NONEXCHANGEABLE AMMONIUM NITROGEN CONTRIBUTION TO PLANT AVAILABLE NITROGEN

Recent European research has shown that nonexchangeable .**GRAPHIC**. may be released from soil and utilized by crops. Surface and subsoil samples were collected from agriculturally important soils of Coastal Plain, Piedmont, and Ridge and Valley regions of Virginia [USA] to determine the relative importance of different soil N fractions for supplying N to wheat (Triticum aestivum L. em Thell) grown in the greenhouse. Exchangeable and nonexchangeable .**GRAPHIC**. .**GRAPHIC**. total N, organic matter contents and an index of organic N availability were measured on all samples. Plant available N was measured by N uptake of successive wheat crops grown in the greenhouse. Multiple linear regression models for different groups of samples were used to determine the significance of the different soil N fractions to the plant available N supply, and to predict N uptake by wheat. Best models were selected considering fit, significance of the regression coefficients, and predictive ability. Collinearity effects on the models were reduced by using Principal Components regression. All the soils provided significant amounts of N to the wheat in both the first and second crops. Exchangeable .**GRAPHIC**. and .**GRAPHIC**. were the major initial sources of plant available N. Nonexchangeable .**GRAPHIC**. contents were highest in fine-textured soils, and/or soils with large amounts of previously-added N fertilizers. The soil N fraction was a significant contributor to the plant available N supply for all soil groups, and amounts up to 129 mg kg-l soil were measured. Interaction of nonexchangeable .**GRAPHIC**. with readily-available N was found in all soil groups. Nonexchangeable .**GRAPHIC**. should be considered as a potential plant available N source in future research.