| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
CRC for Greenhouse Accounting and NR&M Indooroopilly, Brisbane QLD 4068, Australia
weijin.wang{at}nrm.qld.gov.au
Abbreviations: CF, chloroform fumigation • CFE, chloroform fumigation extraction • CFI, chloroform fumigation extraction • IRT, infrared temperature sensor • MBC, microbial biomass C • MT, mercury thermometer • MW, microwave • MWE, microwave extraction • MWI, microwave incubation • ODE, oven-dry equivalent • SIR, substrate-induced respiration • TOC, total organic C • WFP, water-filled porosity
We welcome the response from Dr. Weil and Dr. Islam regarding the microwave (MW) irradiation method for measuring soil microbial biomass C (MBC).
Islam and Weil (1998) initially determined the optimum level of MW irradiation with 100 g soil packed in a 50-mL (original number) beaker, but recommended using twelve 10-g sample tubes per batch for routine operation, without description on how the tubes were arranged in the rack. We found that MW delivery to 24 tubes arranged in a square rack had significant corner and edge heating effect and the sample temperatures after irradiation varied from 60 to 88°C (Wang et al., 2001). Contrary to the critiques by Weil and Islam (2003), we cannot see any inappropriateness in the irradiation time for this test (e.g., no boiling), and the spatial variation is irrelevant to MW oven calibration. Islam and Weil (1998) calibrated a manufacturer-rated 650-W MW oven with a procedure that differed from the International Standard, IEC 705, and obtained an output of 640 W (J s-1). But they irradiated 100 g soil for 60 s to achieve 400 J g-1 soil MW application, which could only have been achieved using a 670-W MW oven. In fact, we calibrated our MW ovens using the method of IEC 705 as well as that described by Islam and Weil (1998) and found that the latter gave lower wattage values than the former.
Islam and Weil (1998) found that sample temperatures after each MW treatment at 400 J g-1 soil were 
82°C, regardless of soil water content that varied from 15 to 32% for three different soils and from 80% water-filled porosity (WFP) to almost dry following a series of irradiation. However, we found that soil water content had a significant effect on the temperature rise (Wang et al., 2001). Using Soils 30 and 31 of Wang et al. (2001) and a MW oven of 888 W as calibrated following the method of Islam and Weil (1998), we irradiated 100 g oven-dry equivalent (ODE) soil at 80% WFP (i.e., 0.31 and 0.38 mL g-1, respectively; initial temperature 23°C) at 400 J g-1 (for 45 s) in a 100-mL beaker covered with a Petri dish and obtained resultant temperatures of 96 to 97°C as measured with an inserted mercury thermometer (MT). Abrupt steam emission from the beaker was sometimes observed. The lower temperature measured by Islam and Weil (1998) might have resulted from the use of a thermistor (see below).
Islam and Weil (1998) operationally used 80% WFP as the standard moisture for MW irradiation of all soils, but did not give any rationalization. The absolute amount of water at the same percentage WFP or water-holding capacity could differ considerably for different soils. Because the amount of water in soil significantly affects the resultant temperature when the samples were exposed to the same MW dosage (J g-1 soil), we operationally moistened soils with same amount of water (50%) on the basis of mass instead of percentage WFP. Weil and Islam (2003) were concerned by this modification because many of the samples were irradiated at >100% WFP. However, there seems no reason to keep soil moisture at <100% WFP during MW irradiation.
Weil and Islam (2003) found only a weak relationship between the temperatures measured with a thermistor probe (unspecified) and those with an infrared temperature sensor (IRT) for centrifuge tubes containing 10 g moist soil each. We irradiated 10 g ODE soil at 50% water content in centrifuge tubes, then inserted a MT into the sample, and at the same time scanned the samples with an IRT from the outer surface of the tubes. The readings (T = temperature) by the MT and the IRT agreed very well (TMT°C = 1.02TIRT - 1.26, r2 = 0.994, n = 11). The significant disagreement between the readings measured with a thermistor and an IRT by Weil and Islam (2003) might have resulted from a few factors: (i) the temperature of 10-g sample could decrease markedly before a slowly reacting thermistor probe gave a stable reading; (ii) the depth of the 10-g sample might be insufficient for the probe to give a correct reading; (iii) the metal probe could absorb some heat from the surrounding soil and thus underestimate the temperature; (iv) the operator failed to keep a reasonably close distance between the sample and the IRT sensor. The IRT readings at different spots of an irradiated sample tube could vary a few degrees, due to uneven MW delivery to the soil sample rather than the problem of the IRT. An average temperature should be determined by twisting and scanning the tube quickly.
Weil and Islam (2003) reprocessed our data using 20 out of 30 soils with clay content < 50% and pH < 7.5, and concluded that "MWE was better than CFE for predicting MBC by the standard CFI" (CFE = chloroform fumigation extraction; CFI = chloroform fumigation extraction; MWE = microwave extraction) without showing the r values between MWE and CFI. In fact, there were 21 soils with clay < 50% and pH < 7.5. Analyses with the 21 soils showed: rCFE vs. CFI = 0.55 (P < 0.01) > rMWE1 vs. CFI = 0.31, and rCFE vs. SIR = 0.81 (P < 0.01) > rMWE1 vs. SIR = 0.67 (P < 0.01) (SIR = substrate-induced respiration). Contrary to their claim, CFE was apparently superior to MWE in terms of their correlations with CFI and SIR. However, the practice of excluding the nine or 10 soils in data analysis should be avoided, as it removed all soils with MBC > 250 mg kg-1 as estimated with CFI. Weil and Islam (2003) criticized these soils as being "highly unusual", but it is very important to test the MW method using soils representing a wide range of properties.
Weil and Islam (2003) assessed our MBC estimates in terms of the percentage of total organic C (TOC) as MBC. However, they did not include the soils that had >250 mg kg-1 MBC as estimated with CFI. Averaged across all soils, the three conventional methods, CFE, CFI, and SIR, consistently gave values lower than the MWE-MBC. Because the soils had been air-dried, stored, and remoistened, it was normal for some soils to have low MBC. Apparently, it was inappropriate to use percentage of TOC as a benchmark to evaluate the MBC estimates.
The optical densities at 410 nm of the 0.5 M K2SO4 extracts following MW irradiation were significantly higher than their counterparts following chloroform fumigation (CF) for a number of soils (Wang et al., 2001). Weil and Islam (2003) argued that the difference in optical density between the MW (0.061 ± 0.072) and CF treatments (0.052 ± 0.067) was minor when averaged across all soils. However, the numbers after "±" indicate variations among soils rather than measurement errors. Assuming that soil microbial constituents released by MW and CF were equally extractable in 0.5 M K2SO4 solution, the difference in the amounts of organic C extracted after MW and after CF (CMWE - CCFE) would be an approximate indication of the extra amount of nonbiomass C released by MW. Figure 1 shows that CMWE was larger than CCFE for most soils tested and the difference of CMWE - CCFE was positively correlated to TOC content. This is consistent with the common knowledge that physical disruption of soil, such as air-drying and freezing/thawing, could result in release of otherwise physically protected nonbiomass organic matter.
|
Weil and Islam (2003) presented the evaluation results of Doran on the MW incubation (MWI) method. Although the results estimated with 10 and 20-d incubations following MW were closely correlated and the ratio of "MBC" estimated by the 10- and 20-d incubations were very similar in two different management plots, these findings do not warrant their conclusion that "the MWI method ... offers great promise ... for routine measurement of MBC."
REFERENCES
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 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 | |||
