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 sam ples with 0.5 M K2SO4, followed by determination of total N in the ext racts. Total N is typically determined by Kjeldahl digestion (TKN) wit h the inclusion of NO2- and NO3- present. The determination of total N by TKN has several procedural constraints such as an initial concentr ation step, long digestion times, and considerable bumping caused by c atalyst salts. In contrast, a persulfate oxidation method (TPN) common ly used for determination of total N in sea- and freshwater samples do es not require a concentration step and is much simpler and faster tha n TKN. The objective of this study was to evaluate and adapt the TPN m ethod to measure total N in 0.5 M K2SO4 extracts for determination of soil microbial biomass N. Studies were conducted to optimize reagent/s ample (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 m in in extracts containing as much as 250 mg glucose-C L-1 at a C/N rat io of 10:1. A comparison of the optimized TPN method to TKN for extrac ts of 12 CHCL3-fumigated soils indicated that the methods gave the sam e N recoveries. The use of the TPN method in 0.5 M K2SO4 extracts shou ld facilitate more rapid and efficient measurements of soil microbial biomass N.