Pollutant management in agro-ecosystems is a complex challenge that requires two key questions to be answered: (i) how to improve identification and quantification of pollutant sources, transport pathways and fate on the catchment scale? and (ii) how to prioritise and combine soil and water management practices to minimise the use and impact of agro-pollutants? An integrative book such as this one, focusing on the stable H, C, N, O and S isotope compositions of a variety of agro-pollutants, may help to address the first question and enable increasingly precise and robust evaluation of pollution in agro-ecosystems. After understanding pollution sources and dispersal, the farm operators and land managers can attempt to answer the second question by considering local conditions and agriculture practices.

This book has highlighted that stable isotope tracers have great potential to improve the qualitative and quantitative assessment of soil and water pollution, which is crucial for ecosystem protection plans worldwide. However, stable isotope analyses should be considered as complementary to conventional concentration analysis and mass load calculations. Stable isotope and ion ratios are usually independent of solute concentrations and provide primarily information about pollution sources and transformation processes but not about pollution levels. Pollution levels need to be determined using hydrochemical analyses and load calculations, respectively, to volume of water and pollutant concentrations.

Various stable isotope tracers are often used individually, but the best results are achieved by adopting a multi-tracer approach to obtain more reliable characterisation of pollution sources and to tease apart the water and solute budgets. Thus, combination of the stable isotope and hydrochemical analyses of specific solutes with optimal sampling design, sediment source apportionment and stable isotope composition of water to estimate retention time and evaporation can reinforce the evaluation of sources and transport of pollutants in agro-ecosystems and help to reduce uncertainties of study outcomes. Here, we also provide examples of case studies to show how the stable isotope approaches helped to estimate sources and transport of agro-pollutants and inform decision-making on the catchment scale to control diffuse agricultural pollution and define adapted policies. The ability to enhance comprehension, identification, and quantification of sources and processes related to agro-pollutants is crucial in a global context. Indeed, chemical pollution and nutrient (nitrogen, phosphorus) flows into the biosphere and oceans have exceeded safe limits, breaching the boundaries of global sustainability, which poses a direct threat to humanity (Steffen et al. 2015).

This book also aims to present a scientific toolbox that gathers optimal designs of sampling programmes, considering possible mixing scenarios and multiple isotope tracers of the origin, transport and transformation for a variety of agro-pollutants (Table 8.1). The book’s focus on sampling protocols, current analytical techniques, standard operating procedures, and field examples can contribute to the integration of multi-tracer approaches into routine studies and research practices. It has been demonstrated that establishing a balance between grab sampling and time-integrated sampling is essential in each study to mitigate biases when interpreting agro-pollutant sources and behavior. In the interpretation phase, providing an estimation of uncertainties related to source apportionments and process extent is advisable whenever possible. This includes error propagation to ensure a rigorous interpretation.

Table 8.1 Current development and future prospects to evaluate agro-pollutants in the field using stable isotope techniques
Full size table

Most importantly, the book also addresses requirements and limitations associated with the use of each type of isotope tracer (Table 8.1). A good knowledge of stable isotope signatures of the pollution sources is an essential requirement for efficient application of all stable isotope tracers in mixing or fractionation models. In future, extensive databases for source identification and apportionment of sediment or single agro-pollutant compounds may be available, following Findable, Accessible, Interoperable, Reusable (FAIR) principles. However, agro-pollutants, whose source identification seems possible and is targeted (e.g., PO4, NO3), should be distinguished from agro-pollutants, whose degradation is the main process considered (e.g., pesticides). In many cases, sampling and sample handling are sensitive operations that may cause undesired isotope fractionation, as for the δ(18O)PO4 method to investigate P cycling in the soil–plant continuum. In addition, the stable isotope composition of the agro-pollutant of interest needs to be considered in relation to the other chemical compounds present in the environment, which may contribute to stable isotope exchange, equilibration or chemical synthesis, thus altering the original signatures of the pollution source. In situations where degradation or transformation is anticipated, understanding the associated isotope fractionation is crucial for interpreting field data. If this information is not yet available in the literature, isotope fractionation may be deduced from reference experiments (e.g., biodegradation in native sediment, photolysis in river water). Moreover, separation fractionation from mixing is the primary challenge and may sometimes lead to ambiguous results. Finally, the interpretation of agro-pollutant transformation is constrained by the absence of reference field studies, encompassing vegetation and ecosystem dynamics. Therefore, laboratory microcosms and glasshouse studies will be instrumental in advancing the application of stable isotope tracers in agro-ecosystems. Additionally, they will contribute to the development of novel, mechanistic mixing concepts and models to enhance studies in complex and dynamic agro-ecosystems.

Despite the acknowledged limitations, stable isotope techniques provide additional dimensions to the interpretation of agro-pollutants, enabling advanced studies that are not achievable through traditional hydrochemical methods alone. This book contributes to the systematization, standardization, and optimization of stable isotope methods in routine studies of agro-pollutants on a catchment scale. We anticipate that, in the future, stable isotope methods will steadily become the routine and preferred approach in many research areas, including the tracing of agro-pollutants.