[Effect of Biochar with Different Particle Sizes on Soil Available Phosphorus]

Biochar can change the effectiveness of soil phosphorus， and most of the available studies have focused on the effects of biochar dosage and allotment on soil available phosphorus content. However， how the particle size of biochar affects soil phosphorus transformation driven is still poorly understood. A pot experiment was used in order to study how different particle sizes of biochar [C0， C1（>1 mm）， C2（0.5-0.25 mm）， and C3（<0.01 mm）] affected soil chemical properties， soil enzyme activities， and soil bacterial communities and consequently， soil available phosphorus content， at different phosphorus levels [P0（0 g·kg^-1）， P1（0.16 g·kg^-1）]. The results showed that soil organic carbon and ammonium nitrogen contents were significantly higher （P<0.05） after biochar application. Soil available phosphorus content increased after biochar application at the P0 level compared with that in the C0 treatment， and it decreased and then increased with the decrease in biochar particle size； soil available phosphorus content decreased after biochar application at the P1 level and increased with the decrease in biochar particle size. Soil enzyme activity and phoD gene abundance increased after biochar application and showed a tendency of increasing and then decreasing with the decrease in biochar particle size at the P1 level. The corresponding bacterial diversity changed， especially increasing the relative abundance of the Proteobacteria. By constructing a random forest prediction model， the soil phosphorus level， organic carbon content， alkaline phosphatase activity， and the Chao1 index were identified as the main drivers of soil available phosphorus content. In summary， the effect of biochar on soil phosphorus availability varies depending on its particle size and the initial soil phosphorus level. Therefore， when using biochar for soil amendment， it is essential to comprehensively consider the influence of biochar particle size and soil phosphorus level on soil nutrients and bacterial communities.