134 A.Sh. Mamilov et al. Applied Soil Ecology 16 2001 131–139
3. Results and discussion
3.1. Bacterial, fungal and microbial biomass content and respiratory activity
The application of both alfalfa meal and starch led to the macroscopically visible development of mycelium
on the soil surface of all microcosms in the presence of the fauna. In defaunated soil, the mycelium was
only visible in the soil treated with alfalfa meal but not with starch. This indicates that soil fauna apparently
Fig. 1. Microbial M, fungal F and bacterial B biomass and fungalbacterial ratio FB in native soil left side, and the effect of fauna on microbiological characteristics compared to defaunated soil right side during the course of decomposition of starch, wheat straw and
alfalfa meal; bars indicate standard deviations.
stimulated fungal growth when substrate containing no nitrogen was added.
Amendment of soil with alfalfa meal and starch led to significant increases of total microbial and fungal
biomass in native and defaunated treatments Fig. 1. The effect was more pronounced for alfalfa meal indi-
cating the high substrate quality. The dynamics of fun- gal biomass in the treatment with starch was similar
to the dynamics of the total microbial biomass during wheat straw decomposition. When starch was added,
the fungal and total biomass and also the respiration
A.Sh. Mamilov et al. Applied Soil Ecology 16 2001 131–139 135
Fig. 2. Microbial respiration rates CO
2
and metabolic quotient qCO
2
in native soil left side, and the effect of fauna on microbiological characteristics compared to defaunated soil right side during the course of decomposition of starch, wheat straw and alfalfa meal; bars
indicate standard deviations.
rates in the native soil Fig. 2 exceeded those values of the defaunated soil by approximately 20–30 after
10 days. In contrast, soil amended with N-rich alfalfa meal showed an approximately 25 higher level of
fungal biomass in the absence of animals. Throughout the whole experiment, the rate of CO
2
evolution was also higher in the defaunated treatments Fig. 2.
At the end of the experiment, the bacterial biomass was approximately 40 higher in the presence of
soil nematodes and microarthropods when soil was amended with starch and alfalfa meal Fig. 1. No sig-
nificant changes in bacterial biomass occurred in the absence of soil fauna after three days of incubation
data not shown.
In the treatments with wheat straw, the microbial biomass was lower in native than in defaunated soil
until day 7 Fig. 1. After day 10, the presence of nematodes and microarthropods permitted microbial
biomass values of up to 1300 mg C g
− 1
soil, which was approximately 25 higher than in the defaunated
soil. A similar, more pronounced, trend was found for the respiration rate Fig. 2. The respiration rate
was higher in the defaunated soil in the first week of incubation. The effect of straw addition resembled
more that of starch than that of alfalfa meal. 3.2. Dynamics of fungal-to-bacterial ratio
The ratio of fungal to bacterial biomass decreased in all treatments independently of the substrate composi-
tion during the course of the experiment Fig. 1. In the presence of soil animals, the fungalbacterial ratio de-
creased from 5.3 and 5.6 to 2.0 and 2.5 for treatments with starch and alfalfa meal, respectively. This was
mainly due to bacterial growth. Considering that the presence of fauna support N liberation from consumed
microbial cells, our findings agree with observations of Bardgett and McAlister 1999 who showed that
high N availability in grassland soils favoured bacteria more than fungi and, thus, decreased fungalbacterial
ratio. In defaunated soil, the fungal biomass de- clined with no significant changes in bacterial
biomass.
3.3. Faunal abundance The abundance of soil fauna is shown in Tables 2
and 3. The animal populations increased most ob- viously when soil was amended with alfalfa meal.
The growth of soil fauna seems, therefore, to have controlled the growth of microbial components. The
136 A.Sh. Mamilov et al. Applied Soil Ecology 16 2001 131–139
Table 2 Abundance of soil micro- and mesofauna during decomposition of alfalfa meal and starch in a soddy-podzolic soil
Treatment Day
16 25
Nematodes number per 10 g
− 1
soil Native + alfalfa meal
280 ± 38
a
1816 ± 47 2564 ± 62
Defaunated + alfalfa meal 8 ± 1
254 ± 44 Native + starch
280 ± 38 427 ± 52
726 ± 57 Defaunated + starch
5 ± 1 72 ± 12
Microarthropods number per 90 g
− 1
soil Native + alfalfa meal
32 ± 4 ND
b
52 ± 7 Defaunated + alfalfa meal
ND Native + starch
32 ± 4 ND
61 ± 5 Defaunated + starch
ND
a
Standard deviation.
b
ND: not determined.
numbers of nematodes were more enhanced than those of soil mites and Collembola. Thus, the varia-
tion of nematode biomass appears more interrelated to microbial biomass. Grazing of microarthropods may
certainly affect microbial biomass and respiration ac- tivity but probably mainly in decomposing faeces of
macrofauna Van der Drift and Jansen, 1977 or fresh leaf litter Hanlon and Anderson, 1979; Ineson et al.,
Table 3 Abundance of soil micro and mesofauna during decomposition of wheat straw in a soddy-podzolic soil
Treatment Day
16 25
Nematodes number per 10 g
− 1
soil Native soil + wheat straw
43 ± 12
a
282 ± 27 1517 ± 83
Defaunated soil + wheat straw
78 ± 17 480 ± 54
Control 43 ± 12
36 ± 8 223 ± 31
Microarthropods number per 90 g
− 1
soil Native soil + wheat straw
Collembola 2
ND
b
3 ± 1 Mites
9 ± 3 ND
61 ± 6 Defaunated soil + wheat straw
Collembola ND
Mites ND
8 ± 3 Control
Collembola 2
ND Mites
9 ± 3 ND
12 ± 5
a
Value of standard deviation.
b
ND: not determined.
1982; Leonard and Anderson, 1991 and poorly in mineral soil horizons. The observation of Klironomos
and Kendrick 1996 and Kaneko et al. 1995 also indicated that many arthropods seem preferentially to
graze in litter rather than in the deeper soil layers and Heneghan et al. 1999 observed that microarthro-
pods did not strongly modify N mineralisation under specific conditions.
A.Sh. Mamilov et al. Applied Soil Ecology 16 2001 131–139 137
3.4. Interrelation between bacteria, fungi and fauna Soil fauna positively influenced the total microbial
biomass, fungal biomass and CO
2
evolution in soil amended with starch and wheat straw after seven days
of decomposition. The presence of soil animals may have increased the content of available nutrients In-
eson et al., 1982; Setälä and Huhta, 1991; McGo- nigle, 1995. Mechanisms of the stimulatory effects
of the fauna on microbial growth and respiration may involve 1 the release of nutrients from the grazed,
either consumed, non-digested, or digested microbial biomass Coleman et al., 1983; Clarholm, 1985, and
2 the transport of limiting nutrients by animals to- wards decomposing plant residues. Migration of ne-
matodes feeding on bacteria towards plant residues may accelerate the microbial biomass turnover Grif-
fiths and Caul, 1993. The grazing of soil animals may affect the physiological status of micro-organisms
Hedlung et al., 1991, which was indicated here by the metabolic quotient. Anderson 1987 considered
that one third of total immobilised N might be re- leased as the result of faunal activity. The release of
compounds from microbial biomass by grazing can explain accelerated decomposition rate of substrates
when microbial growth depleted previously available nutrients. In addition, the fauna may kill rather than
digest biomass, which is consistent with the observa- tion that microbial grazers have high consumption but
low assimilation rates Anderson et al., 1981.
Soil microbiological parameters were not appar- ently affected by predation during the first few days.
However, microfauna significantly stimulated the res- piration rate of soil amended with starch and wheat
straw after seven days Fig. 2. The opposite effect of the microfauna occurred when microbial production
was not restricted by available N, as indicated by the treatment with alfalfa meal. The CO
2
evolution rates and fungal growth were reduced approximately 25
by predation. Larsen and Jakobsen 1996 observed that the collembolan Folsomia candida reduced P
transport by 75 in the mycelium of Glomus caledo- nium when soil was enriched with dry yeast. In the
absence of soil Collembola, yeast addition increased AM-hyphal P transport by the development of an ex-
tended hyphal net. They proposed that the interaction between F. candida and the external mycelium of
G. caledonium was controlled by nutritional condi- tions. It should be considered that invertebrates also
consumed micro-organisms in non-amended soil but the predation under enriched conditions led to sup-
pression of microbial growth. Schlatte et al. 1998 observed no significant effect of microarthropods on
microbial production but did find increasing NH
4 +
content during decomposition of forest litter. The lack of stimulatory effects of soil fauna in their study may
be due to ecological conditions favouring microbial growth, which became less controlled by grazing.
Thus, nutrient status seems to be one of the most important factors determining the effect of soil fauna
on microbial composition and activity.
The microbial biomass was also stimulated by the soil fauna when straw was added. Adding N-poor and
N-free substrates to soil leads to microbial N immobil- isation. Cochran et al. 1988 demonstrated that micro-
bial production was limited by N when leached wheat straw was added. Glucose amendment decreased the
growth of pine seedlings due to microbial immobili- sation of available N and P Bååth et al., 1978. Thus,
available N may have initially restricted microbial growth during starch decomposition Fig. 1. After
seven days, predation on N-limited fungal and bac- terial biomass probably accelerated nutrient turnover
and increased mineralisation rate. When soil was sup- plemented with alfalfa meal, microbial growth was
less restricted by available N. Under those conditions, preferential consumption of active microbial cells as
described by Klironomos and Kendrick 1995 led to the suppression of total soil respiration and fungal
biomass. This is confirmed by the metabolic quotient Fig. 2. The amount of CO
2
respired per unit of micro- bial biomass was higher in the presence of microfauna
after seven days when N-poor substrates were decom- posed but not modified when alfalfa meal was added.
In soil containing nematodes and microarthropods, the bacterial portion of total microbial biomass in-
creased during decomposition. This is caused by an increase in the bacterial component by approximately
40 ww. These results resembled those observed during the succession initiated by re-wetting Zvyag-
intsev et al., 1981. Neely et al. 1991 demonstrated that the ratio of fungal to bacterial biomass decreased
during decomposition of plant residues. Our results underline that succession either initiated by re-wetting
or by amendment of fresh plant residues similarly modifies the food web. An increase in the bacterial
138 A.Sh. Mamilov et al. Applied Soil Ecology 16 2001 131–139
component of total microbial biomass by the micro- faunal grazing agrees with the effect of soil macro-
fauna Anderson et al., 1985. Passage through the gut of the earthworm Lumbricus rubellus did not change
significantly the total microbial biomass but did in- crease the bacterial component Daniel and Anderson,
1992.
4. Conclusions
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