Justus Liebig
University
Giessen
Research Centre for
Bio Systems, Land
Use and Nutrition
IFZ - Department of Animal Ecology
Introduction
0,16
Treatment:
nematodes
lumbricids
lumbricids & nematodes
0,14
32 microcosms (10 cm diameter, 23 cm height)
were filled with soil substrate and inoculated
with earthworms and nematodes in 3 densities
and in combination according to a D-optimised
incomplete design.
The microcosms were kept outside for 58 days
and were subjected to near natural weather
conditions. Temperature and moisture were
partially manipulated to create a wider
spectrum of microclimates.
CO2 was measured daily by means of gas
chromatography.
Soluble
nitrogen
was
measured at the end of the experiment.
Treatment:
lumbricids
nematodes
lumbricids & nematodes
0.14
0.12
Tab. 1: GRM analysis
0,06
0,04
0,02
0,00
-0,02
-0,04
25
30
35
40
45
50
55
soil water content [% WHC]
0.08
Soluble carbon and nitrogen concentrated
within earthworm casts are more readily
accessible in a water filled pore space because
diffusion is largely facilitated in the thicker water
films present in such pores (Fig. 3a).
0.06
0.04
0.02
0.00
-0.02
-0.04
20
25
30
35
40
45
50
55
60
soil water
content
WHC]
soil water
content [%[%
WHC]
Fluctuations in soil moisture affected the impact
of earthworms and nematodes on soil CO2
evolution as well as the nitrogen availability in
the opposite way (Tab. 1). Temperature effects
were not significant.
0,08
0.10
-0.06
15
Results
0,10
Fig. 2: Nitrogen concentration in the soil solution at
the end of the experiment as dependent on
soil moisture and faunal composition
0.16
Residual
CO2
Residual
ofofCO
2
Methods
At the end of the experiment, nitrogen
concentration in the soil solution was strongly
reduced at higher soil moistures in the
presence of lumbricids, while nematodes
induced a slight increase (Fig. 2).
0,12
Residuals of N sol
We analysed whether changes in soil microclimate modulate in a different way the impacts of two
faunal groups on soil organic matter transformation. Earthworms were chosen as representative
of those parts of the soil fauna that excrete stable pellets with a relatively high carbon content.
Nematodes, in contrast, were chosen as a model for soil fauna that creates liquid excreta with
high nitrogen content.
Fig. 1: Soil CO2 evolution as dependent on soil
moisture and faunal composition
Discussion
We interpret the diverging trends of CO2
evolution
and
nitrogen
concentration
remaining at the end of the experiment to
originate from the same consumption
process: Carbon is respired, and nitrogen is
assimilated by the micro-flora.
We assume that the opposing effects of
earthworms and nematodes on microbial soil
organic matter transformation can be largely
explained by opposing effects of increased
soil water content on the accessibility of
carbon and nitrogen from animal excreta for
micro-organisms.
In contrast, nitrogen dissolved in liquid
nematode excreta diffuse further away from the
inner pore surface colonised by the microorganisms, if pores are water filled (Fig. 3b).
As a consequence of their opposing effects,
the combination of lumbricids and nematodes
exhibited less dependence on water content.
This was particularly visible under dry soil
conditions in our experiment.
Conclusions
The results illustrate how different functional
groups of soil organisms can create
complementary effects and thereby stabilise
soil processes in the presence of a diverse soil
community.
(n.a. = not available; n.s. = not significant)
Higher moisture strongly augmented soil CO2
evolution in the presence of earthworms. In
contrast, soil CO2 evolution nearly ceased
under higher moisture in the presence of
nematodes. Both faunal groups together
exhibited an additive superimposition of the
single effects (Fig. 1).
Fig. 3a: Diffusion of soluble nutrients out of an
earthworm cast is largely facilitated if water films
are thick or pores are water saturated in the
surrounding soil.
Fig. 3b: Nutrients from liquid nematode excreta
are diluted and removed away from the microbial
film residing at the inner pore surface (orange) in
water saturated pores.
Acknowledgement: This research was conducted within the Priority Programme “Soil as sink and source of CO2”, funded by the Deutsche Forschungsgemeinschaft.
O. Fox, S. Vetter, K. Ekschmitt and V. Wolters
Justus Liebig University, Department of Animal Ecology
Heinrich-Buff-Ring 26-32 (IFZ), D-35392 Giessen, Germany
E-mail: [email protected]