Towards circular phosphorus: The need of inter- and transdisciplinary research to close the broken cycle

Phosphorus (P) is an essential element to all living beings but also a finite resource. P-related problems center around broken P cycles from local to global scales. This paper presents outcomes from the 9th International Phosphorus Workshop (IPW9) held 2019 on how to move towards a sustainable P management. It is based on two sequential discussion rounds with all participants. Important progress was reported regarding the awareness of P as finite mineable resource, technologies to recycle P, and legislation towards a circular P economy. Yet, critical deficits were identified such as how to handle legacy P, how climate change may affect ecosystem P cycling, or working business models to up-scale existing recycling models. Workshop participants argued for more transdisciplinary networks to narrow a perceived science-practice/policy gap. While this gap may be smaller in reality as illustrated with a Swiss example, we formulate recommendations how to bridge this gap more effectively. Supplementary Information The online version contains supplementary material available at 10.1007/s13280-021-01562-6.


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Illustration of an analysed abstract
The abstract below was selected such as to provide an instructive example of the procedure.
In the text, key words pointing to the elements of the dimensions Inter-/Transdisciplinarity (compartments, disciplines, domains, sectors) and Research orientation (spatial scale, temporal scale, knowledge type) are highlighted with according to the colour code given in this description. However, we also considered implicit information, on the domains for example (here: natural and engineering sciences).
The quantitative evaluation for this abstract is illustrated in Tab. S3 below.
Abstract text: The Flemish Land Agency (VLM) initiated a research project (2015)(2016)(2017)(2018) in order to select the best soil P test method, to set the target soil P availability for both agriculture and environment, and to formulate fertiliser and management recommendations for sustainable P use.
Six soil P tests were evaluated: (1) extraction with ammonium lactate and acetate at pH 3.75 (P-AL, Egnér et al., 1960); (2) extraction with 0.5 M NaHCO3 (P-Olsen); (3) extraction with 0.01 M CaCl2 (P-CaCl2); (4) extraction with ammonium oxalate (P-Ox), with possibility to calculate the phosphate saturation degree (PSD, 5) and (6) diffusive gradient in thin film technique (DGT). In general, no large differences in soil P test performance for explaining yield (losses) were observed in 11 long term field studies across Europe (Nawara et al., 2017) and in a greenhouse P depletion trial (Nawara et al., 2018). The capacity of the soil P tests in estimating soil P leaching losses were evaluated by a soil column leaching experiment, in which the PSD slightly outperformed P-AL and P-CaCl2. P-Ox performed worst for both yield and P leaching estimations. Taking the method performance and the familiarity into account, it is reasonable to keep using the P-AL method in Flanders.
It was investigated which P-AL values are acceptable for both agriculture and environment. First of all, field trials were studied to look for the minimum P-AL value necessary for acceptable crop yields. A Mitscherlich fit revealed that for all crops together and at optimal soil pH, a P-AL value of 110 mg P/kg is necessary to reach 95% relative yield (yield compared to the maximal yield in that field). Secondly, it was investigated which P-AL in the topsoil is related to 25% PSD in the soil profile, limiting the P concentration leaching to the ground water to 0,1 mg ortho-P/L (P standard for surface waters in Flanders). This maximum P-AL acceptable for the environment is 160 mg P/kg. Consequently, the agro-environmental target zone for Flanders is between 110 and 160 mg P/kg measured in the topsoil.
On soils with P-AL values lower or higher than this target zone respectively positive or negative soil P balances (= inputs by P application minus outputs by crop removal) should be applied. A multiple linear regression with data from 33 long term field trials revealed that the higher the initial P-AL value, the more P fixation in time and the larger the net P-AL change per kg P added or removed from the soil.
To stay in the target zone, a small positive soil P balance is necessary. Interestingly on average only 27% of the net amount of P added or removed is measured as change in the P-AL fraction of the topsoil.
The results of the project will nourish the Flemish legislation and recommendations regarding P fertiliser strategies (quantities and management) are issued to the farmers.