HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Wang, W. Articles by Moody, P. W. Search for Related Content PubMed Articles by Wang, W. Articles by Moody, P. W. Agricola Articles by Wang, W. Articles by Moody, P. W.

DIVISION S-3 - SOIL BIOLOGY & BIOCHEMISTRY

Evaluation of the Microwave Irradiation Method for Measuring Soil Microbial Biomass

Dep. of Natural Resources, 80 Meiers Rd, Indooroopilly, Brisbane, Qld 4068, Australia

* Corresponding author (weijin.wang{at}dnr.qld.gov.au)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Soil microorganisms are an important and labile component of soil organic matter. We investigated the feasibility of measuring soil microbial biomass using microwave irradiation as a rapid and nontoxic alternative to chloroform fumigation. The efficacy of microwave irradiation was affected by the distribution pattern and location of soil samples in the oven, the amount of soil, soil moisture content, irradiation time, and soil properties. To ensure uniform and consistent microwave application to soil samples, we modified the microwave treatment procedure (i) by using a round sample rack to improve the uniformity of microwave delivery; (ii) by adjusting the moisture of different soils to the same percentage of their weight; (iii) by using a constant sample weight and irradiation time; and (iv) by grouping soil samples according to their clay contents. The results using 30 Australian soils, with a wide range in soil properties, showed that organic C released by microwave irradiation had weak or no correlation with the microbial biomass C measured by chloroform-fumigation extraction, chloroform-fumigation incubation, and substrate-induced respiration methods; while the correlations between the measurements of the three conventional methods were generally close. The increase in extractable C after microwave treatment was often accompanied by an increase in the optical density of the soil extract at 410 nm, which was indicative of the liberation of humified substances by microwave irradiation. We conclude that the organic C liberated from soil by microwave irradiation may contain an appreciable portion of nonbiomass compounds.

Abbreviations: MBC, microbial biomass C • MBN, microbial biomas N

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
SOIL MICROORGANISMS are the living component of soil organic matter. Critical evaluation of its significance is hampered by the lack of reliable techniques of quantification. Among a number of current methods to measure microbial biomass, including chloroform-fumigation (Jenkinson and Powlson, 1976; Vance et al., 1987a) and substrate-induced respiration (Anderson and Domsch, 1978), microwave irradiation has been proposed as a rapid and nontoxic alternative (Islam and Weil, 1998).

Microwave radiation provides an effective means of killing soil microorganisms (Wainwright et al., 1980; Ferriss, 1984). Speir et al. (1986) found that the biocidal effect of microwave radiation could be similar to that achieved by chloroform fumigation. They also found that mineral N production during incubation of the irradiated soil increased as microwave radiation time increased from 10 to 90 s, which coincided with the decrease in microbial biomass C immediately after irradiation treatment. Speir et al. (1986) postulated that the extra amount of N liberated from the irradiated soil could be of microbial origin.

The possibility of using microwave radiation to estimate soil microbial biomass C (MBC) and microbial biomass N (MBN) has been investigated by Hendricks and Pascoe (1988), Zagal (1989), Monz et al. (1991), and Puri and Barraclough (1993). In these studies, the microwave irradiation-released C and N were generally compared with MBC and MBN determined with the conventional chloroform-fumigation techniques (Jenkinson and Powlson, 1976; Vance et al., 1987a). The results obtained by different researchers have been inconsistent. For example, Hendricks and Pascoe (1988) showed that microwave irradiation and chloroform fumigation of two coarse-textured soils resulted in similar CO₂ production flushes during subsequent incubation, and therefore gave similar MBC estimates. However, Monz et al. (1991) found that microwave radiation released significantly less organic C and N from a sandy loam soil than chloroform fumigation. In contrast, Zagal (1989) demonstrated that MBN values estimated with microwave radiation were consistently higher than those with chloroform fumigation.

Variations in the dosage of microwave radiation applied to soil samples may have contributed to the discrepancy. Insufficient energy application would result in incomplete killing of microbes (Speir et al., 1986), whereas over irradiation and heating would liberate otherwise unextractable humified organic materials (Islam and Weil, 1998). The above studies were generally based on a few soils, with a limited range in soil properties, and therefore, may not be applicable to a wider range of soils. More recently, Islam and Weil (1998) evaluated the microwave method using 62 soils. Moisture of soil samples was adjusted to 80% water-filled porosity, and the soil samples exposed to microwave irradiation at 800 J g^-1 dry weight. They found that although the net flushes of organic C from irradiation-incubation and irradiation-extraction were significantly lower than those from fumigation-incubation, the C values from irradiation and fumigation methods were closely correlated.

However, the soils used by Islam and Weil (1998) in their studies had only a narrow range of clay contents (100–300 g kg^-1, with one exception of 350 g kg^-1), which may be an indication of small variations in other soil properties. Since Zagal (1989) showed that clay content of soil significantly influences the relative amount of MBC estimated by microwave irradiation and chloroform-fumigation methods, further studies are needed using soils with a wide range of properties. Moreover, the effect of radiation is influenced by the energy output of a microwave oven, amount of soil used, duration of irradiation treatment, location of samples in the oven, soil moisture content, and other soil properties. Appropriate techniques are required to ensure that microwave energy is delivered to soil samples evenly and consistently.

The objectives of this study were: (i) to evaluate the influences of irradiation rate, soil properties, soil moisture, and sample position in the oven on the efficacy of microwave treatment; (ii) to modify and optimize the operational procedures to ensure uniform and consistent microwave radiation delivery; and (iii) to evaluate the microwave irradiation method in relation to the chloroform-fumigation extraction, chloroform-fumigation incubation, and substrate-induced respiration techniques for measuring soil microbial biomass.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Soils and Properties
Thirty-two soils were taken from three states of Australia to provide a wide range in pH, texture, and total organic C and N contents (Table 1). The soils were airdried, ground to pass a 2-mm sieve, and stored in sealed plastic containers. Total organic C and N in soil were determined by dry combustion using a LECO CNS-2000 analyzer (LECO Corp., St. Joseph, MI). Calcareous soils were pretreated with H₃PO₄ to remove carbonate (Jalil et al., 1996). Particle-size distribution was measured by the pipette method after soil organic matter oxidation with H₂O₂ and dispersion with sonication (Day, 1965). Water-holding capacity was measured by packing soil into plastic cylinders that were fitted with fine nylon cloth at the bottom, then immersing the cylinder in water for 24 h. Soil moisture content was measured after allowing the cylinder to freely drain on a funnel for 30 min (Choudhary et al., 1995). Soil pH was measured on a 1:5 soil/water suspension using a glass electrode. All the results are expressed on an oven-dry (105°C) weight basis.

View larger version (18K): [in this window] [in a new window]   Fig. 1. Temperature variation of water samples after microwave irradiation at 266 J mL-1, as affected by sample location and distribution in the oven.

Soil Moisture Content
Two methods of controlling soil moisture content were examined for their effect on uniform microwave treatment of different soils. In the first method, 10 g of oven-dry weight equivalent of three soils, Soil 30 (sandy), 31 (loamy), or 32 (clayey) was weighed into centrifuge tubes. The soil samples were then moistened with deionized water to 80% of their water-holding capacity, thus resulting in the same amount of soil but different amounts of water in three soils. Eight tubes of the same soil were evenly placed in a round plate and were exposed to microwave radiation for 18 s, equivalent to an energy application of 248 J g^-1 soil. Four tubes were randomly chosen and measured for temperature with the noncontact infrared thermometer. In the second method, the 3 soils were moistened with the same amount of water at 50% (w/w), then irradiated and the temperature was measured as above.

Soil Texture
To examine the influence of soil texture on microwave radiation, 10 g of Soil 30 (sandy), 31 (loamy), or 32 (clayey) was placed into centrifuge tubes, and then soil moisture content adjusted to 50% (w/w). In the first treatment, five tubes of Soil 30, six tubes of Soil 31, and five tubes of Soil 32 (a total of 16 tubes) were randomly placed in a round plate and exposed to microwave radiation for 36 s. In the second treatment, the three soils were irradiated separately, i.e., 16 tubes of the same soil were heated for 36 s. Temperature after irradiation of soil samples was measured for four randomly selected tubes of each soil.

Duration of Microwave Irradiation
The effects of microwave radiation time on soil temperature, extractable C, and optical density of soil extract were determined with Soils 29 and 32 that had been preincubated for 7 d (see below). Ten grams of oven-dry equivalent of moist soil was weighed into a 50-mL centrifuge tube and adjusted to 50% (w/w) of moisture by adding deionized water. Eight tubes for the same soil were placed in a round rack and exposed to microwave radiation at full power (1100 W) for 0, 8, 16, 32, 64, and 96 s. To avoid too high a temperature, those samples irradiated for 64 and 96 s received a series of 32-s bursts. After each 32-s exposure to microwave, the soils were allowed to cool down to room temperature. Soil temperature was measured with a noncontact infrared thermometer after each irradiation treatment. The tubes were weighed before and after each treatment to determine moisture loss. The soils were then extracted with 45 mL of 0.5 M K₂SO₄ by shaking for 60 min. The soil suspension was centrifuged at 1700 x g (3000 rpm) for 5 min and filtered through a Whatman no. 42 filter paper (Whatman International Ltd., Kent, UK). The filtrates were stored at 4°C and analyzed within 1 wk. Organic C in the solution was analyzed by digesting with acid dichromate at 130°C for 30 min. To determine the relative amount of humic substances solubilized by microwave treatment, the optical density of the yellowish-brown colour of the 0.5 M K₂SO₄ extracts was measured at 410 nm using a spectrophotometer (Islam and Weil, 1998).

Preincubation of Soils
The soils were weighed into polystyrene jars and adjusted to 55% water-holding capacity by adding deionized water. Each jar was covered with a piece of plastic film that was held in place with a rubber band. The plastic film was pricked with two holes to allow aeration. The jars were then put in an incubator at 25°C and 70% relative humidity for 7 d. Moisture loss from the soils during incubation was found to be negligible.

Modification of the Microwave Irradiation-Extraction Method
Based upon the results of the above experiments, the method proposed by Islam and Weil (1998) was modified. Detailed description of the modified method is presented below in the Results and Discussion section.

Microbial Biomass Determination by Chloroform- Fumigation Incubation Technique
The method of Jenkinson and Powlson (1976) was used to determine MBC for 30 soils (Soils 1–30; Table 1). Briefly, each preincubated soil was weighed into duplicate beakers and fumigated with ethanol-free chloroform at 25°C for 24 h. After removal of chloroform vapour by repeated evacuation, the soils were inoculated with 1% unfumigated soils and placed in 1.5-L Mason jars. The jars were flushed with compressed air and then put in an incubator at 25°C for 10 d in the presence of NaOH to absorb CO₂ released from soil. Unfumigated soils were also incubated in duplicate under the same conditions as the fumigated soils. The CO₂ production was determined by HCl titration of NaOH after incubation. Microbial biomass C was calculated as follows:

[1]

where F_C is the difference in CO₂–C evolution between fumigated and unfumigated soils, and 0.45 is the proportion of MBC considered to be released by fumigation followed by incubation (Jenkinson, 1988).

Microbial Biomass Determination by Chloroform- Fumigation Extraction Technique
This was undertaken following the method of Vance et al. (1987a). The preincubated soils (Soils 1–30) were fumigated as described above. The soils were then extracted with 0.5 M K₂SO₄. Controls were prepared by extracting soil without fumigation. The soil suspension was filtered through a Whatman no. 42 filter paper (Whatman Ltd., Kent, UK). Total organic C content in the soil extracts was measured with a dichromate digestion method. Microbial biomass C was calculated from the difference in extractable organic C between the fumigated and unfumigated soil, as follows:

[2]

where F_EC refers to the difference in extractable organic C between fumigated and unfumigated treatments; and 2.64 is the proportionality factor of MBC released by fumigation extraction (Vance et al., 1987a).

Microbial Biomass Determination by Substrate- Induced Respiration Technique
The method of Anderson and Domsch (1978) was modified for this measurement. Twenty grams of air-dried soil were weighed into 285-mL glass jars in four replicates and preincubated as described above. Thereafter, soil in each jar was amended with 600 mg of 1:4 glucose/talcum powder mixture and mixed thoroughly with a spatula. After stabilizing for 1 h, the flasks were flushed with compressed air and covered with lids fitted with a rubber seal. The start time for each flask was recorded and the soil was incubated at 22°C for 1 h. Then 10 mL of a gas sample was taken from each flask with an airtight syringe and analysed for CO₂ concentration using a gas chromatograph (Shimadzu Scientific Instruments, Maryland, USA), and MBC calculated as follows:

[3]

where the conversion factor k varies with temperature and moisture. Here a value of 30 was used as proposed by Kaiser et al. (1992).

Statistical Analysis
The ANOVA and the regression procedures of Genstat 5, Release 4.1 (Payne, 1997) were used. Treatment means were compared using the least significant difference test at P < 0.05 and P < 0.01.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Factors Affecting Microwave Irradiation Treatment Results
Sample Location
Temperature was used as an indicator of the amount of microwave energy received by each sample. The temperature of water samples in the square rack varied greatly after microwave radiation, with the coefficient of variation being 13.8% (n = 12, Fig. 1). The samples located at the four corners had the highest temperature, while those in the middle had the lowest temperature. This variation was largely due to the difference in the surface areas of samples exposed to microwave radiation. Samples in the corners of the rack received the most direct radiation, while those in the middle received the least. This result could be extrapolated to soil samples as soil temperature and extractable C were found to increase concomitantly with increasing microwave energy application (Islam and Weil, 1998). To improve the evenness of microwave delivery to soil samples, the square rack was modified into a round plate. The round rack reduced the coefficient of variation of samples to 1.8% (n = 12, Fig. 1) and hence, it was employed for all the subsequent experiments.

Soil Weight
When different amounts of soil were exposed to microwave at the same radiation rate (s g^-1 or J g^-1 soil), the extent of increase in soil temperature differed with soil weight (P < 0.01; Fig. 2) . The larger the amount of soil used, the higher the temperature was after radiation. This result was consistent with the finding by Ferriss (1984) and may be because of more efficient interception of microwave energy by larger amounts of soil. It implies that adjusting the period of irradiation in proportion to soil weight, as suggested by Islam and Weil (1998), may create inconsistent irradiation effects.

View larger version (28K): [in this window] [in a new window]   Fig. 2. Effect of soil weight on soil temperature after microwave irradiation at 248 J g-1. LSD0.01 = 9.0.

View larger version (49K): [in this window] [in a new window]   Fig. 3. Effect of soil texture on soil temperature after microwave irradiation separately (one soil type) or together (all three soil types).

View larger version (19K): [in this window] [in a new window]   Fig. 4. Increases in soil temperature, extractable C, and optical density of soil extracts after different periods of microwave irradiation.

Microwave-Irradiation Method 1
Duplicate portions of the preincubated soils, each containing 10 g of oven-dry soil, were weighed into 50-mL plastic centrifuge tubes. The water content of soil samples was adjusted to 50% (w/w) by adding deionized water. The centrifuge tubes were covered with caps that had a pinhole. Sixteen samples with similar clay contents were irradiated with an 1100-W microwave oven (Sharp Corp., Osaka, Japan) for 50 s. The samples were allowed to cool down to room temperature, rotated and irradiated for an additional 50 s. The samples were irradiated twice because one exposure under similar conditions was found inadequate to cease microbial activity (Islam and Weil, 1998). After cooling to room temperature, 45 mL of 0.5 M K₂SO₄ was added to each tube, the tube capped and shaken for 60 min. The soil suspension was centrifuged at 1700 x g (3000 rpm) for 5 min and filtered through a Whatman no. 42 filter paper (Whatman Ltd., Kent, UK). Total soluble C in the filtrate was determined using the method described in the chloroform-fumigation extraction method of Vance et al. (1987a). Controls were prepared at the same time without microwave treatment.

Microwave-Irradiation Method 2
The soils were treated with similar procedures as in Method 1, except that 12 samples in a batch were irradiated with a 600-W microwave oven (Samsung Electronics, Malaysia) for 70 s. This method was designed to allow longer irradiation time to minimize variations in temperature between samples because of uneven energy distribution within the oven cavity. Furthermore, this method was designed to use a microwave oven with similar wattage to achieve similar temperatures to those proposed by Islam and Weil (1998), but with modifications as described above to allow the comparison of results.

Comparison of the Modified-Microwave Methods with Other Methods
After microwave irradiation with Method 1, the temperature of soil samples reached 88.7 ± 3.3°C. According to the formula given by Islam and Weil (1998) and using the output specified by the manufacturer, the microwave energy was applied at 2 x 1100 J s^-1 x = 688 J g^-1. The microwave output calibrated by heating deionized water with the procedure given by Islam and Weil (1998) was 888 W. If this wattage is used, the energy application rate to soil samples would be 2 x 888 J s^-1 x = 555 J g^-1. Regardless of the calculation basis, the energy application rates in this study were much lower than the 800 J g^-1 suggested by Islam and Weil (1998), but the temperature of soil samples after irradiation with Method 1 was higher than those (82°C on average) reported by Islam and Weil (1998). Likewise, the energy application in Method 2 was 2 x 600 J s^-1 x = 700 J g^-1, but the temperature reached was 82.0 ± 8.2°C. Method 1 resulted in slightly higher temperature than Method 2 probably because of the effect of soil weight as discussed above. Therefore, in an attempt to apply similar rates of energy and to achieve similar temperature increases to those proposed by Islam and Weil (1998), Method 1 was a compromise between irradiation rate and temperature. Method 2 was designed to apply microwaves to achieve similar temperatures to those suggested by Islam and Weil (1998). It is generally considered that heat is the mechanism of the biocidal effect of microwave on microorganisms (Vela and Wu, 1979; Ferriss, 1984; Ou et al., 1985; Hendricks and Pascoe, 1988). Thus controlling the level of microwave irradiation on the basis of temperature appears more sensible than on the basis of the mass of soil.

With the chloroform-fumigation incubation method, Soils 1, 2, 3, 5, 8, 9, 11, and 16 had higher CO₂ evolution from the unfumigated samples than from their fumigated counterparts. Negative biomass would have been estimated for these soils using this method. The higher CO₂ evolution from the control was because the large natural population in the unfumigated soil was more capable of decomposing soil organic materials than the recolonized population in the fumigated soil (Jenkinson, 1988). Similar results have been reported in the literature and this usually occurs in acidic soils or soils enriched with organic substrates (Martens, 1985; Vance et al., 1987b). Only the soils with positive biomass measurements were used for comparison with the microwave irradiation methods.

The biomass C measured by chloroform-fumigation extraction method was highly correlated with that measured by chloroform-fumigation incubation method (Fig. 5a) . The correlation between the chloroform-fumigation extraction method and the substrate-induced respiration method was also highly significant (Fig. 5b). However, the substrate-induced respiration method seemingly underestimated CO₂ production for soils with pH > 7 (Fig. 5b), possibly because of absorption of CO₂ by the moist alkaline soils. If the CO₂ evolved from soil had been instantly removed from the headspace of the incubation jars, a closer relationship might have been achieved between the chloroform-fumigation extraction and substrate-induced respiration methods (Martens, 1987).

View larger version (28K): [in this window] [in a new window]   Fig. 5. Relationships between microbial biomass C (mg kg-1) measured by different methods. MIE: microwave irradiation-extraction method; SIR: substrate-induced respiration method; CFE: chloroform-fumigation extraction method; and CFI: chloroform-fumigation incubation method. r0.05 = 0.36, r0.01 = 0.46 when n = 30; r0.05 = 0.42, r0.01 = 0.54 when n = 22. The open circles represent soils with pH > 7.

View larger version (19K): [in this window] [in a new window]   Fig. 6. Optical densities of 0.5 M K2SO4 extracts of soils following microwave irradiation or chloroform fumigation. The vertical bars on top of each column refer to standard errors of two replicates. LSD0.05 = 0.0068.

We conclude that the microwave irradiation methods, under the experimental conditions specified in this study, could not selectively and unequivocally quantify microbial biomass in soil.

	ACKNOWLEDGMENTS

 
This research was supported by a CRC Greenhouse Accounting grant. We are grateful to Dr. P.M. Chalk (the University of Melbourne, presently IAEA) for providing a number of soil samples.

Received for publication February 27, 2001.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 

• Anderson, J.P.E., and K.H. Domsch. 1978. A physiological method for the quantitative measurement of microbial biomass in soils. Soil Biol. Biochem. 10:215–221.
• Choudhary, M.I., A.A. Shalaby, and A.M. Al-Omran. 1995. Water holding capacity and evaporation of calcareous soils as affected by four synthetic polymers. Commun. Soil Sci. Plant Anal. 26:2205–2215.
• Day, P.R. 1965. Particle fractionation and particle-size analysis. p. 545–567. In C.A. Black et al. (ed.) Methods of soil analysis. Pt. 1. 1st ed. Agron. Monogr. 9. ASA, Madison, WI.
• Ferriss, R.S. 1984. Effects of microwave oven treatment on microorganisms in soil. Phytopathology 74:121–126.
• Hendricks, C.W., and N. Pascoe. 1988. Soil microbial biomass estimates using 2450 MHz microwave irradiation. Plant Soil 110:39–47.
• Islam, K.R., and R.R. Weil. 1998. Microwave irradiation of soil for routine measurement of microbial biomass carbon. Biol. Fertil. Soils 27:408–416.
• Jalil, A., C.A. Campbell, J. Schoenau, J.L. Henry, Y.W. Jame, and G.P. Lafond. 1996. Assessment of two chemical extraction methods as indices of available nitrogen. Soil Sci. Soc. Am. J. 60:1954–1960.[Abstract/Free Full Text]
• Jenkinson, D.S. 1988. Determination of microbial biomass carbon and nitrogen in soil. p. 368–386. In J.R. Wilson (ed.) Advances in nitrogen cycling in agricultural ecosystems. C.A.B. International, Wallingford, UK.
• Jenkinson, D.S., and D.S. Powlson. 1976. The effects of biocidal treatments on metabolism in soil. V. A method for measuring soil biomass. Soil Biol. Biochem. 8:209–213.
• Kaiser, E.A., T. Mueller, R.G. Joergensen, H. Insam, and O. Heinemeyer. 1992. Evaluation of methods to estimate the soil microbial biomass and the relationship with soil texture and organic matter. Soil Biol. Biochem. 24:675–683.
• Martens, R. 1985. Limitations in the application of the fumigation technique for biomass estimations in amended soils. Soil Biol. Biochem. 17:57–63.
• Martens, R. 1987. Estimation of microbial biomass in soil by the respiration method: Importance of soil pH and flushing methods for the measurement of respired CO₂. Soil Biol. Biochem. 19:77–81.
• Monz, C.A., D.E. Reuss, and E.T. Elliott. 1991. Soil microbial biomass carbon and nitrogen estimates using 2450 MHz microwave irradiation or chloroform fumigation followed by direct extraction. Agric. Ecosys. Environ. 34:55–63.
• Ou, L.T., D.F. Rothwell, and M.V. Mesa. 1985. Soil sterilization by 2450 MHZ microwave radiation. Soil Crop Sci. Soc. Fla. Proc. 44: 77–80.
• Payne, R.W. 1997. Genstat 5, Release 4.1. Oxford University Press, Oxford.
• Puri, G., and D. Barraclough. 1993. Comparison of 2450 MHz microwave radiation and chloroform fumigation-extraction to estimate soil microbial biomass nitrogen using ¹⁵N-labelling. Soil Biol. Biochem. 25:521–522.
• Speir, T.W., J.C. Cowling, G.P. Sparling, A.W. West, and D.M. Corderoy. 1986. Effects of microwave radiation on the microbial biomass, phosphatase activity and levels of extractable N and P in a low fertility soil under pasture. Soil Biol. Biochem. 18:377–382.
• Vance, E.D., P.C. Brookes, and D.S. Jenkinson. 1987a. An extraction method for measuring soil microbial biomass C. Soil Biol. Biochem. 19:703–707.
• Vance, E.D., P.C. Brookes, and D.S. Jenkinson. 1987b. Microbial biomass measurements in forest soils: The use of the chloroform fumigation-incubation method in strongly acid soils. Soil Biol. Biochem. 19:697–702.
• Vela, G.R., and J.F. Wu. 1979. Mechanism of lethal action of 2450 MHz microwave radiation on microorganisms. Appl. Environ. Microbiol. 37:550–553.[Abstract/Free Full Text]
• Wainwright, M., K. Lillham, and M.F. Diprose. 1980. Effects of 2450 MHz microwave radiation on nitrification, respiration and S-oxidation in soil. Soil Biol. Biochem. 12:489–493.
• Zagal, E. 1989. Effects of microwave radiation on carbon and nitrogen mineralization in soil. Soil Biol. Biochem. 21:603–605.

This article has been cited by other articles:

				R.R. Weil and K.R. Islam Comments on "Evaluation of the Microwave Irradiation Method for Measuring Soil Microbial Biomass" Soil Sci. Soc. Am. J., March 1, 2003; 67(2): 674 - 675. [Full Text] [PDF]

				W.J. Wang, R.C. Dalal, and P.W. Moody Response to "Comments on 'Evaluation of the Microwave Irradiation Method for Measuring Soil Microbial Biomass'" Soil Sci. Soc. Am. J., March 1, 2003; 67(2): 676 - 677. [Full Text] [PDF]

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Wang, W. Articles by Moody, P. W. Search for Related Content PubMed Articles by Wang, W. Articles by Moody, P. W. Agricola Articles by Wang, W. Articles by Moody, P. W.