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Basic Plant and Soil Process Measurements for Range Ecosystem Modeling and Management—Updates for 2005

Soil microbial biomass: an estimate for the Missouri Coteau mollisols


Although soil microbial biomass typically accounts for about 1-3% of total soil carbon and about 5% of total soil nitrogen (Horwath and Paul, 1994), this small labile carbon pool is very important in the nutrient turnover and storage of the soil-plant system. Soil microbial biomass in natural ecosystems usually varies greatly, both spatially and temporally. In prairie mollisols, there are 108 to 109 bacteria per gram of dry soil (Metting, 1993). A study by Tracey and Frank (1998) indicates that soil microbial nitrogen varied from etween two periods are not statistically different if they are labeled with a common letter (a,b,or c).50 μg/g to about 800 μg/g dry soil in the grazing grasslands near Yellowstone National Park. What is the average size of the soil microbial biomass in the Missouri Coteau grassland? How does it change in relation to animal grazing? These questions must be answered to study the carbon and nitrogen flows in the grasslands of this area.

For this purpose, we collected soil samples three times in 2004 (June 26 to July 12, July 28-31, and September 15-23) from differently grazed pastures (three pastures with moderate grazing, three pastures with heavy grazing, and one ungrazed exclosure). We used the chloroform fumigation method to estimate soil microbial biomass according to Horwath and Paul (1994). This method uses chloroform vapor to lyse the majority of the cells of the soil microorganisms and uses the flushes of the CO2 and NH4+ from the lysed cells as the basis for biomass calculation. We also measured soil mineral NH4+ from the same pastures that we used for microbial biomass sampling. The results of the microbial biomass measurement showed a small pool size in this grazing system. Some of the biomass data from the first two sampling periods had negative values if calculated using the standard method used by Horwath and Paul (1994). In addition, we noted a strong influence from the moisture content of the soil samples. To deal with these problems, we modified the standard method of calculation and used regression analysis to obtain the statistical average for the biomass data corresponding to each of the three sampling periods in 2004. This way the biomass results are the ones that would be obtained from soils with the common (averaged) water content.

According to Table 4, the soil microbial biomass N decreased in late July, compared with the values from late June to mid-July and from mid-September. The measured soil mineral seemed to decrease in late July also, though it was not statistically different from the previous (late June to mid July) period. Soil bacteria, probably the dominant functional group of microorganisms in the prairie mollisols, can have a generation time of hours to days under good substrate supply, but it can have a generation time of weeks or months under substrate or water limitation (Bolton, et al., 1993). The rainfall pattern in 2004 growing season was characterized by a wet May, dry July and August and wet September. The depressions in both the soil mineral NH4+ and microbial biomass in late July may be the result of summer moisture stress. When compared with another report (Tracy and Frank, 1998) obtained from Yellowstone National Park, the data presented in Table 4 are lower. Our averaged biomass N is 71 μg/g dry soil. If a C/N ratio of 4.3 is used, this translates to a soil microbial carbon of about 305 μg/g dry soil. We originally had expected more from this measurement. It turns out that we can obtain only an averaged estimation for our grazing grasslands to use in our ecosystem model.

Table 4. Soil microbial biomass nitrogen and soil mineral NH4+ for the grazing pastures at the CGREC in 2004. Biomass nitrogen was averaged from 31 samples taken from three grazing treatments (ungrazed exclosure, moderately grazed area, and heavily grazed area) for each sampling period (based on a common moisture content of 23%). Soil mineral NH4+ was averaged from 21 samples for each sampling period. Data of NH4+ between two periods are not statistically different if they are labeled with a common letter (a, b, or c).


Sampling period

Microbial biomass N (μg/g)

Soil NH4+ (μg/g)

June 26 to July 12



July 28-31



September 15-23




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