Comparison of Direct and Indirect Methods of Measuring Fertilizer N Uptake in Winter Wheat

Two methods of estimating fertilizer N uptake (FNU) were compared using 11 site-years of data from four winter wheat experiments comparing different N sources, application methods and application times. Direct ( 15 N) and indirect (difference) estimates of FNU were significantly correlated in 10 of the 11 site-year combinations. When plant uptake of applied N increased, both methods provided similar estimates of FNU and allowed treatment effects to be separated with comparable resolution. Neither method nor time of application had any apparent effect on the ability of the two methods to estimate FNU.