Tracer methods to assess nutrient uptake distribution in multistrata agroforestry systems J. Lehmann 1, 2, * and T. Muraoka 3 1 Institute of Soil Science and Soil Geography, University of Bayreuth, 95440 Bayreuth, Germany; 2 Federal Research Institute for Forestry and Forest Products, Hamburg, Germany; 3 Centro de Energia Nuclear na Agricultura CENA, University of Sao Paulo, 13400-970 Piracicaba, Brazil (*Author for correspondence: Current address: Department of Crop and Soil Sciences, Cornell University, Ithaca NY 14853, USA, E-mail: CL273@cornell.edu) Key words: biological N 2 fixation, nitrogen, phosphorus, radioisotopes, root activity, stable isotopes Abstract Separate assessment of nutrient uptake by individual plants in mixed cropping with trees is impossible without tracer techniques. The different 15 N-to- 14 N isotope ratio of atmospheric and soil N can be used to study the contribution of biologically fixed N to the nutrition of associated trees. In most cases, the assessment of nutrient uptake distribution is an appropriate way of evaluating how to improve the transfer of biologically fixed N. Radioisotopes (e.g., 32 P), stable isotopes (e.g., 15 N) and rare elements (e.g., Sr) can be used to determine relative root activity distribution by applying the tracer to different soil depths or distances from trees. A broadcast application of the tracer instead of point application makes it possibe to calculate uptake values per unit area. The direct determination of nutrient pathways with such robust experiments offers considerable advantages for improving nutrient use efficiency and complementarity in multistrata agroforestry systems. Agroforestry Systems 53: 133–140, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. Introduction Complementarity of nutrient uptake is an impor- tant goal for intercropping different trees and crops, and in order to increase nutrient use effi- ciency and production. On the other hand, nutrient competition may decrease crop production (Schroth et al., 2001). However, improved N nutrition and growth of a non-legume species may be achieved if it has access to biologically fixed N from an associated legume tree. Advantages or disadvantages of intercropping for plant nutrition are difficult to assess in agroforestry systems due to their spatial and/or temporal discontinuity. Multistrata agroforestry systems with perennial plants have an especially high complexity, because they may comprise several different tree species in complex spatial patterns with contrasting above and below-ground biomass distribution. Tracer techniques can help identify and also quantify spatial and temporal patterns of nutrient uptake, but they have rarely been used (Thomas et al., 1998; Kumar et al., 1999) in multistrata agro- forestry systems. The use of tracers in agroforestry research has been principally limited to alley cropping systems (Haggar et al., 1993; Rowe et al., 1999). In this paper, we highlight and discuss the applicability of tracer techniques to study nutrient uptake pathways in multistrata agro- forestry systems.