Contrary to the prevailing view, cereal crops derive the majority of their nitrogen from the soil, not fertilizer. Soils differ considerably in microbial activities that determine nitrogen-supplying power, and these differences must be taken into account if nitrogen fertilizers are to be used efficiently.
The Illinois Soil Nitrogen Test (ISNT) was developed for this purpose, and involves estimation of gaseous ammonia liberated by heating the soil with strong alkali in a Mason jar. Several studies have provided evidence that the ISNT is predictive of yield response by corn to nitrogen fertilization, but there have also been negative evaluations in which concern has been raised that test values represent a constant proportion of total soil nitrogen rather than a microbial fraction that would be potentially available.
A study was conducted from 2004 to 2006 at the University of Illinois to clarify the chemical nature of what the ISNT measures and its relationship to microbial growth in agricultural soils. A multifaceted approach was taken, involving recovery tests with pure organic nitrogen compounds, statistical analyses of different nitrogen fractions measured for 26 Illinois agricultural soils, and incubation studies to determine incorporation of labeled nitrogen into soil nitrogen fractions.
Recovery tests did not support the concept that the ISNT estimates total soil nitrogen. Rather, the results confirmed that the ISNT is selective for certain forms of microbial nitrogen, and differs from conventional acid-hydrolyzable fractions in the proportions of these compounds that are detected. Specifically, amides and the amino sugars in bacterial cell walls were detected, but not alpha-amino acids or fungal chitin. When the findings were applied in a statistical analysis of data from soil nitrogen fractionation, the ISNT was estimated to recover 95% of the nitrogen in bacterial amino sugars and 43% of amide-nitrogen. The incubation studies showed that labeling was more rapid for nitrogen recovered by the ISNT than in hydrolyzable amino sugars, again indicating a dominance of bacterial over fungal nitrogen recoveries by the ISNT.
Taken together, these findings suggest that the ISNT mainly detects bacterial amino sugars and implicates this form of soil nitrogen in the test’s effectiveness for predicting the response of corn to nitrogen fertilization. Because the ISNT also detects some amide nitrogen, the bacterial amino sugar nitrogen signal can be obscured in soils where these components are highly variable.
The research was funded by the USDA and the Illinois Council on Food and Agricultural Research (C-FAR).
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