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

Abstract

AbstractIncreased 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‐Pi 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‐Pi 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 (4 days) increased soil NaHCO3‐Po content and then decreased it with longer incubation time (30 days). The decrease in NaHCO3‐Po 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.