Straw-Derived Biochar Alters Soil Phosphorus Availability and Inorganic Phosphorus Fractions: A Global Synthesis

Straw-derived biochar is increasingly used for residue recycling and soil fertility improvement, but its effects on soil inorganic phosphorus (P) fractions and their relationships with available P (AP) remain unclear. Here, we synthesized 1170 paired observations from 51 studies using meta-analysis, partial least squares path modeling, and machine learning to quantify straw biochar effects on soil AP, inorganic P fractions, microbial biomass P (MBP), and phosphatase activities, and to identify important variables related to these responses. Straw biochar increased soil AP by 77.0% and altered the distribution of inorganic P fractions. The relationships between inorganic P fractions and AP differed: Fe-bound P and occluded P showed positive and negative associations with AP, respectively, whereas Ca-bound P and Al-bound P showed no significant associations with AP. Fe-bound P and Al-bound P increased more strongly in soils with low initial AP, while higher biochar total P corresponded to weaker Fe-bound P accumulation but greater Ca-bound P accumulation. Straw biochar showed a positive response in MBP, while acid and neutral phosphatase activities showed decreasing trends, suggesting distinct microbial P cycling responses involving changes in microbial biomass P and phosphatase activities. These findings provide a P-fraction perspective for evaluating straw biochar as a site-specific amendment for soil P management.