Alkaline persulfate oxidation for determining total nitrogen in microbial biomass extracts

The CHCl₃ fumigation-extraction method for determining soil microbial N involves the extraction of CHCl₃-fumigated and CHCl₃-unfumigated samples with 0.5 M K₂SO₄, followed by determination of total N in the extracts. Total N is typically determined by Kjeldahl digestion (TKN) with the inclusion of NO⁻₂ and NO⁻₃ present. The determination of total N by TKN has several procedural constraints such as an initial concentration step, long digestion times, and considerable bumping caused by catalyst salts. In contrast, a persulfate oxidation method (TPN) commonly used for determination of total N in sea- and freshwater samples does not require a concentration step and is much simpler and faster than TKN. The objective of this study was to evaluate and adapt the TPN method to measure total N in 0.5 M K₂SO₄ extracts for determination of soil microbial biomass N. Studies were conducted to optimize reagent/sample (R/S) ratio and autoclaving time, and to evaluate the effects of C and N concentrations. These studies showed that complete N recovery can be obtained with a R/S ratio of 1 and an autoclaving time of 30 min in extracts containing as much as 250 mg glucose-C L⁻¹ at a C/N ratio of 10:1. A comparison of the optimized TPN method to TKN for extracts of 12 CHCL₃-fumigated soils indicated that the methods gave the same N recoveries. The use of the TPN method in 0.5 M K₂SO₄ extracts should facilitate more rapid and efficient measurements of soil microbial biomass N.