Can Organic Matter from Waste-Derived Amendments Limit Phosphorus Losses from Soil to the Aquatic Environment?

When introducing innovative waste-derived amendments, their potential impact on soil retention capacity towards phosphorus (P) should be accounted for to limit its losses and thus water eutrophication. The study was aimed at evaluating P sorption properties of organic waste-derived amendments conifer woodchip biochar (BIO) and brown coal waste (BCW), and assessing their impact on P sorption properties in soil five years after their application in terms of limiting P losses from soils to the water sources. In batch tests samples of amendments and sandy acid soil (Haplic Luvisol) were exposed for 24 h to balancing solutions with a different P content, respectively 0–80 mg P L−1 for amendments and 0–20 mg P L−1 for soil. Three treatments were studied: (1) conifer woodchip biochar (BIO, (2) brown coal waste (BCW) and (0) soil without amendment (control). Organic amendments were applied to soil at a rate equivalent of 10.2 t C ha−1 once at the start of the trial and they amounted to 24.2 t ha−1 for BCW and 12.8 t ha−1 for BIO. Based on the results of the bath tests and the Langmuir and Freundlich equations, sorption parameters of P were determined for BIO, BCW and the soil after their application. The maximum sorption capacities (Smax) of BIO and BCW were 832.3 and 250.4 mg kg−1, respectively, and were controlled mainly by the organic surfaces of their structures. Within the range of concentrations used in the balancing solutions (5–20 mg P L−1), sorption of P was generally higher in unfertilized soil compared to soil amended with BCW and BIO. The application of BIO significantly (p < 0.05) decreased the Smax (17%), while BCW decreased the bonding energy of P (k) by 15% compared to the untreated soil. The content of dissolved P increased significantly in the case of both BCW and BIO applications, which may indicate an increase in soil susceptibility to P losses. However, the effectiveness of these amendments in limiting P losses with the use of other doses and in different soil and climatic conditions cannot be excluded. Further research is recommended to better understand the effect of biochar feedstock and temperature of its production on P retention/losses in various types of soils.