The fate of phosphorus from bone char-based fertilizers in soil pools in a 5-year crop rotation

Bone char (BC) is a promising P-recycling fertilizer but with rather low P-solubility, which can be increased by modifying the BC surface with elemental sulfur (BCplus), but effects on the soil P status have not been tested under field conditions yet. Hence, a long-time field experiment was started 2013 to track the fate of BC and BCplus P into different soil P pools compared to a control and triple superphosphate (TSP) treatment for severely P deficient (iSPTC-A) vs. sufficiently P fertilized (iSPTC-C) soil. The fingerprint of the recent land-use history (six years grassland prior arable land) was reflected by elevated labile-Po and NaOH-Po concentrations at the beginning of the experiment. However, after 3 years, labile Po concentrations converged and stabilized in both soils at a similar level. The formation of this new equilibrium of labile Po suggests that the rate of Po mineralization was, to some extent, controlled by the amount of available legacy P. After the first crop rotation, the effect of P fertilization on soil-P budgets and fractions were small and mostly insignificant. Only TSP increased the available-P pools in the soil. The other pools were not affected by treatments except stable-P increased significantly after BC application in iSPTC-A. The former laboratory results of higher P solubility of BCplus over BC could not be confirmed within the duration of the field trial. However, to prove that BC and BCplus are capable of maintaining adequate long-term crop P supply, the continuation of this unique field trial is highly recommended.