Temporal Variation of N Isotopic Composition of Decomposing Legume Roots and Its Implications to N Cycling Estimates in Tracer Studies in Agroforestry Systems
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. 1. Introduction Pruning of trees is a common practice in agroforestry systems. In legume-based systems the main purpose of pruning is to provide nitrogen to soil and the associated crops from green manure and below-ground residue. Timing and intensity of pruning can be varied to adjust the N inputs with the requirements of the crops, and amount and optimal timing of these inputs have been the interest of numerous studies [1, 2]. Although most studies have concentrated on N release from above-ground biomass, management of N release from roots may be much more important for crop nutrition, since up to 50–60% of total plant N of frequently pruned agroforestry trees may occur in roots [3]. Isotopic techniques are commonly applied for studying the fate of N in agroforestry systems and the mechanisms involved in its cycling. They can be especially useful for studying below-ground N cycling processes that are difficult to trace otherwise. The techniques require measurement or a reasonable estimate of the N isotopic compositions of the N sources. However, recent research suggests that estimating N cycling in intercropping systems with isotopic techniques becomes difficult after management interventions which affect root turnover. After shoot harvest or pruning of the N
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