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A rapid method to estimate soil N mineralization to improve N fertilizer recommendations has long been sought. Over the years, numerous methods to predict N mineralization have been proposed, but no one method has been widely accepted. Recently, researchers observed the concentration of hydrolyzable amino sugar‐N in the soil correlated with crop N response. The objective of this study was to determine if developmental methods that quantify hydrolyzable amino sugar‐N accurately predict N mineralization when compared to net‐N mineralization by anaerobic incubation. Methods evaluated to predict hydrolyzable amino sugar‐N were the Illinois soil nitrogen test (ISNT) by diffusion, and 2, 5, and 10 M NaOH direct steam distillation procedures. It was observed that the hydrolyzable amino sugar‐N was a somewhat accurate predictor of N mineralization (R2 = 0.38). However, when the hydrolyzable amino sugar‐N concentrations were combined with the hydrolyzable NH4–N concentrations, the ability to predict N mineralization improved (R2 = 0.61). Suggesting more labile soil organic N forms along with amino sugar‐N are potentially mineralizable. Rapid analytical procedures like the ISNT diffusion method and the 2, 5, and 10 M NaOH direct steam distillation techniques accurately predicted hydrolyzable amino sugar‐N as well as hydrolyzable (NH4 + amino sugar)‐N. These methods also accurately predicted NH4–N mineralized after anaerobic incubation. It could be assumed that this predictability may increase when soils are analyzed based on soil management, geographic area, and crop rotation. Glucosamine recovery was significant ( >85%, P = 0.05) for both methods showing their ability to quantify amino sugar‐N in the soil as well as estimate the amount of potentially mineralizable‐N. Lastly, in soil‐testing facilities where the ISNT is already implemented as a procedure, the much quicker and equally reliable 10 M NaOH distillation technique may be used to achieve near identical test values.

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.

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Success of alkaline bydrolyzable nitrogen (AHN) to predict N fertilizer needs when soils were sampled over the effective rooting depth of the crop rekindled an interest in more traditional soil procedures including soil total nitrogen (STN) and total carbon (STC). The benefits of a N soil test that can accurately predict N fertilizer needs are not solely about optimizing economic or agronomic returns, but include making environmentally sound N fertilization decisions. This paper presents the results of a study assessing the ability of STN and STC to predict the N fertilizer needs of direct-seeded, delayed-flood rice (Onza saliva L.) produced on silt loam soils in Arkansas. Twenty-five N response trials were conducted from 2004 to 2008 to correlate STN and STC with rice response parameters such as total nitrogen uptake (TNU), check plot grain yield, and percent relative grain yield (RCY) and calibrate STN and STC to predict the fertilizer N rate required to achieve 95% RGY. Relationships with the selected parameters were evaluated for both methods over a series of soil depth increments using linear regression models. Soil TN was significantly and positively correlated with all rice response parameters except check plot grain yield at the 15-to 30-and 45-to 60-cm depths. Coefficients of determination were greatest for percent RGY at the 0-to 45-cm depth for STN (/-= 0.62) and STC (/-= 0.33). Calibration of the fertilizer N rate to achieve 95% RGY mimicked correlation of rice response parameters with soil depth, but with higher /-values. The highest coefficients of determination were observed in the 0-to 45-cm depth where STN explained 74% and STC explained 54% of the variability in the fertilizer N rate required to achieve 95% RGY. These relationships indicate the importance of proper sampling depth for successful correlation and calibration of soil analytical methods.Abbreviations: AHN, alkaline hydrolyzable nitrogen; DSD, direct steam distillation; ISNT, Illinois soil nitrogen test; RGY, relative grain yield; STC, soil total carbon; STN, soil total nitrogen; TNU, total nitrogen uptake. N itrogen fertilizer represents the primary input cost for most crop production systems and most crop plants are often deficient in N especially in the absence of N fertilizers. To prevent N deficiencies and improve crop yields soil fertility .specialists have attempted to predict N rates required to optimize crop yields and maximize economic returns using sound research based on crop response to N fertilizer. To date the majority of N recommendations are based on yield goal approaches (Lory and Scharf, 2003; Dahnke, 1973) that do not account for the N supplying power of the soil which can have extreme variability from site to site within a relatively small geographic region (Stevenson and Cole, 1999). Research has proposed many direct and indirect methods to estimate potentially mineralizable soil-N; however, residual or inorganic-N testing is the only index of N availability that is used on a wide scale (Dahnke and Johnson, 1990).