High-temperature catalytic oxidation method for measuring total dissolved nitrogen in K2SO4 soil extracts

A high-temperature catalytic oxidation (HTCO) method was evaluated and compared with the Kjeldahl method for determining total dissolved N (TDN) in 0.025M K2SO4 soil extracts used to assess soil microbial biomass nitrogen. First the HTCO method was evaluated for routine analysis for TDN in K2SO4 solutions. Nitrogen recoveries measured by HTCO exceeded 98% for compounds containing organic and inorganic N and for concentrations up to 10mgNl-1. The proposed indirect injection of sample into the furnace safeguards the catalyst and quartz combustion tube. In these conditions, the drift of signal sensitivity during sequence analysis was negligible implying that HTCO can be used for the routine analysis of samples in a 0.025M K2SO4 matrix. The HTCO and Kjeldahl methods were then compared for determining soil microbial biomass nitrogen in different types of soil using the fumigationextraction method. The TDN concentrations in K2SO4 soil extracts measured with both methods were well correlated. Some discrepancies however, were, observed for TDN concentrations >10mgNl-1 due to a probable underestimation of N in the soil extracts with the Kjeldahl method. The calculated extractable microbial biomass values resulting from the difference in TDN between control and fumigated soils were in good agreement for both methods. HTCO thus provides a very rapid and reliable method for measuring TDN in K2SO4 soil extracts used to determine soil microbial biomass nitrogen.