15N methodologies for estimating the transfer of N from legumes to non-legumes in crop sequences

Abstract

A suite of 15N-based methodologies have been applied to quantify the transfer of symbiotically-fixed or legume N to succeeding non-legumes in crop sequences. The structure of these methods, their comparative advantages and efficacy are scrutinized in the present review. Methods are either direct or indirect, the former involving labelling of the legume with 15N2, the substrate for symbiotic N2 fixation, while in the indirect methods the legume is either enriched or depleted in 15N through addition of labelled fertilizer or is otherwise unamended at 15N natural abundance. The methods can be classified further according to their yield dependency, and whether they involve the relocation of residues or are located in situ as green manures or harvest residues. The greatest number of studies has involved the relocation of 15N-labelled plant materials produced artificially at locations remote from the actual crop sequence in the field. Depending on the physico-chemical properties of the residues and the environmental and edaphic conditions pertaining at the experimental site, the proportions of non-legume N in phase 2 of the rotation derived from the phase 1 legume have been quite variable. Nevertheless, there are many recorded instances where green manures and harvest residues represent significant sources of N in legume–non-legume crop sequences. The application of 15N-based methods has resulted in a better understanding of the relative contribution of below- and above-ground legume N in N transfer, and the efficacy of management practices such as incorporation of green manures or use of surface mulches.