The relationships of bacterial-feeding nematodes, phoD-harboring bacteria and alkaline phosphomonoesterase activity under the combined application of organic and inorganic fertilizers in an alkaline soil

Bacterial-feeding nematodes have the potential to increase soil alkaline phosphomonoesterase (AlP) activity by affecting the composition of phoD-harboring bacterial community or the abundance of phoD gene. However, the relationships between bacterial-feeding nematodes, composition of phoD-harboring bacterial community, phoD gene abundance and AlP activity with inorganic plus organic fertilization in alkaline soil where AlP is dominant is yet to be properly understood. Four fertilization regimes: non-fertilized control (CK), chemical nitrogen (N), phosphorus (P) and potassium (K) fertilizers (NPK), crop straw plus chemical fertilizers (SNPK), as well as pig manure plus chemical fertilizers (MNPK), were used in determining the variation in soil bacterial-feeding nematodes, composition of phoD-harboring bacterial community, phoD gene abundance and AlP activity under different fertilization treatments as well as their relationships. Results showed that both inorganic and organic fertilizer application significantly enhanced the activity of soil AlP, which significantly decreased under NPK application compared with SNPK and MNPK treatments. Partial least squares path modeling showed that AlP activity was directly and positively affected by composition of phoD-harboring bacterial community and mi-crobial biomass carbon (MBC), and the bacterial-feeding nematode dominant genus Eucephalobus, which was affected by MBC directly, could affect AlP activity in an indirect way by its significant direct positive effect on phoD-harboring bacterial community composition. Of those 12 top bacterial genera that harbored phoD, just the gram-positive bacteria Massilia and Saccharothrix showed significant positive correlation with AlP, while Sac-charothrix showed obvious positive correlation with Eucephalobus. Overall, these results suggest that the domi-nant genus Eucephalobus may be the beneficial indigenous bacterial-feeding nematode that can increase the activity of soil AlP within the alkaline soil, which is affected by MBC directly and can be likely to positively affect the composition of phoD-harboring bacterial community through a passive food ingestion mechanism and then promote AlP activity indirectly, of which Saccharothrix is the intermediary bacteria for the increase in AlP ac-tivity caused by the bacterial-feeding nematode.