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Abstract
A direct extraction method to estimate both soil microbial C and N, using a single extractant and chemical digestion stage was evaluated. Soils were fumigated overnight with EtOH‐free CHCl3, the CHCl3 removed, and the soils extracted in 0.5M K2SO4. The flush of microbial C, measured as organic C, and microbial N, measured as NH4‐N, were estimated after acid‐dichromate treatment of the soil extract.
For a range of 12 New Zealand soils there was good agreement (r = 0.93, p<0.001) between the organic C flush in 0.5M K2SO4 and the microbial C estimated by the substrate‐induced respiration method. Organic C flushes ranged between 78–1057 μg C g‐1 and comprised 33–49% of the microbial C. There was also good agreement between the organic C flush and the CO2‐C flush by the fumigation‐incubation method (r = 0.83 p<0.01), except for one soil of acid pH where the CO2‐C flush was anomalously low. On average, the organic C flush comprised 76% of the CO2‐C flush.
The N‐flushes were compared with those obtained by extraction in 0.5M K2SO4 followed by Kjeldahl digestion, and with the mineral N flush obtained by the fumigation‐incubation method. The NH4‐N flush measured after dichromate oxidation, ranged from 14–209 μg N g‐1 and was on average 10% less than the N flush estimated after Kjeldahl digestion. The N‐flush measured after oxidation or Kjeldahl digestion comprised, on average, 59% and 69%, respectively, of the N‐flush after incubation. When estimating NH4‐N by the indophenol colorimetric method there was the possibility of overestimation (maximum 9%) of the NH4‐N following dichromate treatment, because of interference from Cr3+.
The organic C and N flushes in 0.5M K2SO4 extracts provide a rapid and reliable estimate of soil microbial C and N of a wide range of soils including those of low pH. Kjeldahl digestion of the extracts is the preferred method to estimate the N flush, but a reasonable measure can be obtained by direct estimation of NH4‐N after dichromate oxidation.