Variability of the Illinois soil nitrogen test across time and sampling depth

There is potential for using the Illinois soil nitrogen rest (ISNT) to improve N fertilizer recommendations for crops in the southeastern United States. The ISNT has been previously calibrated to predict N rates for corn (Zea mays L.) in North Carolina. This study evaluated the effects of sampling time, sampling depth, crop rotation, and fertilizer application on soil ISNT-N during a 2-yr period in the humid Coastal Plain and Piedmont regions of North Carolina. Ten sites were repeatedly sampled at 0- to 10-, 10- to 20-, and 20- to 30-cm depths in fall, mid-winter, and spring between October 2006 and May 2007. Illinois soil nitrogen test N, KCl-extractable soil N[O.sub.3]-N and N[H.sub.4]-N, and soil organic matter (SOM) derived by loss-on-ignition (LOI) were evaluated at each sampling. Temporal changes in these soil parameters were evaluated for various crop rotations and N fertilizer applications. Soil ISNT-N decreased with depth and showed significant variation with time at all three depths at all sites. Soil ISNT-N was influenced by crop rotation and tillage but was not significantly affected by N fertilizer applications. Considering all sites together, ISNT-N was well correlated with LOI; however, ISNT-N was not correlated with LOI across time within sites. This suggests that the ISNT measured a fraction of SOM that behaved somewhat independently with time. Abbreviations: ASN, amino sugar nitrogen; ISNT, Illinois soil nitrogen test; LOI, loss-on-ignition; SOM, soil organic matter. doi: 10.2136/sssaj2009.0253