Sustained superiority of biochar over straw for enhancing soil biological-phosphorus via the mediation of phoD-harboring bacteria in subtropical Moso bamboo forests

Straw and biochar are commonly used to enhance soil organic carbon pools and improve soil quality in subtropical Moso bamboo forests. However, their effects on soil biological-phosphorus (P) remain unclear, even though P limitation is common in these forests. This study investigates the impact of these amendments on soil biological-P through a two-year trial with three treatments: control, straw, and biochar addition in a Moso bamboo forest. We measured soil biological-P fractions, including enzyme-, citrate-, CaCl₂-, and HCl-extractable P, along with the activities of alkaline phosphatase, and the abundance and community structures of phoD-harboring bacteria at 3, 12, and 24 months post-treatment. Results showed that both straw and biochar increased the four biological-P fractions by 7.0–134.6 % and 14.4–157.7 %, respectively. Straw addition resulted in a rapid increase in the first principal component of the four biological-P fractions (biological-P_PC1, which represented a composite index of all fractions), with a 291.8 % enhancement initially. However, this effect declined over time, showing a decrease of 113.3 % at 12 months and 25.0 % at 24 months. In contrast, biochar led to a sustained improvement in the biological-P_PC1, with increases ranging from 157.3 % to 184.6 % over the two-year period. Positive correlations were found between the abundance of phoD-harboring bacteria and biological-P_PC1, as well as between phoD abundance and alkaline phosphatase activity, indicating that these bacteria are key in regulating biological-P. Furthermore, dominant phoD-harboring bacterial genera (e.g., Bradyrhizobium, Cupriavidus, and Pseudomonas) primarily governed the regulation of biological-P, rather than rare genera. Overall, this study highlights the potential of straw and biochar as organic amendments for enhancing soil biological-P dynamics. Biochar shows promise for long-term improvements in soil biological-P. These findings contribute to our understanding of soil nutrient dynamics and inform sustainable management practices in Moso bamboo forests.