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 (BCplus), 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 BCplus 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 mu m, SMaA), large microaggregates (53-250 mu m, LMiA), small microaggregates (1-53 mu m, SMiA), and composite building units (< 1 mu m, 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 approximate to 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 BCplus 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 BCplus 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.