microscopy (Jenkinson et al., 1976; Paul and van. Veen, 1980; Tesarova and ... (Anderson and Domsch, 1978) were somewhat lower in soils below pH 5 than in ...
Soil Bid. Bioehem. Vol. 19, No. 6, pp. 6894%. Printed in Great Britain
0038-0717/87 53.00 + 0.00 Fkrgamon Journals Ltd
1987
MICROBIAL BIOMASS MEASUREM~~S IN SOILS: DETERMINATION OF kc VALUES AND HYPOTHESES TO EXPLAIN THE FAILURE CHLOROFORM FUMIGATION-INCUBATION IN ACID SOILS
FOREST TESTS OF OF THE METHOD
E. D. VANCE,* P. C. BROOKS and D. S. JENKINSON Soils and Plant Nutrition Department, Rothamsted Experimental Station, Harpenden, Her&. AL5 UQ, U.K.
(Accepted 20 March 1987)
Summary-The chloroform fumigation-incubation method underestimates the amount of microbial biomass C in strongly acid soils @H < 4.5). Several explanations for the failure of the method were examined using IO forest soils that ranged in pH from 3.4 to 7.2. The hypothesis that chloroform might form compounds toxic to microbial recolonizers in strongly acid soils, but not in soils of higher pH, was tested using an alternative fumigant. carbon disulphide. CO& evolution was very similar with both fumigants and this explanation was rejected. Another possibility is that the proportion of microbial C evolved as CO& from the decomposition of microbial cells killed by fumigation (k,-) is tess in strongly acid soils than in soils of higher pH, so that low values for biomass wilt be obtained if the value of kc commonly used for near-neutral soils (0.45) is used on strongly acid soils. This was tested by measuring the “CO, evolved when “CT-labelled fungi and bacteria were added to the soils, which were then fumigated and incubated. The values of kc thus obtained were indeed less in strongly acid soils; the mean kc for soils below pH 4.5 was 0.30, compared to a mean of 0.46 in soils above pH 4.5. In the fumigation-incubation method, the fumigated soil is inoculated with a few mg of non-fumi~t~ soil after the fumigant is removed but before incubation begins, to increase the size of the microbial recolonizing population. In soils above about pH 5. it makes little difference whether or not the soil is inoculated. However, in some of our strongly acid soils this was not so and a large inoculum increased the quantity of CO, produced after fumigation. Neither the use of an inappropriate kc factor nor inadequate inoculation appear, in themselves, to be sufficient to explain why the fumigation-incubation method underestimates the amount of microbial biomass in strongly acid soils. We suggest the main reason is that the recolonizing population in strongly acid soils is unable to metabolize non-~crobial organic matter as fast as the native population in non-fumigated soit, so that the use of a non-fumigate soil as control witI lead to low values for biomass
INTRODUCIION In the fumjgation-incubation method (Jenkinson and Powlson, 1976b), microbial biomass C (B,) is calculated from the relationship: Bc = Fe/kc, where kc is the fraction of microbial biomass C decomposed to CO*-C in IO days (k, = 0.45 at 25”C, Jenkinson and Ladd, 1981). The flush (F,) is defined as: Fc = [(CO,-C evolved from fumigated soil, following fumigant removal and insulation) - (C02-C evolved from a non-fumigated sample of the same soil)], both measured during a 10 day aerobic incubation at constant temperature. In soils above about pH 5, microbial biomass measured by the fumigation-incubation method correlated well with estimates of biomass by direct microscopy (Jenkinson et al., 1976; Paul and van Veen, 1980; Tesarova and Repova, 1984) and soil ATP concentration (Jenkinson et al., 1979; Oades
*Present address: School of Forestry. Fisheries and Wildlife, University of Missouri, Columbia, MO 65211, U.S.A.
and Jenkinson, 1979; Eiland, 1983; Brookes and McGrath, 1984). However, in strongly acid soils or humus layers with pH values less than about 4.5, biomass estimates by the fumigation-incubation method are often low relative to those measured by other methods (e.g. Jenkinson et al., 1976; Jenkinson er al., 1979; Williams and Sparling, 1984). Three hypotheses were proposed by Jenkinson et al. (1976) to explain why the amount of microbial biomass in a strongly acid soil (PH 3.9) was much less when measured by fumigation-incubation than by direct microscopy: (1) that many of the stained “organisms” observed and counted by direct microscopy in the acid soil were empty and dead, (2) that the killed biomass in the acid soil was less decomposable after fumigation than the killed biomass in soils of higher pH, (3) that the microbial recolonizers in the fumigated acid soil were less biochemically-competent to decompose the fumigant-killed biomass than those in fumigated soils of higher pH.
E. D. VANCEer al.
690
Other possible explanations which Jenkinson et al. (1976, 1979) did not consider are: (4) that chloroform fumigation leaves toxic degradation products in strongly acid soils, but not in soils of higher pH, (5) that the recolonizing population which develops in fumigated acid soils is unable to decompose non-microbia1 soil organic matter to the same extent as does the native microbia1 population in non-fumigated acid soils. (6) that current inoculation techniques, adequate for soils of higher pH, are inadequate for strongly acid soils. Hypothesis (1) can be rejected because the ATP concentration of the microbial biomass from a strongly acid soil was similar to that observed in soils of higher pH (Jenkinson ef al., 1979). using direct microscopic measurements of microbial biomass. Hypothesis (2) cannot be tested directly because it is not yet technically feasible to isolate the whole of the native soil microbial biomass from soil and measure its d~om~sabiiity. This hypothesis must therefore be set aside pending evidence that killed organisms from acid soils are much more resistant to decomposition than those from soils of higher pH. There is some evidence which supports hypothesis (3). It has been shown that decomposition rates of yeast cytoplasm extract (Powlson and Jenkinson, 1976) and added microorganisms grown in vitro (Anderson and Domsch, 1978) were somewhat lower in soils below pH 5 than in soils of higher pH. We have carried out experiments designed to test hypotheses (36). Hypothesis (3) was tested by measuring the d~om~sition of ‘*C-iabeiied fungi and bacteria in a range of fumigated soils varying in pH from 7.2 to 3.4. Hypothesis (4) was tested by using a different fumigant, carbon disutphide (CS,), in place of CHCi,.Hypotheses (5) and (6) were tested by increasing the size and diversity of the microbial recolonizing population in fumigated, strongly acid soils, by adding increasing quantities of the same (but non-fumigated) soil. Our aims were to investigate why the fumigation-incubation method does not work in strongly acid soils and to see if the method could be modified for use in such soils.
SSATERIALS AND METHODS
Soils Ten U.K., forest soils with a wide range of pH values (Table I) were sampled in August 1984 to a depth of IO cm, excluding the litter layer (01 horizon). Soil IOB was taken from the H horizon of soil 10. The soils were picked free of stones, large roots, other pieces of undecomposed plant material and large soil animals, then sieved (~2 mm), adjusted to 40% water holding capacity (WHC) and mixed thoroughly. Before use, soils were given a 20 day aerobic conditioning incubation at 25°C in closed steel drums which also contained trays of water (to maintain humidity) and soda-lime (to absorb CO,). Soils not used within this period were kept in iooseiysealed plastic bags at SC and given a conditioning incubation under the above conditions for several
days before use. All results are expressed on an oven-dry (OD) basis (105”C, 24 h) and, unless stated otherwise, are the means of three replicate determinations. Soif analysis Total C was measured in soii by dichromate digestion (Kalemba~ and Jenkinson, 1973) and total N by Kjeldahl digestion and steam distillation (Bremner, i965), both on air-dried, ground samples. Soil pH was measured with a 1: I soii:water paste (PHtHfl,] or a I: I soil: Lomb CaC& paste b&cao,,], except for soil IOB, where 1:2 ratio was used due to its high WHC. Values given in the text refer to PH,“*~). Clay content was measured as described in Soil Survey Laboratory Methods (1982). Respiration experiments Before respiration experiments began, samples were adjusted to 50% WHC. Moist soil samples containing 25 g OD soil were fumigate with ethanolfree CHCI, for 24 h at 25°C in desiccators confining moist paper and soda lime (Jenkinson and Powlson. 1976b). Non-fumigated soil samples were treated identically except that the desiccators did not contain CHCI,. The CHCI, was removed from soil by repeated evacuation and the fumigated soil samples inoculated with C(I IO mg non-fumigated soil. Carbon disuiphide fumigation was done as described by Kudeyarov and Jenkinson (1976). Soils were incubated in 60 ml glass jars placed in sealed 1 I glass jars containing IO ml water in the bottom and 20 ml I M NaOH in soda-glass vials. Carbon dioxide evolution over the 10 day incubation period was calculated from the quantity of standard HCI required to bring the pH of the NaOH solution from pH 8.3 to 3.7 using a Radiometer autotitrator. “‘CO2evolution was determined by liquid scintillation counting of a 1 ml aiiquot of the NaOH solution mixed with IOmi of Optiphase RIA scintillation cocktail. Oxygen consumption was measured using I I respirometers containing 20 ml 1 M NaOH in soda-glass vials; IOmi H,O was placed in the bottom of the respirometer vessels. Pressure differences were measured for paired respirometers, one containing the soil sample in a glass vial and one containing an equal dry wt of sand, also in a glass vial. Respirometer readings were made three times daily for the initial 2 to 3 days, then twice daily up to 10 days and once daily up to 20 days. Microorganisms used The fungal species, Trichoderma viride, Fusarium oxysporum, Penicillium janrhinellum and Mucor hiemafis were grown individually in liquid media at pH 7.3 on glucose (sp. act. approx 370 Bq mg-’ C), as described by Lynch and Harper (1974). The bacterial species, Bacillus subtilis, Pseudomonas &orescens, Enterobacter cloacae and Arthro batter globiformis, were grown individually in liquid medium fpH 6.6) containing, per litre of water, IOg glucose (sp. act. approx 370 Bq mg-’ C), 7.5 g KH2P04, 7.5 g Na,HPO,, 1.5 g NH,Cl. I g MgSO,. 7H,O, 0.1 g CaC12-2H,O, 2 mg ZnS04+7H,0, 2 mg CuSO,-SH20, 2 mg MnSO,-4H,O, 2 mg CoCi,6H,O and 20mg FeCl,*6H,O.
Pine, birch
Pine. birch
Oak
Oak. buxcl
I
As soil IO
Shirrell Heath
Bssendon
Manod or Dcnbigh
As soil
As soil 2
Ash, oak, hazel
Oak. ash. hawthorn
Harwclt
Oak, haxcl
As soil IO
Typical brown earth or brown podrolic soil Palco-argrillir stagnogky soil Humofcrric pOd2ol
As soil I
Typical argrillie brown earth As soil 2
Pelo-stagnogky
Denchworth
Oak,
hazel
Typical brown podzolic
Stagnogleyic pako-ttrgillic brown earth Typical brown earth or brown ranker
Subgroup
Manod
Malhom or Crwbin
Batcomc
Series
England and Wales classitication
Oak. haxcl
Hawthorn, ush. sycamore Ash. sycamore, hawthorn
Broadbalk Wilderness 7-L 121136 MerkwoodHoneybee wood SD 483896
McrkwoodThwaitc Head wood SD 353905 Alice Holt su 793414 Alice Hott SU 792412 Merlewood Meathop wood SD 436797 Gerscr0n Wilderness TL 131128 McrkwoodSeattle SD 382837 Alice Hot1 su 911409 Alice Holt E Horizon su 856447 Alice Holt H horizon su Il.%447
Major tree species
Site and National Grid nfcnrKX
‘Not Determined.
IO9
IOA
9
II
7
3
2
I
Soil No.
As soil IO
AquicjAlbie pakudalf Placorthod
As soil 3
As soil I
Typic haplaqucpt Typic hapludatf As soil 2
Lilhii udorthent or Lithic dystrochrcpt ~nt~/L~thic haplorthod
Aquic palcudalf
U.S. soil taxonomy subgroup
0.28
5.4 4.0
23
3.2
3.4
3.5
3.8
2.7
2.1
3.0
3.5
3.7
3.8
4.2
4.1
4.6
5.1
6.2 4.3
I4 I
32.9
9.2
26
ND’
3.7
26
25
0.48
s.2
42
5.2
4.6
12.6
25
5.0
5.2
4.9
Ott2
0.087
0.38
0.62
0.29
0.40
0.91
0.46
24
4.1
5.8
0.37
Total N
6.1
f%)
Organic C
I’“,,, 6.9 27
Clay ~H,.zor 7.2
I. Properties of forest soils studied
Soil type
Table
40.1
49.3
16.3
IS.0
12.8
14.3
II.2
13.1
13.7
10.7
II.1
C:N ratio
692
E. D. VANS er al.
Table 2. Percentage carbon and sp. act. of microorganisms used for k, determinations Specific activity Bq mg-’ C
Microbial species
% Carbon’
Fungi T. eiride F. oxyspomm P. jamhinellum M. hiemalis
44.8 5 47.3 f 42.7+ 48.6f
0.94 0.61 1.46 0.93
319 307 332 307
Bacteria B. mbrilis P. jluorescenr E. cloacae A. globt~ormis
39.6+0.66 41.9& 1.52 4l.7~0.60 38.9+ 0.50
340 326 333 354
'Mean + SE of the mean, n = 4.
Cultivation and harvest of “C-labelled
microorganisms
Inocula were prepared from cultures grown on malt agar (fungi and A. globijbrmis) or on nutrient agar (other three bacteria) by shaking with approximately 10ml of the phosphate buffer solutions (pH adjusted) used in the respective fungal and bacterial media. Fungal cultures were shaken with glass beads in order to release spores. The suspensions thus prepared were then used to inoculate the 14Cmedia. Fungi were grown in 250 ml Erlenmeyer flasks with cotton plugs on a rotating shaker maintained at room temperature. Bacteria were grown in 1 I aerated chemostat vessels maintained at 25’C. Fungi and bacteria were both harvested at approximately late log phase; the fungi by pouring the cultures through glass sinters and rinsing with distilled water, the bacteria by repeated centrifugation and rinsing the pellets with distilled water. Harvested organisms were initially frozen at - 15°C and than at -80°C. freeze-dried and stored at - 15°C. Their total C content was measured on four replicates by dichromate digestion (Kalembasa and Jenkinson, 1973) and their 14C content, (three replicates) on a Packard Tri-Carb Sample Oxidizer, using 9 ml CO2 absorber (Carbosorb) and 9 ml scintillant (Permafluor) for approximately 100 mg dried organism. Specific activities of the microorganisms are shown in Table 2. kc determinations
incubated in a sealed glass jar under the same conditions, so that the additional effect of the large inocula on respiration in fumigated soil could be derived. In the second inoculation experiment, increasing quantities of moist non-fumigated inocula of soils 6. 7, 8 and 9 (all with pH values less than 4.5) were mixed with moist fumigated samples of the same soil. Each treatment consisted of SOg (OD basis) of the soil mixture, of which the non-fumigated inoculum made up either ca IO mg (the standard inoculum), IO, 20. 25, 30, 40 or 50g of the total wt of soil. COZ-C evolution was then measured over IO days. The third inoculation experiment was a direct test of whether large inocula increased the size of kc. Two “C-labelled organisms (B. subtifis and M. hiemafis) (about 20 and 40mg, respectively) were thoroughly mixed with separate 20g portions (OD basis) of moist soils I (pH 7.2) and 7 (pH 4.1) and fumigated. After fumigant removal, the soils were inoculated with either the standard (ca IO mg) inoculum or a 4 g inoculum (20% of fumigated soil weight) of the same soils. Following remixing, the soils were incubated (IO days, 25°C. 50% WHC) and the i4C02 evolved trapped in 20 ml I M NaOH and analyzed by scintillation counting, all as described previously. Each treatment was replicated four times.
RESULTS
Respiration in a near neutral and in a strongly acid soil
Broadbalk Wilderness (pH 7.2) and Geescroft Wilderness (pH 4.1) soils (Table I) were used for the initial respiration experiments. Both soils were under arable cultivation until 1883, when they were allowed to revert to natural vegetation (Jenkinson, 1971). Both belong to the Batcombe soil series: they differ in that the Broadbalk soil had been heavily chalked while under cultivation, whereas the Geescroft soil had not. Trends in 0, consumption (Fig. 1) were very similar to those observed for these soils by Powlson Z 5: zccorE7’
(a)
Weighed quantities of individual, freeze-dried microorganisms (approximately 40 mg of fungi and 20 mg of bacteria) were added to separate samples of moist soil (20 g OD basis), mixed thoroughly and fumigated with CHCI, for 24 h. After removal of CHCI, and inoculation (ca 10 mg non-fumigated soil), the quantities of total and labelled COr-C evolved were measured over the O-IO and IO-20 day incubation periods. Each treatment was replicated four times. inoculation
AND DlSCLSSlON
5
15
lo
1 x)
Incubotlon time (days 1
treatments
There were three experiments on the effects of inoculation. In the first, moist non-fumigated samples of a strongly acid soil (soil 7; 25 g, OD basis) were thoroughly mixed with moist fumigated samples of the same soil (25 g, OD basis) and incubated in a sealed glass jar for 20 days. The same quantities of fumigated and non-fumigated soil, but unmixed, were
Incubotlon 11meI days
1
and nonfumigated soils: (a) Broadbalk Wilderness soil [PHtMS, 7.21; (b) Geescroft Wilderness soil [PHoS,, 4. I]. Fig.
I.
Oxygen
consumption
by
fumigated
Microbial biomass measurementsin forest soils
and Jenkinson (1976). Fumigation caused a large immediate increase in O2 consumption in the neutral Broadbalk soil, in contrast to the long lag phase and absence of a flush in the acid Geescroft soil. Respiration in a fumigated soil that does not contain recently added substrate can, in theory, be resolved into two parts. One part of the CO* evolved comes from the decomposition of microbial biomass killed by the fumigant: the other is “basal respiration” and comes from the decomposition of nonmicrobial soil organic matter. In calculating biomass C by the fumi~tion-incubation method, the CO& evolved by the non-fumigated soil (the “control”) is subtracted from that evolved by the fumigated soil to give the “flush”, from which biomass is calculated. This implies (Jenkinson and Powlson, 1976b) that a negligible part of the biomass in the non-fumigated soil dies during the 10 day incubation and furthermore, that decomposition of non-microbial soil organic matter continues at the same rate in both fumigated and non-fumigated soils, despite the reduced diversity (Reber, 1967) and biomass (Jenkinson and Powlson, 1976a) of the microbial recolonizers in the fumigated soil. The increasing Or consumption in the fumi~ted Geescroft samples during the 2-4 day period suggests that, after a prolonged lag, killed organisms were decomposed in the normal way, giving rise to a small, if belated, increase in respiration rate (Fig. lb). However, beyond about 6 days, respiration was much slower in fumigated than in non-fumigated Geescroft soil, indicating that basal soil respiration was not even approximately equal in the fumigated and nonfumigated soils. In contrast, roughly parallel respiration curves were observed in the Broadbalk soil once the flush was over, after about 4 days (Fig. la). A combination of a longer lag phase and a lower basal soil respiration in fumigated soil means that biomass cannot be calculated in a strongly acid soil such as Geescroft, in which respiration in the “control” is greater than that in the fumigated soil. E&cts of soil pN on the mineralization of labelled bacterial and fungal carbon in fumigated soils (kc Cakes) Four %labelled bacteria and four “C-labelled fungi commonly found in soil were added to each of the ten soils, which were then fumigated, incubated and the mineralization of added microbial C determined by measuring the r4COt evolved during a IO day aerobic incubation. Figure 2 shows the influence of soil pH on the proportion of microbial C mineralized (k,) from each fungus (Fig. 2a) and bacterium (Fig. 2b). In soils above pH4.5, the average k, for all fungi and bacteria combined was 0.46, remarkably close to the value of 0.45 proposed by Jenkinson and Ladd (1981). who calculated a weighted mean kc based on data from Jenkinson (1976) and Anderson and Domsch (1978). The k, value for the more acid soils (pH < 4.5) was significantly (P = 0.05) lower for both fungi and bacteria, averaging 0.35 for fungi and 0.25 for bacteria (Table 3). Anderson and Domsch (1978) and Nicolardot ez al. (1984) also observed that kc tended to be lower in acid soils, although the effect
693
0.60- (a)
0.50 -
D s+*
0.40 -
3:; +
+
l
0 * ;*
k, 0.30 -
+’
’
RI 0
ozo0.10 -
I I I I I I I I, 4.0 4.5 5.0 5.5 &O 6.5 7.0 725 80
I
‘?O
Xi
Soit pH
(b)
O.w-
0.50-
.: l.
Q4Q-'
. k,Q30-•
P
; 'I
J
*
*
'A! .*
l
. 0.20-
"
l l
0.10Qcn
;
"1 30 35 40
"1 "1' 4.5 SL) 33 60 65 70 SGilpH
ZS 60
Fig. 2. Influence of soil pH,u,, on kc values for: (a) fungi (O-T. riride, 0-F. oxysporum, +-P. janthinelhun, x--M. hiem& and (b) bacteria (e---8. subtilir, W-P, jhmrescem, A-E. cloarae, %--A_ globiformis.
was markedly less than in our experiments, presumably because our soils were more acid. There was a large variation in k, values among the different fungi and bacteria (Fig. 2). emphasizing the need to use a range of microorganisms to obtain a representative estimate of kc. The ~lationship between soil pH and mean kc for all bacteria and fungi is shown graphically in Fig. 3. kc is substantially independent of pH over the range 5-7.5. Jenkinson et al. (1976) found that microbial biomass measured by fumigation-incubation was seven to ten times smaller than that estimated by direct microscopy in the strongly acid Geescroft soil. Although the mean k, value of 0.35 for bacteria and fungi combined for this soil is lower than the kc normally used for the fumigation-incubation method (0.45), it is not nearly small enough to account for the large discrepancy between biomass as estimated by fumigation-incubation and as estimated by direct Table
Soil No. : 3 4 5 6 7 8 9 IO
3. Average kc valuesfor added microbialC sd
P&W,
k, for fungi
kc for bacmia
7.2 6.1
0.48 0.48 0.49 0.52 0.52 0.36 0.40 0”:; 0.29
0.46 0.38 0.46 0.34 0.41 0.28 0.3 I 0.29 0.10 0.29
0.42 + 0.009
0.33 * 0.012
if 5.1 4.2 4.1 3.8 3.5 3.4
Meanf SE of the mean. n 5 10
694
~ncu~fion time t cbys)
L
I
I
I
I
4
5
6
7
6
Sail PH
fig. 3. relationship between soil pN and mean k, for ali bacteria and fungi. microscopy or from soil ATP (Jenkinson et al., 19%; 1979). These results therefore suggest that the small respiratory flush in the fumigated Geescroft soil, compared to that in the ~~adbalk soil, was only partly due to decreased minerali~tion of biomass killed by the fumigant, so that hypothesis (3) cannot, by itself, satisfactorily explain the failure of the fumigation-incubation method to measure biomass in acid soils. A relatively small propartion (S-19%) of added microbial f*C was mineralized to CC+-C during the 10-20 day incubation falla~n~ fumigation (Table 4& The proportion increased with decreasing soil pH, fram values of 5-10% above pH 5, to almost 20% at the lowest soil pH of 3.4, presumably because more undecomposed substrate remained in the acid soils. The small proportion mineralized from soils of pH > 4.5 during this period implies that only a small part of the total CO& evolved during the IO-20 day period is derived from the original killed biomass in such soils.
Slow microbial colonization of fum~~t~ acid soil could be caused by a specific interaction between Cl-ICI, and soils of low pH that produces toxic compounds not found in soils of higher pH. This was tested using the fumigant carbon disulphide (CS& which has been found to act similarly to CHCI, when
Fig. 4. Oxygen consumption by Geescroft Wifdemw soil [pH,,,, 4. I] following fumigation with either CHCl, or CSz. used in the fumi~tio~in~ubati~n method in nearneutral soils (Kude~rov and J~nk~nson, 1976). Figure 4 reveals similar U2 ~o~urn~t~o~ patterns when CS, and CHCI, were used as fumigants on the strongly acid Geescroft soil, although there is an indication that the lag phase was longer with GSz than with CHCI,, su~~tin~ that CS, is the more effective biocide. This result suggests that the reduced respiratory flush in acid soils is not due to an interaction between CHCI, and soils of low pH, which might produce products (e.g. arganochlorine compounds) toxic to microbial recolonizers. Hypothesis (4) must therefore be rejected. Eficts of inoculation Although in~~s~~g the size of Ihe recoloniziag population by inaculation with fresh soif has little effect an respiration in fumigated, unamended, nearneutral soils (Jenkinson and Powlson, 1936a; Martens, l985), it is possible that inocuiation is more criticai in strongly acid soils. This was tested by mixing fumigated soil (25 g} from Geescroft Wilderness fpH 4.1) with a iarge inoculum (25 g> of the same soil that had not been fumigated. The treatment increased the rate of Oz consumption by the fumigated soil (after subtraction of the 0, consumption of the inoculum alone) and eliminated the lag period (Fig. Sf. This shows that d~am~sition is limited in the Geescroft soil by the size or diversity of the populatian that survives fumigation and indicates
Table 4. Percentage ofadded microbial “C mineralized in fumigated soih durina the W-Z0 day ineubetion w&d Soil
Soil pH,,fl,
Pcrantaae of added microbial-‘% &olved as CO,.(Fb
4.8i: 0.2s
1
i4 4 5 6
;: 9
IO
;f 512 5.2 5.1 4.2
6.7 ** 0.41 6.8 0.33 9.6 2 f.28 9.5 f 0.62 14 * 1.29
4.1 3.8 3.5 3.4
13 f 2.18 11* 1.60 I4 + 3.03 19 ; 2.25
‘Awage for four fungi and four bacteria. bMean f; SE of the
mean,of= 8.
Fig. 5. Influence of inoculum size on oxygen consumption by fumigated Geescroft soil [PH,,, 0.411.Respiratian from inoculum (aon-fu~gat~ Geescroft soil) is subtracted. Two inacula were used: standard (co tOmg non-fumigated soil) or large (I: 1mixture of fumigated and non-fumigated soil).
69s
Microbial biomass measurements in forest soils
l~*--l----
Soil
6
soil
9
Soil
7
..-•--•-•---*__*
