Fate and availability of phosphorus from bone char with and without sulfur modification in soil size fractions after five-year field fertilizations

The agronomic value of fertilizer phosphorus (P) in the soil is determined by how it is incorporated and released during aggregate formation and breakdown. These processes play a vital role in P accessibility, storage, and cycling. Bone char (BC) is a promising substitute for phosphorus fertilizer. Its low P solubility can be increased by modifying the BC surface with elemental sulfur (BC^plus), although the fate of BC-based P within soil aggregates have not yet been tested under field conditions. In this study, soil samples were taken from a five-year field experiment (2013–2018) to track the fate of BC and BC^plus P. These samples were compared to triple superphosphate (TSP) and a control without additional P (No-P) in different soil P pools within soil aggregates in severely P deficient vs. sufficiently P fertilized Cambisol. Soil aggregate distributions were assessed after wet-sieving, centrifugation, and tangential flow filtration to separate small macroaggregates (250–2000 µm, SMaA), large microaggregates (53–250 µm, LMiA), small microaggregates (1–53 µm, SMiA), and composite building units (< 1 µm, BU). Soil P status was assessed after sequential extraction (Hedley scheme). We found that the mass proportions of soil size fractions decreased in the order of SMiA > LMiA ≈ SMaA > BU. The addition of 45 kg P ha⁻¹ year⁻¹ using different fertilizers significantly increased the mass proportion of LMiA from 7% to 22% in comparison to the No-P. This was likely due to stimulated plant growth after fertilization and thus introduction of organic binding agents which increased soil aggregation. The addition of BC and BC^plus showed nearly no significant effect when compared to No-P on soil P pools. However, sulfur modification of BC resulted in higher labile P which was comparable to that found in TSP. Therefore, we conclude that BC^plus behaves similarly to TSP, without any additional positive or detrimental effects on P status. This lends support for the use of BC wastes as a possible substitute for TSP, which would represent a move towards more sustainable agriculture with more closed P cycles.