Direct steam distillation as an alternative to the illinois soil nitrogen test
Development of the Illinois Soil N Test (ISNT) has rejuvenated the search for a soil-based N test to measure potentially mineralizable soil N. Accurate quantification of amino sugar N has been achieved using the ISNT, but issues concerning sample variability and analysis time have led to the discovery of a 10 mol L–1 NaOH direct steam distillation (DSD) procedure. Our primary objective was to determine if DSD could be used as a reliable alternative to the ISNT. Laboratory experiments were conducted to compare the two methods based on recovery of N from pure organic compounds, specificity tests to determine amine group hydrolysis, and recovery of 15N-labeled glucosamine N added to soils. Both methods recovered appreciable amounts of amino sugar N from pure compounds and the ISNT had a higher recovery of N from all amino sugar compounds. Recovery of N from glutamine and asparagine was higher using DSD. Direct 15N techniques for recovery of glucosamine N added to six soils showed no significant difference between the two methods within a soil, but resulted in significant differences among soils. Glucosamine-15N recovery significantly and positively correlated with soil total N. Although the ISNT and DSD measure different amounts of amino sugar N and transition amino acid N, they recover relatively the same amount of hydrolyzable N for a given soil, indicating that differences between the methods may not be that significant as both appear to quantify a pool of potentially mineralizable N. Direct steam distillation appears to be a viable alternative to the ISNT in correlation and calibration of crop response for N-fertilizer recommendations due to the short analysis time per sample (~6 min) and the accurate estimation of potentially mineralizable N.