Assessment of the Amino Sugar – Nitrogen Test on Iowa Soils : I . Evaluation of Soil Sampling and Corn Management Practices

A soil N test capable of measuring the soil organic N fraction that contributes to plant available N would be useful to corn (Zea mays L.) producers as they make N fertilizer rate decisions. The objectives of this study were to evaluate the effects of soil sampling time, sampling depth, long-term crop rotation, and long-term N fertilizer application on the amino sugar–N test (ASNT). Soil samples from 43 N rate trials were analyzed using the ASNT procedure. TheASNTwas also determined on soil samples from two sites having several crop rotations and N fertilizer rates in place for the past 18 and 24 yr. The ASNTresults were consistent over time but show variation from week to week and were more variable at sites with high soil organic C or manure application history. Amino sugar–N test values from the 0to 15-cm soil depth were usually greater than the 15to 30-cm or 0to 30-cm depths. Crop rotations that included alfalfa (Medicago sativa L.) resulted in greater ASNT values than continuous corn and corn rotatedwith soybean [Glycinemax (L.)Merr. ]. Therefore, sampling depth and rotation should be taken into consideration when calibrating the ASNT. Long-term N fertilization rate did not consistently influenceASNTvalues. Differences between soils had larger effects on theASNT than cornmanagementpractices. SamplingbeforeN application in the fall, early spring before planting, or late spring during the growing season should provide similar ASNT results and flexibility in sample collection before N application. SOIL N testing can be an important component of managing N fertilizer for corn in the U.S. Corn Belt. A significant amount of N available to corn during the growing season originates from readily mineralized soil organicN.Researchers have proposed numerous chemical and biological methods for measuring various organic N fractions in soil (Stevenson, 1957a, 1957b; Cornfield, 1960; Keeney, 1965; Keeney and Bremner, 1966a, 1966b; Stanford, 1968; Smith and Stanford, 1970; Fox and Piekielek, 1978a, 1978b; Stanford, 1978). The methods with promising results for estimating mineralizable N have been too complex or time consuming to be adopted bymost soil testing laboratories. For example, the standard procedure for fractionating components of organic soil N requires acid hydrolysis of soil under reflux for 12 to 24 h. Steam distillation is then used for quantitative determination of the N fractions in the soil hydrolysate. Some limitations of the procedure are that amino sugar–N can be partially destroyed by acid hydrolysis (and subsequently underestimated), and the amount of amino sugar–N must be calculated by subtracting the amount of NH3–N recovered in separate distillation procedures (Stevenson, 1996). Researchers in Illinois have been working to correct deficiencies in the determination of amino sugar–N in soil using an improved technique for fractionating N in soil hydrolysates (Mulvaney and Khan, 2001). Mulvaney and Khan (2001) used a diffusion method that is more accurate and specific than steam distillation. Quantitative analysis of a group of diverse Illinois soils for amino sugar–N was 74 to 317% greater with diffusion than with steam distillation. Further work byMulvaney et al. (2001) using the improved fractionation technique revealed that amino sugar–N in soil hydrolysates was highly correlated with check plot yield and N fertilizer response in corn. The amount of hydrolyzable amino sugar–N correctly classified 18 soils as responsive or nonresponsive to N fertilizer. A simpler N soil test was later developed for use as a routine soil test based on direct soil diffusion by heating with NaOH (Khan et al., 2001). The concept of this alkali diffusion procedure is to quickly and easily estimate the amino sugar–N fraction rather than using the difficult and lengthy fractionation process. The procedure was highly correlated with hydrolyzable amino sugar–N. The results of the method also include exchangeable NH4–N. Khan et al. (2001) concluded that the simple N soil test accurately classified 25 Illinois soils as responsive or nonresponsive to N fertilizer application. Hoeft and Nafziger (2002) refer to this simplified N soil test as the Illinois N soil test (amino sugar–N test), but it is referred to in this paper as the amino sugar–N test (ASNT). They also indicated that additional research was needed to establish soil sampling protocols, to assess accuracy of the test over a wide range of soils and growing conditions, and to determine if crop and N management practices affect ASNT values. Few studies have been conducted to ascertain the change in ASNT values over time during the growing season. A temporal study in Minnesota found a small decline in test values during the summer months of the growing season and greater ASNT variability at the 15to 30-cm depth compared with the 0to 15-cm depth (Randall and Vetsch, 2002). Also, the 0to 15-cm depth had consistently greater ASNT values than the 15to 30-cm depth. Hoeft et al. (2005) analyzed soil samples from four U.S. Midwest states and reported a similar trend of ASNT values, with test values decreasing in early summer. However, the change in ASNT values was not consistent across locations. Ellsworth et al. (2005) found the highest ASNT values in early spring and late summer at four Illinois sites. Studies conducted Department of Agronomy, Iowa State Univ., Ames, IA 50011-1010. Received 4 Feb. 2006. *Corresponding author (jsawyer@iastate.edu). Published in Agron. J. 98:1345–1351 (2006).