Co-composted biochar and iron-modified biochar enhance soil bioavailable phosphorus fractions by altering gcd- and phoD-harboring microbial communities

Organic fertilizers and biochar amendments can improve soil quality and facilitate phosphorus (P) availability. However, how co-composted biochar and iron-modified biochar affect soil P fractions and P-functional microbial communities remain uncertain. Herein, pot experiments amended with chemical fertilizer (CF), sole compost from pig manure (OF), co-composted general biochar (BOF), and co-composted iron-modified biochar with different application rates (FeBOFs) were conducted. Variations in bioavailable P fractions, soil stoichiometry, and plant properties were examined, metagenomic sequencing and high-throughput quantitative PCR (qPCR) were also performed to investigate the responses of P-cycling microbial communities and functional genes to the different amendments. Notably, FeBOFs significantly enhanced soil P availability and promoted plant P uptake compared with BOF. Although the BOF treatment had the highest soil Citrate-P, Enzyme-P and HCl-P contents, FeBOFs altered the distribution of bioavailable P fractions, indicated a greater capacity to facilitate the transformation of available P. Importantly, BOF and FeBOFs amendments shifted the composition and diversity of gcd - and phoD -harboring microbial communities. Correlation and partial least squares path analyses revealed contrasting effects of BOF and FeBOFs amendments on gcd - and phoD -harboring microbial communities. In addition, gcd -harboring microbes positively associated with P functional genes and potentially altered soil P availability. In contrast, phoD -harboring microbes directly related to soil bioavailable P fractions. Our findings offer new insights into the responses of gcd - and phoD -harboring microbial community to distinct types of co-composted biochars, highlighting their divergent roles in the microbial transformation of soil P fractions.