Varieties of P fractions in biochar-amended reconstructed soils as impacted by freeze-thaw interference.

With low cost and stable chemical properties, biochar has great potential in environmental pollution control and improving soil quality. Reusing tailings slag to reconstruct soil ecosystems and applying amendments such as biochar to enhance soil quality are significant for restoring waste mine lands. Phosphorus (P) as the restrictive nutrition element for plant growth is easily affected by freeze-thaw cycles (FTCs). However, effective information about FTCs on P dynamics in biochar-amended reconstructed soil is scanty. To further understand the effect of FTCs on P in reclaimed mine soils, three reconstructed soils composed of equal brown soil and tailings slag with the respective application of no amendment, 5% biochar and 5% powder both derived from Gleditsia japonica shells (GS), were prepared to evaluate P fraction changes after FTCs. The results indicated that GS biochar increased soil pH, total organic matter (TOM), and moisture content (MC). GS biomass had a similar impact on TOM and MC but decreased soil pH. The two agricultural amendments increased active P and microbial biomass P (MBP) by 46.13%-101.63% and 162.8%-185.7%, which might be largely contributed by soil organic matter and moisture. FTC numbers (0, 3, 6, 10, 15) significantly decreased MBP contents and slightly converted non-labile P into labile fractions while FTC temperature (-20∼5 °C and -10∼5 °C) hardly influenced soil P behavior. In addition, GS conditioners simultaneously enhanced available P content and P fixation potential by soil under FTCs.