Increasing the Sensitivity of the 15N Root Bioassay Technique: Suggested Procedures

Abstract

The diagnosis of tree nutrient demand for nitrogen (N), phosphorus (P), and potassium (K) is possible using a root bioassay with excised roots, to measure the rate of uptake of the corresponding isotope. In this study, Norway spruce (Picea abies) roots were used to test the uptake response in roots of different diameter. The 15N bioassay has been developed to test for nitrogen demand by plants, where high 15N uptake demonstrates nitrogen limitation. Sometimes sample variability may mask potential treatment differences, but we have shown that when the fine root fraction of <1 mm diameter is selected rather than the whole root, the sensitivity can be significantly increased. Roots of different diameters revealed a 2–50 fold difference in the response to the N bioassay. If samples are restricted to the fine roots, the method is also more comparable to the bioassays for P and K.