Mineral nitrogen release during repeated extraction from 12–14 months old biochar-amended composts.

(A, B, C, D) Nitrate and (E, F, G, H) ammonium released from different biochar-amended composts (Pyreg^®—wood biochar amended compost CB1, Pyreg^®—sewages sludge char amended compost CB2, Kon-Tiki–wood biochar amended compost CB3) and a non-amended control compost (Con) during consecutive, repeated extraction steps with 2 M KCl (A, E) at ambient temperature on a shaker without further modification (“regular”), (B, F) after removing all biochar particles visible to the naked eye (“w/o biochar”), (C, G) after γ-sterilization of the composts (“γ-sterilized”) or (D, H) with a combined 2 M KCl + 3% H₂O₂ solution (“3% H₂O₂”) in the first extraction step. Extractions were performed 12 to 14 months after the beginning of the aerobic composting (2 months in aerobically managed windrows, subsequent storage storage in open plastic bags). Subsequent extraction steps with individual duration from 1 to 96 h are shown with increasing dark colors. Error bars indicate standard errors of triplicate extractions summated for 6 repeated extractions, lower case letters depict significant differences in total extracted amount of the respective N species with one set of extractions. If no lower case letters are shown, the average values were not significantly different by the 0.05 level of an ANOVA.