimated nitrogen
population at Lyons could be in the range of 340 to 1050 g m
−
2
−
1
(Table 6.3). If the actual tissue production and food requirement
values fall in the lower part of this range, as seems likely in a cool temperate soil, then the figures
suggest there is a reasonable match between food supply and demand. However, only a proportion
of the organic matter input is likely to be available to earthworms. Litter bag studies indicated that,
at most, 34% of crop residues are utilized by earthworms (Curry and Byrne 1997), and there is
likely to be severe competition from other organisms for organic inputs in the form of rhizodepo-
sition. Thus, it is unlikely that the level of earthworm population biomass recorded could be
sustained for long under continuous arable cropping.
requirement was 4.6 to 7.1 g m
−
2
