Review
An overview of contemporary advances in the usage of 15N natural abundance (δ15N) as a tracer of agro-ecosystem N cycle processes that impact the environment

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Highlights

  • The role of 15N natural abundance as a tracer in N cycle processes is reviewed.

  • Advances in knowledge over the past two decades is the focus of the review.

  • Individual processes that add or subtract N in agroecosystems are considered.

  • δ15N-derived qualitative or quantitative estimates of N processes are identified.

  • Estimation of symbiotic dependence is the principal quantitative application.

Abstract

During the past 20 years there have been major advances in the application of 15N natural abundance (NA) measurements to trace the pathways and magnitudes of N fluxes in the soil-plant-atmosphere continuum. However, estimates are often not quantitative due to the unknown extent of isotopic fractionation during a particular N transformation under study, when other processes compete simultaneously for substrate. Examples are the estimation of N fertilizer use efficiency or the transfer of biologically-fixed N2 to non-fixing companion species in intercrops or crop sequences. In some cases it has been possible to identify a particular process or source leading to a change in the relative isotopic composition (δ15N signature) of a system component, by innovative selection of experimental conditions that isolate the source or process from confounding factors. Nevertheless, there are examples where significant contemporary advances have occurred in the application of NA as a quantitative tracer, such as in the estimation of the symbiotic dependence of a range of N2 fixing plants. The key is the estimation of isotopic fractionation during N2 fixation and assimilation, and new knowledge has been obtained on factors contributing to variation, and new approaches devised to obtain more accurate estimates of fractionation. A second example is the innovative application of isotopomer measurements of the potent greenhouse gas N2O that enable presumptive identification of the biological and chemical processes resulting in its production under various agricultural scenarios.

Introduction

It is 22 years since Högberg (1997) published a comprehensive review on 15N natural abundance in soil-plant systems, where he examined, isotopic fractionation in N cycle processes, variations in δ15N in soils and plants and applications of δ15N as a tracer in environmental studies. However, significant advances have occurred during the past two decades in the applications of δ15N as a tracer in ecosystem research, particularly in agro-ecosystems, which was not the primary focus of Högberg (1997). Subsequent reviews covered the opportunities for applying δ15N as a tracer in ecological studies (Robinson, 2001), and the potential uses and precautions of using δ15N as a tracer through agricultural landscapes (Bedard-Haughn et al., 2003; Choi et al., 2017). A review of the use of stable isotopes in tracing anthropogenic inputs of nitrogen to ecosystems was presented by Kendall et al. (2007). Recently, Denk et al. (2017) reviewed isotope effects and isotope modeling approaches in nitrogen cycle studies.

The objective of the present review is to explore the usage of δ15N as a tracer from a somewhat different perspective, by focusing on processes that add or remove N from agro-ecosystems, those that specifically have environmental consequences. Removal processes include gaseous N emissions, nitrate transport and crop harvest, while additive processes include N fertilizers, biological N2 fixation and atmospheric accretion through wet and dry deposition. We will demonstrate situations where the use of δ15N as a qualitative tracer of N cycle processes is appropriate, but also where its application as a quantitative tracer is either justified or inappropriate. Applications beyond the agro-ecosystem level per se, such as in food science will not be considered, as these were reviewed by Inácio et al. (2015a) and Inácio and Chalk (2017).

Section snippets

Relative 15N abundance

The reporting of stable isotope data is governed by guidelines set by the Commission on Isotopic Abundances and Atomic Weights of the International Union of Pure and Applied Chemistry (IUPAC). The latest guidelines and recommended terms were set out by Coplen (2011). When samples are close to natural abundance, a relative (δ) value of 15N abundance is used. The expression (Eq. 1) is the isotope ratio of a sample relative to the isotope ratio of the international standard. Since nitrogen has

Processes contributing to N removal from agro-ecosystems

Significant advances have occurred in recent years in using 15N natural abundance to trace the sources of gaseous N emissions in agro-ecosystems that have environmental consequences, including ammonia, nitrous oxide and odd oxides of nitrogen. In addition, the source and movement of nitrate in ground and surface waters have been traced using δ15N measurements on agro-ecosystem components.

N fertilizer additions

Addition of N fertilizers to soils has considerable environmental impact through enhancing the emission of gaseous forms of N to the atmosphere, encouraging soil acidification through long-term additions of NH4+-based fertilizer to poorly buffered soils and through nitrate transport to surface- and groundwater resulting in eutrophication and a potential health hazard in drinking water.

The agro-ecosystem–natural ecosystem interface

The interaction between natural and adjacent agro-ecosystems was examined in several studies. For example, Lindau et al. (1997) studied the impact of runoff from sugarcane cropland to an adjacent forested wetland. Based on the measured concentrations and δ15N values of NH4+ and NO3 in samples of water collected at six sites over a period of 16 months the authors concluded that fertilizer N draining into the wetland was only a small fraction of that applied to sugarcane. However, as previously

Conclusions

There have been rapid advances in the use of 15N natural abundance as a tracer in agro-ecosystems to follow losses and gains of N in soil-plant systems, since the first comprehensive review was published two decades ago. Of particular significance has been the development of innovative isotopomer methodologies to differentiate microbial pathways of N2O formation in soils. The efficacy of this approach is undoubtedly hampered by the paucity of isotopic signatures from representative pure

Acknowledgement

We thank Dr. Fabiano Balieiro for drawing Fig. 1. We also thank the University of Melbourne for supporting this work by providing funds for our participation in the 7th International Nitrogen Initiative 2016 conference in Melbourne, Australia, and the 21st World Congress of Soil Science 2018 in Rio de Janeiro, Brazil, which provided us with opportunities to review progress and to plan the completion of the review.

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