Sustainable Mineral Resource Management—Insights into the Case of Phosphorus

Gerald Steiner 1 ID and Bernhard Geissler 1,2,* ID 1 Department of Knowledge and Communication Management, Danube University Krems, Dr. Karl-Dorrek-Straße 30, 3500 Krems, Austria; gerald.steiner@donau-uni.ac.at 2 Faculty of Geosciences, University of Resources TU Bergakademie Freiberg, Geoengineering and Mining, Akademiestraße 6, 09599 Freiberg, Germany * Correspondence: bernhard.geissler@donau-uni.ac.at


Introduction
Although it is among the most abundant elements in Earth's crust (11th [1]) and water (13th [2]), phosphorus (P) is commonly referred to as "life's bottleneck", as "life can multiply until all the phosphorus is gone, and then there is an inexorable halt which nothing can prevent . . . " [3]. Historically, the first scientific knowledge about P and its impact on agriculture was acquired during the 19th century. Nevertheless, ancient farmers who lacked a deeper scientific understanding utilized macronutrients in manure and bones as fertilizers [4]. Ever since, phosphorus and its impact on agriculture, and therefore on food production, have gained interest and fostered demand. About half of today's crop yield can be directly attributed to the application of modern mineral fertilizers (consisting mostly of P, nitrogen, and potash) [5,6]. This, in addition to its characteristics of being unsubstitutable, having a dissipative structure, and being finite in the form of concentrated deposits, makes phosphorus essential and unique.
Nowadays, P is almost exclusively produced by the mining of phosphate rock (PR) deposits of either sedimentary (87%) or igneous (13%) origin [7]. Currently, a total of 200 to 265 million metric tons [8] (depending on the data source), of marketable PR concentrate is mined and further processed to produce phosphorus fertilizer (83%) or industrial P (17%) [7]. Figure 1 provides a snapshot of the global situation, including production, consumption, imports, and exports, for the end use of phosphate nutrients in fertilizers in 1985 and 2015. It also shows China's dominant position (as part of eastern Asia) in terms of production and consumption, as well as increasing import dependency for regions such as Europe.
All the above make phosphorus and its primary source, phosphate rock, a unique mineral raw material embedded in a complex global supply chain. This complexity is well documented in the literature (e.g., [9,10]), but only a few authors aim toward clearing the smoke of complexity. Still, interest in the field is growing, as numerous special issues with varying foci have shown over the last few years (selected issues):  Our aim is to contribute to a more holistic understanding of the underlying system dynamics along the supply chain for the benefit of scientific peers, industries, the public, and policy makers. The latter, especially, are the key to managing this limited resource in a more sustainable manner.

Essentiality, Criticality, Scarcity, and the Peak Theory
The question regarding available or potential substitutions represents the main distinction between criticality and essentiality. Scholz and Wellmer [11] use the example of energy; whereas energy is essential for society, the lack of primary resources such as uranium can be considered critical, but the basic function of producing energy, heat, motion, light, and power can be fulfilled by every energy source. Phosphorus, by contrast, represents one of the three macronutritional elements besides nitrogen (N) and potassium (K), and because it is not substitutable at all, P must be considered essential.
One major driver of the recent "popularity" of phosphorus is the ongoing discussion on scarcity. The literature reveals several types of scarcity [12][13][14], whereby this distinction is often neglected in discussions of P. The first is absolute scarcity, which equals the physical (un)availability (i.e., physical scarcity) of, in our case, a mineral. Second, relative scarcity deals with (un)availability under economic constraints (i.e., economic scarcity). Typically, economic scarcity occurs long before physical scarcity, as the extraction (in our case) of a mineral might still be possible from a technical stance, but the involved costs would exceed the willingness to pay for the mineral. Third and finally, structural scarcity represents a form of scarcity where the relationship between joint products (i.e., companion products or co-products) or by-products depends on (a) further (primary) product(s).
Most discussions about P or PR scarcity and its future availability have evolved around Hubbert's peak theory [15]. Originally used to model the oil market, the concept was used for phosphate rock, and soon the term "peak phosphorus" was coined, raising great awareness. Much has been written about its highly controversial application to the case of P in recent years, for example [16][17][18][19][20], mostly around lack of availability of URR (ultimate recoverable resources) data and the comparability of oil and PR markets. However, this aspect is not within the scope of this special issue, and we point interested readers to the references mentioned above.

Economy, Circular Principles, and the Role of Innovation
Merriam-Webster defines economy as (1) "the structure or conditions of economic life in a country, area, or period; also: an economic system", respectively; (2a) as the "thrifty and efficient use of material resources: frugality in expenditures; also: an instance or a means of economizing" [21]. In other words, an economy must be seen as a real-world system constraint by either geographic or temporal borders, which deals with resources based on the assumption of the rational choice of all agents [22]. However, circular economy (CE) is a concept that broadens the thoughts on purely economic rational choice in favor of sustainable development. The Ellen MacArthur Foundation defines it as follows: "A circular economy is one that is restorative and regenerative by design and aims to keep products, components, and materials at their highest utility and value at all times, distinguishing between technical and biological cycles. This new economic model seeks to ultimately decouple global economic development from finite resource consumption. A circular economy addresses mounting resource-related challenges for business and economies, and could generate growth, create jobs, and reduce environmental impacts, including carbon emissions" [23].
Whereas we see a CE as a target state to aspire in a future economy, today's challenge must be to consider ways of operationalizing this goal. The application of circular principles, including circular production and use, to ultimately form circular supply chains is inevitable. Nevertheless, the transition from today's mostly linear supply chains requires a change in consumption behaviors, decision-making processes, and respective legislative frameworks. This involves firms and consumers on the microeconomic level, as well as policy makers on the macroeconomic level [22]. Innovation itself may be seen as a vehicle for change, whereby it is important to consider the multiplicity of innovation per se. Thus, rather than focusing solely on product and process innovation, we should also consider structural and social innovations of varying degrees (i.e., radical to incremental innovations) [24].

The Content of This Special Issue
Although in a perfect CE, primary resource consumption (i.e., mining) would be obsolete, there is a long way to go to reach that stage, and for now, mining remains an essential factor. [25]. However, if we break down the goal of attaining an overall circular economy into subgoals, such as a phosphorus circular economy, the complexity can be reduced. This will require an adjustment about how we perceive an economy. As stated above, common definitions foresee a system bound by either geographic or time-frame means. Thus, if we consider a phosphorus CE, we define the value chain of P as the defining system boundary [22]. Below, Figure 2 is a first attempt to build on the linear Extended Phosphorus Supply Chain first introduced by Steiner et al. [26]. The agricultural phosphorus use cycle consists of the main phases of a P supply chain (black), some of the major losses involved in each phase (red), and several key challenges along the chain (green) where principles might be applied toward a circular economy of P and potential contributions made to an overall CE (e.g., the recovery of by-products such as uranium and rare earth elements or waste utilization).  [22] categorizing [27][28][29][30][31][32][33][34][35][36][37][38][39].
Subsequently, Table 1 lists all contributions in the special issue. It is arranged in ascending order according to their main scope along the agricultural P use cycle. Each entry is complemented by core messages in a very brief summary of the main findings, as well as the keywords provided by the authors. Content: Part 2 builds on the contributions of Mehr, Jedelhauser, and Binder by developing three scenarios on landscape, regime, and niche level as potential pathways toward a sustainable P future based on a multilevel perspective of transition theory. While scenarios one and two show the highest implications for primary and secondary P flows, the scenario including urine separation entails fundamental socio-technical shifts of the wastewater system, whereas sewage sludge recovery represents an incremental adaptation. Keywords: phosphorus; Switzerland; scenario analysis; substance flow analysis; socio-technical transition; circular economy; human diets; recycling; sewage sludge; urine separation

Conclusions and Outlook
Great appreciation goes to all participating authors, the numerous anonymous reviewers, and the editorial team of Sustainability. Without each of them, this special issue would never have been possible. In our attempt to provide a clear and coherent storyline across the potential single contributions of this SI, we also organized an authors' workshop in November 2017, the 1st International Göttweig Mineral Resource Summit. This event was organized under the umbrella of the Transdisciplinarity Laboratory Sustainable Mineral Resources at Danube-University Krems in Austria (see www.donau-uni.ac.at/ smr-tdlab).
We are positive that each single contribution on its own contributes significantly to the field of phosphorus research. Thus, in coherent consideration of the form of this special issue, it should be seen as a state-of-the-art starting point for further research contributing to the journey toward sustainable management of this essential resource and, ultimately, to a circular economy of phosphorus.
Author Contributions: Both authors contributed equally to this editorial and in the process of guest editorship.