ReviewAn 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
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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