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NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Characterization of the Organic Nitrogen Fraction Determined by the Illinois Soil Nitrogen Test

^a Soil and Water Science Dep., 2169 McCarty Hall, Univ. of Florida, P.O. Box 110290, Gainesville, FL 32611-0290
^b Univ. of Illinois, 1102 S. Goodwin Ave., Urbana, IL 61801

^* Corresponding author (kwon.hoyoung{at}gmail.com).

The Illinois soil nitrogen test (ISNT) has shown promise as a predictor of corn (Zea mays L.) response to N fertilization in several field trials, but controversy has arisen regarding whether the test detects a particular class of organic N compounds or a constant fraction of the organic N in agricultural soils (SON). The goal of this study is to elucidate the chemical properties of the ISNT-labile N and its relationship to microbial growth in agricultural soils. In the ISNT, NH₃ generated during alkaline hydrolysis of whole soil samples is separated from the hydrolysate by diffusion and quantified by trapping and acidimetric titration. Analyses of pure organic N compounds confirmed that the ISNT detects the N in some components of microbial biomass, such as the monomeric amino sugars (AS) of bacterial cell walls (95% recovery) and amides (AM, 55% of monomers), but not amino acids (AA) or chitin (CTN), a refractory AS polymer. Because the ISNT detects different proportions of these N containing compounds than any other conventional acid hydrolyzable SON fraction, ISNT-N can be considered a fifth fraction that together with the others permits one to distinguish AS of bacterial and fungal origin. Using this approach, the average proportions of SON components observed in 10 manured (and 16 non-manured) agricultural soils from Illinois are 18% (10%) CTN, 11% (5%) bacterial amino sugars (BAS), 10% (17%) AM, 29% (25%) AA, and 15% (20%) unknown. The reason that the sum of CTN and BAS is 50% greater than the conventional AS N fraction (AS-N) of these soils—18% (9.5%) of SON—is the lack of recovery correction conventionally made to AS-N data. To investigate N fractions in living biomass, we conducted studies of ¹⁵NH₄⁺ immobilization in two fresh soils. ISNT-N was more rapidly labeled than AS-N, a fact that is indicative of more rapid labeling of bacteria than fungi. Taken together, these findings suggest that the effectiveness of the ISNT for predicting corn N response is due largely to its ability to quantify N in BAS. Because ISNT also detects some AM-N, the BAS-N signal may be obscured in soil N datasets where these components are highly variable.

Abbreviations: AA, amino acid • AM, amide • AS, amino sugar • BAS, bacterial amino sugar • CTN, chitin • HA-N, hydrolyzable NH₄–N • HUN, hydrolyzable unidentifiable N • ISNT, Illinois soil N test • SOM, soil organic matter • SON, soil organic N • THN, total hydrolyzable N • TIN, total inorganic N • TN, total N • TOC, total organic C • TON, total organic N