Citation: Debicka, M.; Jamroz, E.; Bekier, J.; ´ Cwiel ˛ ag-Piasecka, I.; Kocowicz, A. The Influence of Municipal Solid Waste Compost on the Tranformations of Phosphorus Forms in Soil. Agronomy 2023, 13, 1234. https://doi.org/10.3390/ agronomy13051234 Academic Editor: Francesco Montemurro Received: 3 March 2023 Revised: 19 April 2023 Accepted: 20 April 2023 Published: 27 April 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). agronomy Article The Influence of Municipal Solid Waste Compost on the Tranformations of Phosphorus Forms in Soil Magdalena Debicka * , El ˙ zbieta Jamroz , Jakub Bekier , Irmina ´ Cwiel ˛ ag-Piasecka and Andrzej Kocowicz Institute of Soil Science, Plant Nutrition and Environmental Protection, Wroclaw University of Environmental and Life Sciences, ul. Grunwaldzka 53, 50-357 Wroclaw, Poland * Correspondence: magdalena.debicka@upwr.edu.pl; Tel.: +48-713-205-641 Abstract: Searching for a renewable phosphorus (P) source for agricultural soils has become a challenge of particular importance on a global scale. Depletion of the world’s phosphate resources as well as the risk of environmental problems caused by excessive P mobility have placed this topic among the most important environmental issues. The aim of this study was to analyse the transformation of phosphorus forms in municipal solid waste compost (MSWC) at different maturity stages, to assess the effects of their application on soil phosphorus status and to evaluate the suitability of MSWC for agricultural soils as a potential phosphorus source. The application of MSWC to the soil had a positive effect on the content of available P. The content of potentially mobile P forms increased significantly in the soil, primarily after the introduction of immature MSWC and confirmed by the significant correlations with the applied P indicators. However, the results indicate that the MSWC does not pose a serious environmental risk and could become a potential source of P in the soil, although the material used for this purpose should be mature. This study also indicated that the assessment of using compost as a P source should not be based on P total content alone, but should include the content of different P forms and specific P-related indicators, to reflect both the P availability and the potential environmental risks. Keywords: environmental P indicators; P availability; P behaviour in soil; P mobility; organic waste utilisation; renewable P source 1. Introduction Considering the limited worldwide range of phosphorus (P) reserves [1–4], which as phosphorites were included in the Critical Raw Materials List by the European Commission in 2014 [1,2], the problem of finding new P sources for agricultural soils has become a challenge for modern generations. Hence, reducing the use of mineral fertilisers and making greater utilisation of renewable P sources has become an emerging direction for agriculture. Phosphorus is an essential macronutrient in plant nutrition, and therefore, is an important issue for agriculture. The low efficiency of P fertilisation, i.e., not higher than 15–30%, which results from strong P sorption in soils, is the main reason for the deficiency of available P forms in soil. Therefore, continuous P fertilisation is necessary to meet the requirements of crops. This, in turn, contributes to the accumulation of P in arable horizons of cultivated soils in forms that are strongly fixed and not available to plants. At subsequent P doses, when a certain level of P is exceeded, over-fertilisation can occur, which refers to the level at which the sorption sites for P are no longer available (P saturation is reached). Then, P is poorly held in the soil and the risk of mobilisation and leaching of excess P into the environment rises significantly, causing undesirable effects of eutrophication in aquatic ecosystems. These aspects must be taken into account when P fertilisers are introduced into soil. One of the possible and reasonable solutions is to look for naturally available resources in the environment that can be reused. In particular, this is currently recommended, in Agronomy 2023, 13, 1234. https://doi.org/10.3390/agronomy13051234 https://www.mdpi.com/journal/agronomy