Eliminating Nitrate Interference During Kjeldahl Digestion of Soil Extracts for Microbial Biomass Determination

Abstract

Abstract Nitrate interference in Kjeldahl digestion of soil and plant samples presents a significant problem in establishing N budgets in agricultural systems, where NO − 3 is frequently abundant. During digestion, NO − 3 can be reduced or it can cause volatilization of reduced N, resulting in either over‐ or under‐estimation of Kjeldahl N. A method has been developed to volatilize NO − 3 from K 2 SO 4 extracts of soil prior to Kjeldahl digestion for determination of microbial biomass N. Extracts were pretreated with H 2 O 2 and H 2 SO 4 at 97 °C for 48 h, and then were Kjeldahl digested using a procedure modified for microbial biomass extracts. This method volatilizes >99% of NO − 3 in K 2 SO 4 extracts, as determined by 15 N recovery from spiked samples.