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

Dep. of Vegetable Crops, Univ. of California, Davis, 1636 E. Alisal St., Salinas, CA 93905

P. D. Brooks

Dep. of Soil Science, Univ. of California, Berkeley, CA 94720

*Corresponding author.

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 K₂SO₄ extracts of soil prior to Kjeldahl digestion for determination of microbial biomass N. Extracts were pretreated with H₂O₂ and H₂SO₄ 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 K₂SO₄ extracts, as determined by ¹⁵N recovery from spiked samples.

Received for publication June 4, 1993.

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