Assessing Winter Wheat Response to N Using the Amino Sugar-Nitrogen Test.

The Amino Sugar Nitrogen Test (ASNT) which determines a potentially mineralizable fraction of soil organic matter shows promise for making in season N predictions for crops in the Southeastern USA. The objective of this research was to investigate the relationship between soil ASN verses optimum crop N rate, crop N uptake, crop grain yield over a range of soil types, sampling depths and management conditions.

Field experiments were conducted at a total of 60 site locations across North Carolina during three winter wheat growing seasons from 2006-2008. A randomized complete block design with 5 replications was used with N rate as the main plot treatment. There were 7 fertilizer N treatments applied at in Spring at GS 30 ranging from to 0 to 168 kg ha^-1. Soil samples were collected in mid-Winter at GS 25 to three depth increments (0-10 cm, 0-20 cm and 0-30 cm). These samples were analyzed for amino sugar N (ASN), nitrate, ammonium and soil organic matter content. ASN analysis was conducted using a modified procedure described by Williams et al. (2007). Site information such as soil series, map unit, drainage class and previous crop rotation was gathered at each site. At harvest grain and straw yield data were collected and grain and straw N analysis were conducted in order to ascertain crop N uptake. This data was then analyzed using SAS software. Native soil N uptake was determined from the unfertilized check plots and was modeled using the variables ASN, residual soil nitrate, and ammonium.

ASN and native crop N uptake were highly correlated. ASN and crop yield without N fertilizer were also highly correlated. Residual nitrate and ammonium needs to be included into these models so they can account for sites with significant residual mineral N stemming from previous fertilizer and management. These relationships were dependent on soil drainage classification. The ASNT can be used to measure the potential supply of N through mineralization at different sites for winter wheat production. Further exploration of this relationship between ASNT and crop N uptake was conducted to investigate if the ASNT could be used to predict economic optimum N rates for winter wheat in the Southeastern USA.