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Dear All, Dec 2, 2007
With winter only 3 weeks away we can perhaps resume some aspects of
the earthworm thread.
I read two recent papers in October and from these learned, perhaps
for the second or third time, that not all earthworms are equal. Some
pull litter into the mineral layer (the ones made known to me by their
works) and other species apparently feed mostly above the mineral layer.
One of these papers, Patterns of litter disappearance in a northern
hardwood forest invaded by exotic earthworms, Suarez et al., 2006 can be
downloaded as a pdf file (283 kb) at--
Patrick J. Bohlen, Home Page, Archbold Biological Station; earthworms
<http://archbold-station.org/abs/staff/pbohlen/pbohlen_publications.htm>
Before considering this paper I wish to make some personal comments.
The invasive species that gives me most concern is the one whose members
e.g. lay plans to twin a highway and boast that doing so will decrease
greenhouse gas emissions by shortening the distance between centers of
population (or drivel to that effect).
Also, even in the most simple system, it can be easy to reach
incorrect conclusions. Some decades ago I encountered a good example of
this in some work involving Sodium. To make a long story shorter, after
many attempts, determinations of Sodium in National Bureau of Science
and Technology (or some similar name) standards were consistently too
high but blanks (an empty crucible in each ashing batch that was
intended to detect contamination) were low, indicating no appreciable
contamination from lab sources and I phoned NBST at least twice to ask
if their plant tissue standard might have had a batch with defective
Sodium. And I was assured, correctly, no way. [The former name of this
agency was National Bureau of Standards (USA) and they tended to get
things right.]
Our ashing oven had become contaminated with Sodium (from several
high Na samples) but the blank did not detect this because the blank
crucible had never been used to ash plant material and consequently had
a much lower surface area, for Sodium vapor to partition onto, than did
the crucibles that had become etched through repeated use. Consequently,
a quality control procedure that had served well for many decades to
detect water, air or reagent contamination, failed under these different
circumstances; contamination by Sodium vapor in the ashing oven.
Now to get to the perhaps dry to many, interesting to me, stuff.
Table 1 represents a comparison of 3 trts and 2 litter types on 2
sites (between-subjects effects; with 20 df in the Error term shown
when in fact I think there are at most 2 that could be based on the
3-way interaction of TLS, because there are no true replications of the
plots) at three dates (Within-subject effects; with 100 df in the Error
term whereas I get about 15 depending upon choices) as an analysis of
variance.
This substitution of measurement error for experimental error may be
the norm in Ecological studies but, norm or not, I think 'Honey I
shrunk the Error Mean Square by inflating the Error df " is unsound.
At both sites and in the absence of earthworms, about 60% of
applied leaf litter remained after 340 days and nearly 50% remained
after 540 days (Fig. 1). In the real world, one would not have 50% of
the leaf material that had been shed in year x present in the spring of
year x+2 because this implies limitless accumulation of litter.
Consequently one must conclude that there is something that arises from
the methods used which slows the rate of litter loss in the absence of
earthworm activity.
Anything that keeps litter artificially dry would have this effect
(e.g. by inhibiting feeding by Isopods, Snails and by slowing
decomposition by fungi or bacteria). The 'litter boxes' would do this
because they have a roof of 1-mm mesh, a floor of 1 cm mesh and solid
walls about 12 cm high. The floor mesh would decrease contact between
litter and the underlying soil and the fine roof mesh would entirely
prevent compression of intact leaves into a moist mat by snow. [I moved
some 2007 vintage leaves to pile wood about 2 weeks after our first snow
this year and many in the lower half of the leaf mat were already coated
with fungal hyphae.] The litter boxes were 50 cm x 50 cm and the roof of
1-mm fiberglass screen would have sagged such that during light rain or
snow-melt a high proportion of ppt. would have fallen near the middle of
the box.
Consequently, I think their method, by slowing normal loss, will
tend to overestimate the impact of earthworms on litter loss. But based
on their methods preamble (Study sites) there appears to be a definite
effect of earthworms ("...heavily invaded sites have no forest
floor..."). [Or do earthworms preferentially colonize sites on which
litter is more rapidly lost ? Or with forest floor depleted by repeated
fires ?].
It must help to be there.
The comment, in the introduction, to the effect that earthworms
cause "...reductions in soil organic matter content..." is puzzling
because soil organic matter usually refers to the residual fraction
that is relatively resistant to decomposition except in cultivated soil,
where better aeration and high nutrient flow speed loss. Is the better
profile aeration brought about by earthworm holes and the higher
nutrient flow due to returning fines to the surface sufficient to
deplete the normally stable soil organic matter ? Are earthworms absent
in the Ukraine and in other areas of Chernozem soils ?
Yours truly, Dave Webster, Kentville
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