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NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Changes in Alkaline Hydrolyzable Nitrogen Distribution with Soil Depth: Fertilizer Correlation and Calibration Implications

Crop, Soil, and Environmental Sciences Dep., 115 Plant Science, Univ. of Arkansas, Fayetteville, AR 72701

^* Corresponding author (tlrobert{at}uark.edu).

Alkaline hydrolyzable N (AHN) quantified by the Illinois Soil N Test (ISNT) or direct steam distillation (DSD) may index potentially mineralizable N in the soil. Limited success has been achieved utilizing the ISNT to predict corn (Zea mays L.) yield response when soils are sampled to a depth of 30 cm, and a strong correlation of ISNT to total N (TN) has raised concerns about the sensitivity of the method in determining potentially mineralizable N. A study was implemented to determine the effects of site and soil depth on AHN. Soil samples were collected from 0 to 60 cm in 15-cm depth increments and analyzed for AHN (by ISNT and DSD) and TN. Analyses of variance for AHN and TN each showed a significant site x soil depth interaction. Alkaline hydrolyzable N ranged from 22 to 280 mg N kg^–1 soil and the highest values were not always in the top 15 cm of the soil. Total N ranged from 191 to 1542 mg N kg^–1 soil and the highest values were always in the top 15 cm. Soil TN from the 15- to 60-cm depths was generally uniform among 15-cm increments within each soil, but AHN was not. Alkaline hydrolyzable N accounted for 11 to 38% of soil TN and was variable across sites and depths. Variation in AHN and the fraction of TN quantified as AHN with site and soil depth indicates the importance of proper sampling depth (i.e., to the rooting depth of a given crop) for correlation and calibration of crop response using either the ISNT or DSD. Significant changes in AHN with depth could influence crop-available N and thus the reliability of a soil test to predict crop response to N fertilization.

Abbreviations: AHN, alkaline hydrolyzable nitrogen • DSD, direct steam distillation • ISNT, Illinois Soil Nitrogen Test • TN, total nitrogen