Nonadditive effects of biochar amendments on soil phosphorus fractions in two contrasting soils

Increased soil total phosphorus (P) or available P has been reported in biochar-amended soils, although the underlying mechanisms need to be fully understood. In the present study, two contrasting soils (acidic Haplic Luvisol and alkaline Calcaric-Fluvisol) amended with wheat straw biochar were sequentially extracted with modified Hedley method to study the P transformation in the soils. Our results showed that biochar application significantly increased (positive effects) P fractions (except for NaHCO3-P-i and residual-P) content in Haplic Luvisol. The increased soil microbial activity and reduced soil acidity or increased cation exchange capacity may be accounted for enhanced P transformation. The reduced NaHCO3-P-i content may be related to P immobilization with increased soil microbial activity induced by biochar addition because the high C:P ratios of biochar (ranged from 234 to 357) suggested net P immobilization occurred when biochar was incorporated into soil. Biochar application first (4days) increased soil NaHCO3-P-o content and then decreased it with longer incubation time (30days). The decrease in NaHCO3-P-o suggested that the labile organic P was converted into nonlabile inorganic or organic P. In comparison with those in Haplic Luvisol, almost all P fractions showed negative effects with biochar addition into Calcaric-Fluvisol. The results may be caused by the P precipitation or sorption with increased soil pH with biochar application. On the basis of P transformation, biochar is recommended to be used in Haplic Luvisol (acidic soil) and not to Calcaric-Fluvisol (alkaline soil) because of the positive effects and negative effects observed on P fractions in biochar-amended soils.