Effects of the reduction in chemical fertilizers on soil phosphatases encoding genes (phoD and phoX) under crop residue mulching

Phosphorus (P) within crop residues is released to soils and thus is expected to partially replace the application of chemical P. However, whether and how P availability responds to a reduction in chemical P under crop residue mulching is still poorly understood. In this study, conventional chemical fertilizer (NPK), NPK plus crop residue (NPKStr), NPKStr combined with 20% reduction in only chemical P (NPmKStr) and NPKStr combined with 20% reduction in chemical NPK fertilizers (mNPKStr) were used. The abundance and diversity of the phoD and phoX genes encoding phosphatases were evaluated using quantitative PCR and high-throughput sequencing. The re-sults showed that variations in the abundance and alpha-diversity were detected in phoD rather than phoX, while the composition of phoX-harbouring bacterial communities changed between treatments. Specifically, NPmKStr resulted in an increase in the abundance and alpha-diversity of phoD while mNPKStr stimulated phoD abundance. In addition, NPmKStr decreased the activities of alkaline phosphomonoesterases (ALPs) and mNPKStr increased phosphodiesterase (PDE) activities. Moreover, phoD- and phoX-harbouring bacterial species were highly corre-lated, especially bridged by particular species belonging to Alphaproteobacteria and Cyanobacteria, in response to reduced amounts of chemical fertilizer. Last, structural equation modelling suggested that the activities of both ALP and PDE showed negative feedback towards increased available P by virtue of phoX-harbouring bacterial species. These results indicated that a balanced reduction in chemical fertilizers may be more reasonable for maintaining the ability to hydrolyse organic P under crop residue mulching and that phoX played an interme-diary role in this process.