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Applied and Environmental Soil Science
Volume 2012, Article ID 506302, 14 pages
Research Article

Temporal Variation of N Isotopic Composition of Decomposing Legume Roots and Its Implications to N Cycling Estimates in Tracer Studies in Agroforestry Systems

1Department of Forest Sciences, University of Helsinki, P.O. Box 27, 00014 Helsinki, Finland
2Bioversity International, P.O. Box 236, UPM Post office, Serdang, 43400 Selangor Darul Ehsan, Malaysia
3UR1321 ASTRO Agrosystèmes Tropicaux, INRA, 97170 Petit-Bourg, France

Received 8 May 2012; Revised 13 July 2012; Accepted 29 July 2012

Academic Editor: William R. Horwath

Copyright © 2012 Riina Jalonen and Jorge Sierra. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Below-ground residue of agroforestry trees is an important N source for associated crops. Several studies have shown that its isotopic signature (δ15N) may change after tree pruning, which makes it difficult to study below-ground N inputs from pruned trees by isotopic techniques. We studied how temporal variation of legume root residue δ15N could be explained by considering differential decomposition kinetics and 15N content of residue fractions. A mathematical model on the isotopic patterns of soil and a N recipient plant during root decomposition was developed and applied for testing assumptions about residue characteristics against two experimental datasets. Observed 15N patterns of the recipient plants could be satisfactorily simulated only when the residue was assumed to consist of at least two fractions with distinct δ15N and decomposition rates depending on their C : N ratio. Assuming δ15N of residue constant over time resulted in substantial underestimates of N derived from low-quality residue (%Ndfr) by the recipient plant when compared with experimental data. Results of this study suggest that residue fractionation can help improve estimation of %Ndfr in isotopic studies, as an alternative or complementary method to assuming or aiming at homogenous isotopic composition of N sources.