INFLUENCE OF ADDED NITROGEN INTERACTIONS IN ESTIMATING RECOVERY EFFICIENCY OF LABELED NITROGEN

The addition of N fertilizer to soil has been shown to stimulate the uptake of native soil N via a priming effect recently termed added nitrogen interaction (ANI). This ANI, due to pool substitution, can substantially affect the N-recovery efficiency (NRE) by plants as calculated by the N-15 isotopic dilution technique. We evaluated NRE in a pot study using 'Edwall' spring wheat (Triticum aestivum L.) with three soils (Palonse, a fine-silty, mixed mesic Pachic Ultic Haploxeroll; Ritzville, a coarse-silty, mixed, mesic Calciorthidic Haploxeroll; and Shano, a coarse-silty, mixed mesic Xerollic Camborthid) of varying organic C (OC) levels and five levels of N-15-labeled fertilizer. Data obtained at 60 d after emergence (DAE) showed that NRE generally increased with fertilizer additions, was lowest in the soil having the greatest OC, and ranged from 57 to 79%. The NRE values estimated by the isotopic method averaged 20% lower than those estimated by the difference method, although the two estimates were strongly related. Although the magnitude of ANI showed no direct relationship to OC levels, it was strongly related to N rates, OC, soil C/N ratio, and N lost, which together explained 73% of the relationship. The magnitude of occurrence of ANI appears to be influenced by several factors and thus may be soil specific. Regression of NRE with soil properties showed that NRE was influenced primarily by the same factors that influenced ANI. This study suggests that, for accurate comparisons of nitrogen recovery efficiency between soils or treatments using the isotopic method or for comparing methods of determining N-recovery efficiency, the role of added N interaction must be considered.