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Effects of 2450 MHz microwave radiation on nitrification, respiration and S-oxidation in soil
so,/ B,,,,. B,a‘Iw,,r Vol. 12. pp 4x9 10 4v.1 0 Pergmon Pros Lid I’M1 Prmted I” Great Bnlaln
EFFECTS OF 2450 MHz MICROWAVE RADIATION ON NITRIFICATION, RESPIRATION AND S-OXIDATION IN SOIL M. WAINWRIGHT, K. KILLHAM and M. F. DIPROSE* Deparknrnts of Microbiology. and *Electronic and Electrical Engineering,
University of Sheffield, SIO 2TN. U.K. (Accepred 13 March 1980) Summary-A 20 s exposure to 2450 MHz microwave radiation had a marked differential e&t on the viable count. of soil micro-organisms, had little influence on numbers of heterotrophic bacteria, but reduced fungal colonies on dilution plates to zero. The growth of fungi from soil particles was also reduced following treatment. Prolonging the exposure to microwave radiation progressively inhibited nitrification and S-oxidation, but stimulated ammonification. Brief exposures (20s) also stimulated S-oxidation and increased the numbers of thiobacilli in soil. Most of these effects are explained by reference to the marked increase in soil temperature resulting from microwave treatment.
Respiration
INTRODL’CTION
MHz) has been used to control Botrytis cinerea in greenhouse soils (Lyon et al., 1979). and to kill weeds in the field (Davis et al., 1971) little is known about the effects of this type of radiation on soil micro-organisms and their activities. Our aim was to determine the effects of 2450 MHz microwave radiation on viable counts of micro-organisms and on rates of nitrification, respiration and S-oxidation in .a garden loam and two brown earth soils. Although
microGave
radiation
(2450
MATERIALS AND METHODS
Soils Three soils, an organic loam (pH 6.9; organic C, 6.2%: organic N, 0.7%) and two brown earths (Fitzwiliiam: pH 5.4: organic C, 7.6; organic N. O.l9”/‘,and Chapeltown: pH 3.9; organic C, 12.2% and organic N, 0.2%) were used (sieved to < 2 mm). Incubation experiments
The effects of microwave radiation on nittification and S-oxidation were determined by amending soil with substrate 100 pg NH; g- ’ as (NH,), SO, and I% (w/w) So, respectively; adjusting to 28% (v/w) moisture and incubating at 28°C in Erlenmeyer flasks covered with perforated parafilm. Concentrations of N and S-ions were determined at intervals before and after irradiation. Chemical analysis Water extracts of soil (I:5 soil:extractant) were filtered and analysed for NO;-N using chromotropic
acid, and KC1 extracts (1.5 N, 1:5 soil:extractant) for NH; -N using the indophenol blue assay (Wainwright and Pugh, 1973). LiCl, (0.1 M) extracts (1 :lO) were analyzed turbidimetrically for SO:--S (Hesse, 1971) and calorimetrically for S20:--S and &0:--S (Nor and Tabatabai, 1971). 489
O2 uptakes of irradiated and control soils (2g) amended with 2ml 1% (w/v) glucose were measured for 6 h in a Warburg respirometer. Microbial analysis
Viable microbes were counted by the dilution technique. Soil (1 g) was shaken in l/4 strength Ringer’s solution (1OOml) for 15 min on a reciprocal shaker (100 throws min- I). Serial dilutions of the suspensions (0.1 ml) were then aseptically spread onto the surface of plate count agar (Oxoid); Czapek-Dox agar (Oxoid) (+ SOmg 1-r streptomycin) and thiosulphate medium (Postgate, 1966); to count heterotrophic bacteria, fungi and thiobacilli respectively. Small crumbs of soil were also spread onto the surface of CzapekDox agar to determine the effects of microwave radiation on the numbers of fungal colonies derived directly from the soil. Microwave irradiation Samples of soil (20g) were adjusted to 40% (v/w) moisture (dry wt basis) and irradiated for a known period in 6cm dia glass Petri dishes with the lid removed. The soils were exposed in a microwave cavity constructed of stainless steel having the dimensions of 46 x 30 x 34.5 cm (length x height x width). A Pyrex shelf spanned the middle of the cavity and the Petri dishes were placed at the centre of the shelf. Microwave power was supplied by a magnetron operating at a frequency of 2450 f 30 MHz with an output of 1 kW. Since complex electric field distributions occur in the cavity a rotating metal blade was used as a mode stirrer to ensure an even energy distribution throughout the sample. Power density measurements were made by determining the rate of rise in temperature of water (40 ml) in an open 6 cm Petri dish placed at the centre of the oven. A rise of 41°C occurred in 20 s which corresponds to 8.6 J cm- 3 s- ’ being dissipated in the water load.
490
M. WAIYH'HIGHT. K. KILLHAM
im
and Xl. F. DIPHOSL
NH;-N
T 80 -
Doys
Fig. I. Effect of microwave
radiation
on nitrification from control
A 50 p sample of Fitzwilliam soil was autoclaved at l2O.C for I h on three successive occasions. It was then amended with steamed So (I y, wjw) and divided into two subsamples and one subsample irradiated for 20s. Fifty g of non-sterile Fitzwilliam soil was then added to each subsample. mixed thoroughly. and incubated at 25’C. The concentrations of SOi- in the subsamples were determined over 28 days.
RESL'LTS
m an organic P = 0.05).
loam soil. (‘Significantly
different
20 days before irradiation for 20s (Chapeltown soil) or for various times (Fitzwilliam soil). Figure 2 shows that S-oxidation decreased with duration of exposure to microwave radiation. although SOi--S concentrations increased towards the end of the incubation in soils exposed for IO and 20 s. A marked stimulation of S-oxidation occurred in the Chapeltown brown earth irradiated (20s) 20 days after being amended with S”; with the concentrations of SO:-. S20$-. and $0: increasing over 3.2.5 and 2 times over the control respectively (Fig. 3). Similar increases in S-ions were also seen in the organic loam which was irradiated for 20s immediately after adding So (Fig. 4).
Samples of organic loam wcrc amended with (NH&SO, incubated for 3 days and then irradiated for various times (Fig. I). Exposure to microwave radiation inhibited nitrification during the next 5 days with inhibition increasing with duration of exposure. There was a concomitant increase in NH;-H in irradiated soils. but no increase in NO; -N (concentration was detected by spot test).
The rates of O2 uptake Fitzwilliam brown earth lowing a 20s exposure none of the decreases (P = 0.05).
q/ tnicrowxr
E&v
rudictriori
011 microhid
c0unr.s
A 20s exposure to microwave radiation reduced the number of fungi on plates prepared from the organic loam from 3O.ooOg~ I to zero. while the so$s
2500 t
by the organic loam and the were depressed slightly folto microwave radiation. but was statistically significant
S-ouidtrrim Two types of experiments were performed to determine the effects of microwave radiation on S-oxidation. In the first. organic loam was amended with So and immediately irradiated for 20 s: while in the second experiment, brown earth soils were amended with S” and allowed to oxidize the element for I4 or
0
Fig. on-S
7
14
21 28 ooys
35
42
2. ElTect of period of exposure IO microwave radiation oxidation ;n brown ea;th soil (Fitzwilliam). Soil incu-
bated
at 30-C. (*Significantly P = 0.0.0.
different
from
control
Microwave radiation-soil
0
I,
5
IO
IS
20
25
30
491
micro-organisms, nitrification, S-oxidation
35
40
45
0
5
IO
IS
20
25
30
35
40
45
Days
Days
250 - s.o,%
Fig. 3. EtTect of microwave radiation (20 s) on S-oxidation in a brown earth soil (Chapeltown).
so:=s
IO-
oa06-
0
s. o:=s
02
-
0.1
-
0
*hL::
;
I 7
Control I
I 21
I 26
o&s Fig. 4. Effect of microwave radiation on S-oxidation in an organic loam soil. Soil irradiated directly after S-amendment (*Significantly different from control P = 0.05). SBB.I? 5. r
492
M. WAINWRIGHT, K. KILLHAM and M. F. DIPROSE Table
I. Effect of microwave
radiation
of micro-organisms
in an orgamc
loam soil
0
7
Days 14
21
6.6 * 0.7 7.0 k 0.6
6.9 k 0.8 7.8 + 1.0
9.6 + 1.2 6.8 + 1.0
9.8 k 0.9 10.8 k 1.3
30 * 8 0
51 + 4.7 3 f 0.5
32 f 4 5 + 0.9
30 * 5 IO f 2
122 + 12 8 + I2
138 k 15 20 + 3
139 + 17 41 +8
I40 + 14 70 + 9
139 * I5 81 +9
IO * 0.9 20 f 3.2
30 f 5 50 + 7.2
50 & 7 80 f 9.2
92 k 10 420 + 52
89 k I2 430 & 47
Viable count Heterotrophic bacteria (IO’ 8-l) Control Irradiated Total fungi (IO-’ g- ‘) Control Irradiated Fungal colonies from soil particles Control Irradiated Thiobacilli ( IO4 g- ‘) Control Irradiated
on viable count
Soil was amended with So and immediately irradiated (L-standard replicate). Counts expressed as number g- ’ oven dry soil.
numbers of heterotrophic bacteria were not affected (Table 1). The viable count of fungi then continued to remain low throughout the post-irradiation period, while bacterial numbers remained similar in control and treated soils. The number of fungal colonies derived from soil particles was also markedly reduced following irradiation (Table I). Thiobacilli on the other hand increased in numbers following exposure, so that treated soils contained nearly 5 x as many of those organisms than did the control, 28 days after treatment. The same pattern of reduction in fungal count and stimulation of thiobacilli also occurred in Chapeltown soil microwaved (20s) following a 20-days oxidation (Table 2). DISCUSSION
Our results show that exposure to 2450MHz microwave radiation (20s) has a marked differential effect on the viable count of soil micro-organisms, eliminating the fungal count while having little influence on numbers of heterotrophic bacteria. Diprose rt al. (1978) also showed that microwave irradiation (30s) eliminated the fungal count while a 60 s exposure was needed to halve the bacterial count but some bacteria survived 360s exposure. These observations suggest that fungal spores are more susceptible to microwave irradiation than are bacteria,
Table 2. Effect of microwave
radiation
deviation,
means
of three replicates,
Fungi (lO”g--‘)
0
23 + 5.4
10 20
26 + 4.2 28 k 4.5
30 40
O(31 + 4.1) 2(35 k 5.6)
13.9 f 10.1 f
1.4 1.0
29 5 3 9 * 1.2
10 plates
per
since the fungal plate count is usually considered as a count of these propagules. Lyon et al. (1979) demonstrated the marked effect of microwave irradiation on fungal spores when they showed that the viability of Botrytis cinerea spores was reduced by 98% when added to soil subsequently exposed to microwaves for 8 s. Our observed reduction in the number of fungal colonies derived from soil particles suggests that fungal hyphae are also susceptible. On exposure to microwaves the temperature of soil increases substantially (Table 3), and Vela and Wu (1979) state that it is thermal rather than direct radiation effects which kill micro-organisms. Such increases in soil temperature account for the reduction in fungal count decrease in nitrification and S-oxidation after prolonged exposures. Bollen (1969) showed that plate counts of fungi were reduced by 98% when soil was heated to 60°C while bacteria were largely unaffected. Nitrification is slow in soils heated above 4o”C, while the optimum temperature for ammonification, in contrast to most microbial transformations in soil is not in the mesophillic range, but above 40°C and the process can occur in manure piles held at 65°C (Alexander, 1977). S-oxidation is also inhibited at high temperatures (Li and Caldwell, 1966). The increase in numbers of thiobacilli and rate of S-oxidation following short exposure to microwave radiation is difficult to explain, but was not due to a marked
(20 s) on viable count
of micro-organisms
in a brown earth soil (Chapeltown)
Days
28
Heterotrophic bacteria (IO’ g- I)
Thiobacilli (IO’g-1)
Pre-microwaved soil 80 k 12.3 29 + 6.1 82 k 9.4 92 + 13.0 Post-microwaved 110 k 25(97 + 17) 140 * 21(110 + 20)
39 30 soil 350 530
+ 4.8 f 5.1 + 42 (39 + 5.8) + 66 (39 + 6.1)
The figures in brackets are the numbers of micro-organisms in control soils. Counts expressed as number g-’ oven dry weight soil. The soils were microwaved for 20 s on day 20 (+ standard deviation, means of three replicates, IO plates per replicate).
Microwave radiation-soil
493
micro-organisms, nitrification, S-oxidation
Table 3. The effects of microwave irradiation (20 s) on availability of S,, to S-oxidizing micro-organisms and the increase in soil temperature following irradiation Treatment
0
7
Days 14
21
28
Control Irradiated
195 245
400 455
775 800
850 800
1000 950
0 21
10 52
20 70
30 82
50 83
Exposure time (s) Temp. (“C)
so:so:-
pgg-‘2 pgg-’
* Means of three replicates. in the availability of the element following treatment (Table 3). Possibly micro-organisms which normally inhibit thiobacilli were killed or some agent ,of bacteriostasis was removed by microwave radiation; both allowing for an increase in thiobacilli. Only limited use has been made of microwave radiation in agriculture, despite its marked advantage over pesticides or fumigants in not leaving potentially harmful residues. Baker and Fuller (1969) showed that the ability of microwave radiation to destroy plant pathogens in soil depends on the moisture content of soil, and conclude that such treatment is commercially impractical. Vela et al. (1976) state that the amount of energy absorbed by soil during a 5 s exposure to microwave radiation (2450MHz) is 5000 J cm-‘, which is much greater than the amount of energy needed to kill weeds in the field (180-360 J cm-‘). The exposure times used by us are unlikely therefore to be used in the field, but similar exposures could be used in greenhouse soils to control fungal pathogens. Under these circumstances microwave treatment is unlikely to have a detrimental effect on soil fertility, while increases in NHf-N and SOi--S resulting from treatment may stimulate seedling emergence and growth. increase
REFERENCES
ALEXANDERM. (1977) Introduction to Soil Microbiology, 2nd edn. Wiley, New York. BAKERK. F. and FULLERW. H. (1969) Soil treatment by microwave energy to destroy plant pathogens. Phytopathology 59, 193-197.
BOLLENG. J. (1969) The selective effect of heat treatment on the microflora of a greenhouse soil. Netherlands Journal of Plant Pathology 15, 157-163. DAVIS R. S., WAYLANDJ. R. and MERKLEM. G. (1971) Ultra-high frequency electromagnetic fields for weed control: phytotoxicity and selectivity. Science 173, 535-537. DIPR~XEM. F., LYONA. J. E., HACKAMR. and BENSONF. A. (1978) Partial sterilization and soil and leaf moisture content measurement by microwave radiation. Proceedings British 491-498.
Crop
Protection
Conference-Weeds.
HENCEP. R. (1971) A Textbook of Soil Chemical Analysis. Murray, London. Lt P. and CALDWELLA. C. (1966) The oxidation of elemental sulphur in soil. Soil Science Society of America Proceedings 30, 370-372.
LYONA. J. E., DIPR~~EM. F., HACKAMR. and BENSONF. A. (1979) The effect of 2450 MHz radiation on two soilborne pathogenic fungi (Botrytis cinerea, Fusarium). Proceedings of the! 14th Microwave Power Symposium, pp. 90-92. IMPI, Monaco. NOR I. M. and TABATA~AIM. A. (1977) Extraction and calorimetric determination of thiosulphate and tetrathionate in soils. Soil Science 122, 171-175. POSTGATE J. R. (1966) Media for sulphur bacteria. Laboratory Practice 15, 1239-44. VELAG. R. and WV J. F. (1979) Mechanism of lethal action of 2450 MHz radiation on micro-organisms. Applied and Environmental Microbiology 37, 550-553.
VELA G. R., Wu J. F. and SMITH 0. (1976) Effect of 2450MHz microwave radiation on some soil microorganisms in situ. Soil Science 121, 44-57. WAINWRIGHT M. and PUGH G. J. F. (1973) Effect of three fungicides on nitrification and ammonification in soil. Soil Biology & Biochemistry 5, 577-584.
EFFECTS OF 2450 MHz MICROWAVE RADIATION ON NITRIFICATION, RESPIRATION AND S-OXIDATION IN SOIL M. WAINWRIGHT, K. KILLHAM and M. F. DIPROSE* Deparknrnts of Microbiology. and *Electronic and Electrical Engineering,
University of Sheffield, SIO 2TN. U.K. (Accepred 13 March 1980) Summary-A 20 s exposure to 2450 MHz microwave radiation had a marked differential e&t on the viable count. of soil micro-organisms, had little influence on numbers of heterotrophic bacteria, but reduced fungal colonies on dilution plates to zero. The growth of fungi from soil particles was also reduced following treatment. Prolonging the exposure to microwave radiation progressively inhibited nitrification and S-oxidation, but stimulated ammonification. Brief exposures (20s) also stimulated S-oxidation and increased the numbers of thiobacilli in soil. Most of these effects are explained by reference to the marked increase in soil temperature resulting from microwave treatment.
Respiration
INTRODL’CTION
MHz) has been used to control Botrytis cinerea in greenhouse soils (Lyon et al., 1979). and to kill weeds in the field (Davis et al., 1971) little is known about the effects of this type of radiation on soil micro-organisms and their activities. Our aim was to determine the effects of 2450 MHz microwave radiation on viable counts of micro-organisms and on rates of nitrification, respiration and S-oxidation in .a garden loam and two brown earth soils. Although
microGave
radiation
(2450
MATERIALS AND METHODS
Soils Three soils, an organic loam (pH 6.9; organic C, 6.2%: organic N, 0.7%) and two brown earths (Fitzwiliiam: pH 5.4: organic C, 7.6; organic N. O.l9”/‘,and Chapeltown: pH 3.9; organic C, 12.2% and organic N, 0.2%) were used (sieved to < 2 mm). Incubation experiments
The effects of microwave radiation on nittification and S-oxidation were determined by amending soil with substrate 100 pg NH; g- ’ as (NH,), SO, and I% (w/w) So, respectively; adjusting to 28% (v/w) moisture and incubating at 28°C in Erlenmeyer flasks covered with perforated parafilm. Concentrations of N and S-ions were determined at intervals before and after irradiation. Chemical analysis Water extracts of soil (I:5 soil:extractant) were filtered and analysed for NO;-N using chromotropic
acid, and KC1 extracts (1.5 N, 1:5 soil:extractant) for NH; -N using the indophenol blue assay (Wainwright and Pugh, 1973). LiCl, (0.1 M) extracts (1 :lO) were analyzed turbidimetrically for SO:--S (Hesse, 1971) and calorimetrically for S20:--S and &0:--S (Nor and Tabatabai, 1971). 489
O2 uptakes of irradiated and control soils (2g) amended with 2ml 1% (w/v) glucose were measured for 6 h in a Warburg respirometer. Microbial analysis
Viable microbes were counted by the dilution technique. Soil (1 g) was shaken in l/4 strength Ringer’s solution (1OOml) for 15 min on a reciprocal shaker (100 throws min- I). Serial dilutions of the suspensions (0.1 ml) were then aseptically spread onto the surface of plate count agar (Oxoid); Czapek-Dox agar (Oxoid) (+ SOmg 1-r streptomycin) and thiosulphate medium (Postgate, 1966); to count heterotrophic bacteria, fungi and thiobacilli respectively. Small crumbs of soil were also spread onto the surface of CzapekDox agar to determine the effects of microwave radiation on the numbers of fungal colonies derived directly from the soil. Microwave irradiation Samples of soil (20g) were adjusted to 40% (v/w) moisture (dry wt basis) and irradiated for a known period in 6cm dia glass Petri dishes with the lid removed. The soils were exposed in a microwave cavity constructed of stainless steel having the dimensions of 46 x 30 x 34.5 cm (length x height x width). A Pyrex shelf spanned the middle of the cavity and the Petri dishes were placed at the centre of the shelf. Microwave power was supplied by a magnetron operating at a frequency of 2450 f 30 MHz with an output of 1 kW. Since complex electric field distributions occur in the cavity a rotating metal blade was used as a mode stirrer to ensure an even energy distribution throughout the sample. Power density measurements were made by determining the rate of rise in temperature of water (40 ml) in an open 6 cm Petri dish placed at the centre of the oven. A rise of 41°C occurred in 20 s which corresponds to 8.6 J cm- 3 s- ’ being dissipated in the water load.
490
M. WAIYH'HIGHT. K. KILLHAM
im
and Xl. F. DIPHOSL
NH;-N
T 80 -
Doys
Fig. I. Effect of microwave
radiation
on nitrification from control
A 50 p sample of Fitzwilliam soil was autoclaved at l2O.C for I h on three successive occasions. It was then amended with steamed So (I y, wjw) and divided into two subsamples and one subsample irradiated for 20s. Fifty g of non-sterile Fitzwilliam soil was then added to each subsample. mixed thoroughly. and incubated at 25’C. The concentrations of SOi- in the subsamples were determined over 28 days.
RESL'LTS
m an organic P = 0.05).
loam soil. (‘Significantly
different
20 days before irradiation for 20s (Chapeltown soil) or for various times (Fitzwilliam soil). Figure 2 shows that S-oxidation decreased with duration of exposure to microwave radiation. although SOi--S concentrations increased towards the end of the incubation in soils exposed for IO and 20 s. A marked stimulation of S-oxidation occurred in the Chapeltown brown earth irradiated (20s) 20 days after being amended with S”; with the concentrations of SO:-. S20$-. and $0: increasing over 3.2.5 and 2 times over the control respectively (Fig. 3). Similar increases in S-ions were also seen in the organic loam which was irradiated for 20s immediately after adding So (Fig. 4).
Samples of organic loam wcrc amended with (NH&SO, incubated for 3 days and then irradiated for various times (Fig. I). Exposure to microwave radiation inhibited nitrification during the next 5 days with inhibition increasing with duration of exposure. There was a concomitant increase in NH;-H in irradiated soils. but no increase in NO; -N (concentration was detected by spot test).
The rates of O2 uptake Fitzwilliam brown earth lowing a 20s exposure none of the decreases (P = 0.05).
q/ tnicrowxr
E&v
rudictriori
011 microhid
c0unr.s
A 20s exposure to microwave radiation reduced the number of fungi on plates prepared from the organic loam from 3O.ooOg~ I to zero. while the so$s
2500 t
by the organic loam and the were depressed slightly folto microwave radiation. but was statistically significant
S-ouidtrrim Two types of experiments were performed to determine the effects of microwave radiation on S-oxidation. In the first. organic loam was amended with So and immediately irradiated for 20 s: while in the second experiment, brown earth soils were amended with S” and allowed to oxidize the element for I4 or
0
Fig. on-S
7
14
21 28 ooys
35
42
2. ElTect of period of exposure IO microwave radiation oxidation ;n brown ea;th soil (Fitzwilliam). Soil incu-
bated
at 30-C. (*Significantly P = 0.0.0.
different
from
control
Microwave radiation-soil
0
I,
5
IO
IS
20
25
30
491
micro-organisms, nitrification, S-oxidation
35
40
45
0
5
IO
IS
20
25
30
35
40
45
Days
Days
250 - s.o,%
Fig. 3. EtTect of microwave radiation (20 s) on S-oxidation in a brown earth soil (Chapeltown).
so:=s
IO-
oa06-
0
s. o:=s
02
-
0.1
-
0
*hL::
;
I 7
Control I
I 21
I 26
o&s Fig. 4. Effect of microwave radiation on S-oxidation in an organic loam soil. Soil irradiated directly after S-amendment (*Significantly different from control P = 0.05). SBB.I? 5. r
492
M. WAINWRIGHT, K. KILLHAM and M. F. DIPROSE Table
I. Effect of microwave
radiation
of micro-organisms
in an orgamc
loam soil
0
7
Days 14
21
6.6 * 0.7 7.0 k 0.6
6.9 k 0.8 7.8 + 1.0
9.6 + 1.2 6.8 + 1.0
9.8 k 0.9 10.8 k 1.3
30 * 8 0
51 + 4.7 3 f 0.5
32 f 4 5 + 0.9
30 * 5 IO f 2
122 + 12 8 + I2
138 k 15 20 + 3
139 + 17 41 +8
I40 + 14 70 + 9
139 * I5 81 +9
IO * 0.9 20 f 3.2
30 f 5 50 + 7.2
50 & 7 80 f 9.2
92 k 10 420 + 52
89 k I2 430 & 47
Viable count Heterotrophic bacteria (IO’ 8-l) Control Irradiated Total fungi (IO-’ g- ‘) Control Irradiated Fungal colonies from soil particles Control Irradiated Thiobacilli ( IO4 g- ‘) Control Irradiated
on viable count
Soil was amended with So and immediately irradiated (L-standard replicate). Counts expressed as number g- ’ oven dry soil.
numbers of heterotrophic bacteria were not affected (Table 1). The viable count of fungi then continued to remain low throughout the post-irradiation period, while bacterial numbers remained similar in control and treated soils. The number of fungal colonies derived from soil particles was also markedly reduced following irradiation (Table I). Thiobacilli on the other hand increased in numbers following exposure, so that treated soils contained nearly 5 x as many of those organisms than did the control, 28 days after treatment. The same pattern of reduction in fungal count and stimulation of thiobacilli also occurred in Chapeltown soil microwaved (20s) following a 20-days oxidation (Table 2). DISCUSSION
Our results show that exposure to 2450MHz microwave radiation (20s) has a marked differential effect on the viable count of soil micro-organisms, eliminating the fungal count while having little influence on numbers of heterotrophic bacteria. Diprose rt al. (1978) also showed that microwave irradiation (30s) eliminated the fungal count while a 60 s exposure was needed to halve the bacterial count but some bacteria survived 360s exposure. These observations suggest that fungal spores are more susceptible to microwave irradiation than are bacteria,
Table 2. Effect of microwave
radiation
deviation,
means
of three replicates,
Fungi (lO”g--‘)
0
23 + 5.4
10 20
26 + 4.2 28 k 4.5
30 40
O(31 + 4.1) 2(35 k 5.6)
13.9 f 10.1 f
1.4 1.0
29 5 3 9 * 1.2
10 plates
per
since the fungal plate count is usually considered as a count of these propagules. Lyon et al. (1979) demonstrated the marked effect of microwave irradiation on fungal spores when they showed that the viability of Botrytis cinerea spores was reduced by 98% when added to soil subsequently exposed to microwaves for 8 s. Our observed reduction in the number of fungal colonies derived from soil particles suggests that fungal hyphae are also susceptible. On exposure to microwaves the temperature of soil increases substantially (Table 3), and Vela and Wu (1979) state that it is thermal rather than direct radiation effects which kill micro-organisms. Such increases in soil temperature account for the reduction in fungal count decrease in nitrification and S-oxidation after prolonged exposures. Bollen (1969) showed that plate counts of fungi were reduced by 98% when soil was heated to 60°C while bacteria were largely unaffected. Nitrification is slow in soils heated above 4o”C, while the optimum temperature for ammonification, in contrast to most microbial transformations in soil is not in the mesophillic range, but above 40°C and the process can occur in manure piles held at 65°C (Alexander, 1977). S-oxidation is also inhibited at high temperatures (Li and Caldwell, 1966). The increase in numbers of thiobacilli and rate of S-oxidation following short exposure to microwave radiation is difficult to explain, but was not due to a marked
(20 s) on viable count
of micro-organisms
in a brown earth soil (Chapeltown)
Days
28
Heterotrophic bacteria (IO’ g- I)
Thiobacilli (IO’g-1)
Pre-microwaved soil 80 k 12.3 29 + 6.1 82 k 9.4 92 + 13.0 Post-microwaved 110 k 25(97 + 17) 140 * 21(110 + 20)
39 30 soil 350 530
+ 4.8 f 5.1 + 42 (39 + 5.8) + 66 (39 + 6.1)
The figures in brackets are the numbers of micro-organisms in control soils. Counts expressed as number g-’ oven dry weight soil. The soils were microwaved for 20 s on day 20 (+ standard deviation, means of three replicates, IO plates per replicate).
Microwave radiation-soil
493
micro-organisms, nitrification, S-oxidation
Table 3. The effects of microwave irradiation (20 s) on availability of S,, to S-oxidizing micro-organisms and the increase in soil temperature following irradiation Treatment
0
7
Days 14
21
28
Control Irradiated
195 245
400 455
775 800
850 800
1000 950
0 21
10 52
20 70
30 82
50 83
Exposure time (s) Temp. (“C)
so:so:-
pgg-‘2 pgg-’
* Means of three replicates. in the availability of the element following treatment (Table 3). Possibly micro-organisms which normally inhibit thiobacilli were killed or some agent ,of bacteriostasis was removed by microwave radiation; both allowing for an increase in thiobacilli. Only limited use has been made of microwave radiation in agriculture, despite its marked advantage over pesticides or fumigants in not leaving potentially harmful residues. Baker and Fuller (1969) showed that the ability of microwave radiation to destroy plant pathogens in soil depends on the moisture content of soil, and conclude that such treatment is commercially impractical. Vela et al. (1976) state that the amount of energy absorbed by soil during a 5 s exposure to microwave radiation (2450MHz) is 5000 J cm-‘, which is much greater than the amount of energy needed to kill weeds in the field (180-360 J cm-‘). The exposure times used by us are unlikely therefore to be used in the field, but similar exposures could be used in greenhouse soils to control fungal pathogens. Under these circumstances microwave treatment is unlikely to have a detrimental effect on soil fertility, while increases in NHf-N and SOi--S resulting from treatment may stimulate seedling emergence and growth. increase
REFERENCES
ALEXANDERM. (1977) Introduction to Soil Microbiology, 2nd edn. Wiley, New York. BAKERK. F. and FULLERW. H. (1969) Soil treatment by microwave energy to destroy plant pathogens. Phytopathology 59, 193-197.
BOLLENG. J. (1969) The selective effect of heat treatment on the microflora of a greenhouse soil. Netherlands Journal of Plant Pathology 15, 157-163. DAVIS R. S., WAYLANDJ. R. and MERKLEM. G. (1971) Ultra-high frequency electromagnetic fields for weed control: phytotoxicity and selectivity. Science 173, 535-537. DIPR~XEM. F., LYONA. J. E., HACKAMR. and BENSONF. A. (1978) Partial sterilization and soil and leaf moisture content measurement by microwave radiation. Proceedings British 491-498.
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HENCEP. R. (1971) A Textbook of Soil Chemical Analysis. Murray, London. Lt P. and CALDWELLA. C. (1966) The oxidation of elemental sulphur in soil. Soil Science Society of America Proceedings 30, 370-372.
LYONA. J. E., DIPR~~EM. F., HACKAMR. and BENSONF. A. (1979) The effect of 2450 MHz radiation on two soilborne pathogenic fungi (Botrytis cinerea, Fusarium). Proceedings of the! 14th Microwave Power Symposium, pp. 90-92. IMPI, Monaco. NOR I. M. and TABATA~AIM. A. (1977) Extraction and calorimetric determination of thiosulphate and tetrathionate in soils. Soil Science 122, 171-175. POSTGATE J. R. (1966) Media for sulphur bacteria. Laboratory Practice 15, 1239-44. VELAG. R. and WV J. F. (1979) Mechanism of lethal action of 2450 MHz radiation on micro-organisms. Applied and Environmental Microbiology 37, 550-553.
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