Study On The Mechanism And Application Potential Of Soil Organic Phosphorus Mineralization Regulated By Bacteria-feeding Nematodes Under Different Fertilization System

Bacterial-feeding nematodes can affect the composition of Al P-producing bacteria community harboring phoD genes or the abundance of phoD gene through predation,thereby affecting soil alkaline phosphomonoesterase（Al P）activity and the soil organic phosphorus（P）mineralization process mediated by Al P,which would help to promote soil P cycling and improve soil P availability.However,the mechanism by which bacterial-feeding nematodes increase the activity of Al P by affecting the Al P-producing bacteria harboring the phoD gene under the combined application of organic and inorganic fertilizers as well as its effect on soil organic P mineralization is still unknown.In this study,the alkaline purple soil,which was widely distributed in the hilly area of central Sichuan,was used as the research object.Four fertilization treatments: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 to analyze the relationships between bacterial-feeding nematode genera,the composition of Al P-producing bacterial community harboring the phoD gene,phoD gene abundance and Al P activity under different fertilization regimes by using field in situ experiments and high-throughput sequencing technology.This study aims to find out the key indigenous bacterial-feeding nematode that can increase soil Al P activity under the combined application of organic and inorganic fertilizers,and explore the mechanism of bacterial-feeding nematode in increasing soil Al P activity.On this basis,this study continue to study the effects of the key indigenous bacterial-feeding nematode on Al P activity and composition and content of organic P fraction as well as the mechanism of their interaction influencing P availability by nematode addition experiment and Hedley P fractionation method.The main results and conclusions are as follows:（1）Eleven bacterial-feeding nematode genera were identified,among which Acrobeloides,Eucephalobus,Paramphidelus and Heterocephalobus were the dominant bacterial-feeding nematode genera（relative abundance>10%）,with relative abundances of 33.2%,26.0%,14.1%and 11.1%,respectively.Abundances of Eucephalobus and Heterocephalobus increased significantly upon SNPK treatment compared with those upon CK treatment（P<0.05）.In comparison with non-fertilized controls,the fertilizer application treatments significantly increased the soil organic carbon（SOC）,microbial biomass carbon（MBC）,total nitrogen（TN）,total phosphorus（TP）,available phosphorus（AP）content and Al P activity but decreased soil p H（P<0.05）.Compared to the NPK treatment,the SNPK and MNPK treatments significantly increased the soil TN content and Al P activity（P<0.05）.（2）As revealed by comparison analysis of theβ-diversity,composition of phoD-harboring bacterial community was significantly different among the different fertilization treatments（P=0.001）.Aggregated boosted trees（ABT）analysis showed that the bacterial-feeding nematode dominant genus Eucephalobus had the most significant effect on composition of phoD-harboring bacterial community.Partial least squares path modeling（PLS-PM）showed that Al P activity was directly and positively affected by composition of phoD-harboring bacterial community and MBC,and the bacterial-feeding nematode dominant genus Eucephalobus,which was affected by MBC directly,could affect Al P activity in an indirect way by its significant direct positive effect on phoD-harboring bacterial community composition.Among the five phoD-harboring bacterial dominant classes,Eucephalobus was significantly negatively related to Actinobacteria but remarkably positively related to Bacilli（P<0.05）.Of the twelve top bacterial genera that harbored phoD,Saccharothrix and Massilia showed distinct positive relation with Al P activity（P<0.05）,while Saccharothrix showed obvious positive relation with Eucephalobus（P<0.05）.Overall,these results suggest that the dominant genus Eucephalobus is the beneficial indigenous bacterial-feeding nematode that can increase the activity of soil Al P within the alkaline soil,which can be likely to positively affect the phoD-harboring bacterial community composition through a passive food ingestion mechanism and then promote Al P activity indirectly,of which Saccharothrix is the intermediary bacteria for the increase in Al P activity caused by the bacterial-feeding nematode.（3）The addition of bacterial-feeding nematode genus Eucephalobus significantly increased soil Al P activity（P<0.05）,SOC and TN content.Al P was significantly positively correlated with SOC,TN,labile inorganic P,labile organic P,moderately stable inorganic P and highly stable organic P,and labile organic P was significantly positively correlated with NH₄Cl-Pi（P<0.05）.The PLS-PM showed that Al P could affect P availability in an indirect way by its significant direct positive effect on labile organic P and highly stable organic P fractions.Of which,both labile organic P and highly stable organic P,which were directly affected by Al P,could affect NH₄Cl-Pi directly or indirectly by their effect on labile inorganic P and highly stable inorganic P,respectively.Overall,these results suggest that the increased Al P activity under the addition of bacterial-feeding nematode promotes the mineralization of labile organic P and highly stable P fractions,which may be the main mechanism by which bacterial-feeding nematode affect on P availability.