| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
,b
a University of Maryland, Dep. Natural Resource Sciences and L.A. College Park, MD 20742
b The Ohio State University, South Centers Piketon, OH 45661
Abbreviations: CF, chloroform fumigation • CFE, chloroform fumigation extraction • CFI, chloroform fumigation incubation • IRT, infrared thermometry • MBC, microbial biomass C • MW, microwave • MWE, microwave extraction • MWI, microwave incubation • OD, optical density • TOC, total organic C • WFPS, water-filled total pore space
Microwave (MW) irradiation has been proposed as a nontoxic, rapid biocide treatment in place of chloroform fumigation (CF) in estimating microbial biomass C (MBC). We appreciate the interest shown by Wang et al. (2001) in critiquing and modifying the MW method of Islam and Weil (1998). However, we respectfully disagree with their conclusions that the MW method lacks correlation with standard CF methods and is unsuitable to quantify soil MBC.
The first criticisms in Wang et al. (2001) focus on variability in temperatures in MW irradiated plastic tubes containing water or moist soil. Islam and Weil (1998) used 12 polycarbonate centrifuge tubes (containing 10 g oven dry equivalent moist soil samples) in every other hole of a polycarbonate rack and irradiated them with two MW bursts of 400 J g-1 each in a calibrated MW oven with turntable. Soil temperatures ranged from 75 to 85°C using a thermistor inserted into the soil samples. Wang et al. (2001) apparently packed 24 tubes in a rack (plastic unspecified) and did not calibrate their MW oven to determine proper irradiation times, but found only a slightly greater range of temperatures among tubes irradiated. While we agree with Wang et al. (2001) that a circular rack may allow more uniform irradiation of soil samples, Islam and Weil (1998) found temperatures within this range to have little effect on MBC estimates. Other temperature anomalies in Wang et al. (2001) may be related to using soil water contents and sample sizes outside the ranges tested by Islam and Weil (1998).
Wang et al. (2001) made all temperature measurements by noncontact infrared thermometry (IRT), which indicates temperature of the target container outer surface, not of the water or soil inside. We found a close relationship between thermistor probe and IRT (ST model with laser circle aiming, Raytech, Shelton, CT) readings for 1 L of water in 1.5-L beakers after MW exposure (°CThermistor = 1.2 + 0.97 x °CIRT, R2 = 0.97, N = 10). However, for 50-mL centrifuge tubes containing 10 g of moist soil in a rack, we found only a weak relationship (°CThermistor = 58 + 0.26 x °CIRT, R2 = 0.37, N = 10). Like Wang Weijin (10 Jan. 2002, personal communication), we found that aiming the IRT at slightly different angles or at different parts of a tube gave radically different (±15°C) temperature readings, possibly due to tube material surface emissivity, infrared-absorbing humid air inside the MW cavity, small target size, and/or variable soil-tube contact. We conclude that IRT may not be appropriate for measuring soil temperature in tubes after MW irradiation.
Wang et al. (2001) criticized the Islam and Weil (1998) method based on results obtained with substantially modified versions of the MW method, but did not present any results using the method as published. For example, they failed to adjust samples to 80% water-filled total pore space (WFPS) as specified by Islam and Weil (1998). Instead, they initially adjusted to 80% of drained water holding capacity. Depending on soil texture, the difference could be considerable. In comparing MW methods with standard methods, Wang et al. (2001)(Table 2) added water equivalent to 50% of sample dry weight, resulting in many of samples being irradiated while at >100% of WFPS.
Wang et al. (2001)(p. 1696, 1700) point out that their results may have differed from ours because their samples covered a greater range of clay contents. Because we have had not used soils with clay contents > 50% or with pH values > 7.5, we cannot speculate about the performance of the MW method on such soils. We therefore reexamined the Wang et al. (2001) data for the 20 soils (out of 30) with clay 
500 g ha-1 and pH 7.5 (using data kindly supplied by Wang Weijin, 10 Jan. 2002, personal communication) and found that for MBC in these 20 soils, the relationship between MW and CF extraction (MWE and CFE, respectively) methods (r = 0.82, Fig. 1a)
was much closer than between CF incubation (CFI) and CFE (r = 0.24, without seven negative values by CFI) and about the same as between CFE and a substrate-induced respiration method (r = 0.81). Thus, within clay and pH ranges studied by Islam and Weil (1998), MWE was better than CFE for predicting MBC by the standard CFI.
|
|
While Wang et al. (2001) experienced difficulties with MW irradiation, others have found it useful. Using the Islam and Weil (1998) microwave incubation (MWI) method, Liebig et al. (2002) reported MBC ranging from 1.7 to 2.0% TOC in Typic Argiudolls (310 g kg-1 clay, pH 6.1–6.9) under various crop rotations and N fertilizer rates. They found the MBC results to be repeatable, more sensitive to management than TOC, and to follow trends in agreement with other measures of active C.
The CO2 produced after biocide treatment and reinoculation is a straightforward measure for comparing effects of CF and MW treatments, leaving aside questions about which conversion factors to use or which control values to subtract, if any (Franzluebbers et al., 1999). USDA/NRCS National Soil Biology Lab in Lincoln, NE, (S. Samson-Liebig, 1 Feb. 2002, personal communication) compared headspace CO2 concentrations after 10 d of incubation following either MWI (Islam and Weil, 1998) or CF (Jenkinson and Powlson, 1976) using soil samples from Ohio (100 to 490 g kg-1 clay). The two methods gave closely correlated results (R2 = 0.97, Fig. 1b). They also compared MBC (kg ha-1) estimates for soils from Ohio, Texas, and Oregon and found close agreement between MWI and CFI methods using published conversion factors and recommended controls (Fig. 1c).
Before the Lincoln laboratory began using MWI in 1998, several evaluations were made (John Doran, 5 May 2002, personal communication) comparing CFI with the MWI method of Islam and Weil (1998) modified slightly to use five 50 g soil samples at 80% WFPS in 100-mL glass beakers (250 g total soil) per run. They used 10 and 20-d incubations for soils from the U.S. Great Plains region (clay 30–350 g kg-1, pH 6–7) that were remoistened for 1 wk before analysis. They found both methods highly repeatable (CV 3–6%), and closely correlated (r = 0.87), but the results differed in magnitude by 30 to 60%, depending on whether the 10 or 20-d controls were subtracted. The ratio of MBC in the alternative management plots to that in the conventional management plots was very close by the two methods. They concluded that "the MWI method, as proposed by Islam and Weil (1998) offers great promise as an alternative ... for routine measurement of MBC ... This method is safe, simple, and less time consuming than the CFI method. Further research is needed however for...understanding the proper conversion factor (0.341) to use in calculations MBC." We believe this conclusion is still valid, but also suggest that more research is needed to evaluate the MW method on calcareous, high clay soils.
NOTES
REFERENCES
This article has been cited by other articles:
|
|
 |
|
  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]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | Crop Science | |||
| Journal of Natural Resources and Life Sciences Education |
Vadose Zone Journal | ||||
| Journal of Plant Registrations | Journal of Environmental Quality |
The Plant Genome | |||
