Addition of bacterial-feeding nematodes contributes to soil phosphorus availability by affecting the mineralization of moderately labile organic phosphorus

Improving the soil residual organic phosphorus (P) utilization rate is important for alleviating P resource shortages and reducing water pollution. Bacterial-feeding nematodes can impact soil alkaline phosphomonoesterase (AlP) activity and mediated organic P mineralization through affecting AlP-producing bacteria that harbor the phoD gene. However, it is unclear which organic P fraction mineralization is promoted with increasing AlP activity after bacterial-feeding nematodes are added, and whether this promotion effect varies with fertilization regimes. Microcosm experiments without (control; -Nematode) and with adding bacterial-feeding nematode Eucephalobus (+Nematode) were carried out in the present study to investigate impacts of bacterial-feeding nematodes on organic P mineralization by using soils collected from four fertilization regimes: non-fertilized control (CK), chemical nitrogen (N), P and potassium (K) fertilizers (NPK), crop straw plus chemical fertilizers (SNPK), pig manure plus chemical fertilizers (MNPK). After 30 days, +Nematode treatment significantly increased soil AlP activity under all the fertilization regimes, and also increased the richness index of AlP-producing bacteria as well as the relative abundances of the gram-positive (G+) bacteria Actinomycetia, Streptomyces and Amycolatopsis. Meanwhile, these three G+ bacteria showed a significant positive correlation with AlP activity, and contributed the most to the different AlP-producing bacterial community compositions under -Nematode treatment versus +Nematode treatment. Moreover, +Nematode treatment only significantly increased the labile organic P (NaHCO₃-Po) content in CK and SNPK soils, and the moderately labile organic P (NaOH I-Po) content in SNPK soil. Correlation analysis showed that the AlP activity had positive relationships with the AlP-producing bacterial richness index, community composition and NaOH I-Po, water-soluble P (NH₄Cl-Pi) contents. Partial least squares structural equation modeling (PLS-SEM) revealed that NaOH I-Po, under the direct influence of AlP, indirectly and positively influenced NH₄Cl-Pi by affecting moderately labile inorganic P (NaOH I-Pi). Overall, this study increases our understanding of the effect mechanism of bacterial-feeding nematodes on organic P mineralization, and finds that the addition of bacterial-feeding nematodes under straw returning may have a better effect in improving the soil P availability compared with their addition under other fertilization regimes.