Effect Of Organic Fertilizer On Red Soil Phosphorus Supply And Immobilization And Its Mechanism

Phosphorus（P）,one of the three main elements of plant nutrition,is crucial to crop growth and yield.Red soil,widely distributed in southern China,has a low P content as well as strong P adsorption capacity.Under the situation of great demand and continuous application of organic fertilizers in red soil,how to achieve P efficient utilization while reducing the risk of its loss is an important issue facing the sustainable agriculture development in this region.Depending on the five-year experiment of inorganic and organic fertilization and the seven-year experiment of swine manure application with different rates in Yueyang Agricultural and Environmental Science Experiment Station and the laboratory cultivation experiments,the objectives were to explore the effects of organic fertilizers on P supply and immobilization and the main nonbiological and biological driving factors.The main results were as follows:1.The differences in soil P fractions and availability and the P dynamic resupply process were investigated during summer maize growth under the treatments of no fertilizer,chemical fertilizer,combined application of chemical fertilizer with bone meal organic fertilizer,crop straw and bio-organic fertilizer respectively after five-year continuous application.The results showed that chemical fertilizer combined with bone meal organic fertilizer and chemical fertilizer combined with bio-organic fertilizer treatments had better effects on the content of Olsen-P,organic P（P_o）,inorganic Ca₂-P,Ca₈-P,Al-P,Fe-P and phosphatase activities.The inorganic non-stable P fractions had obvious positive effect on Olsen-P,but the shallow contribution of P_o was observed.The organic fertilization treatments had higher solid P resupply capacity.Maize plant P was affected by soil P resupply and inorganic P supply pools,which was mainly regulated by soil organic matter SOM input directly.2.The diversity and composition of the P-solubilizing bacteria community harboring pho D（soil organic P_o mineralization）and gcd（inorganic P dissolution）at maize seedling stage were further analyzed in above different treatments.The results showed that the diversity of pho D-and gcd-harboring bacterial communities increased under fertilization treatments,especially under chemical fertilizer combined with bio-organic fertilizer treatment significantly by 106.5%and 62.58%,respectively.Fertilization treatments also significantly changed the community composition and structure.The dominant bacterial genus of pho D-and gcd-harboring bacterial communities in soils was the Bradyrhizobium which had no positive effects on nonstable P fractions.Most key pho D-and gcd-harboring bacterial communities significantly correlated with nonstable P fractions were low-abundance genera.Especially,the key genera,positively correlated with P fractions,played more important role in P fractions and alkaline phosphatase activity.Applying organic fertilizer increased Olsen-P level through SOM input,which can also regulate the diversity of bacteria carrying the pho D gene,the community structure of bacteria carrying the gcd gene,and alkaline phosphatase activity,thereby affecting P activation.3.The P fractions accompanied with organic carbon components changes and the driving factors in soils with swine manure applied with different rates(0,7.5,15,30,45 t·hm^-2)for seven-year application were investigated.The results showed the contents of nonstable P fractions in swine manure applied soils increased significantly.Under 15-45 t·hm^-2 manure application,a large amount of Ca₈-P was formed,and the increase rate was higher than that of Fe/Al-P,which had a more significant impact on Olsen-P;however,the accumulation of difficult-to-utilize P was limited.Swine manure application improved the abundance of functional genes related to C-and P-cycling and enhanced related enzymes activities by increasing the content of available N and P;however,the main factors affecting the contents of P fractions were soil organic carbon SOC and p H.The relative contents of ketone-C and O-aryl-C,regulated by the abundance of genes related to C-fixation and the activities of C-degrading enzymes affected by SOC content,were also closely associated with the nonstable P fractions.In addition,the continuous application of 45 t·hm^-2 swine manure inhibited the increasing trend of functional genes abundance and greatly reduced the proportion of dominant genera harboring P cycling functional genes.4.With the increasing year of swine manure application,the increase of total P content and Olsen-P content leveled off under high-rate swine manure application;when the total P or Olsen-P content of the low-rate manure applied soil was approximately or even higher than that of the high-rate swine manure applied soil,the total P input was much less.The swine manure continuous application decreased the P adsorption amount under different P concentration but had little effect on Q_max,indicating that swine manure continuous application changed the types of P adsorption sites rather than the amount of adsorption sites in soil.The soil properties of especially SOM and p H and soil P status were more closely related to P adsorption capacity.The threshold value of P loss in our soils was DPS_ox>25.72%or Olsen-P>67.13 mg·kg^-1;the 15-45 t·hm^-2 swine manure application for years had high risk of P loss.5.The characteristics of adsorption and mineralization for seven P compounds,includingβ-disodium glycerophosphate（BGP）,D-glucose 6-disodium phosphate（G6P）,adenosine disodium monophosphate（AMP）,adenosine disodium triphosphate（ATP）,ribonucleic acid（RNA）,sodium phytate（IP6）)and orthophosphate（Orthor P）,were investigated in five different fertilized soils（S1-S5）.The results showed that under the different P concentration,the adsorption amounts of different soils for seven P compounds were showed as S1≈S2>S3≈S4>S5,especially for ATP,RNA,IP6,and Orthorp.Only RNA,ATP and IP6 had better adsorption amounts than Orthorp in S1 and S2;the adsorption amounts of all organic P compounds in soil S3-S5 were superior to that of Orthorp.The Q_max of RNA was the highest in S1 and S2,but RNA adsorption in S5 was limited,and Q_max was the lowest.The contents of total Fe,Al and Ca and the P accumulation were essential factors affecting the P adsorption between soils.The contents of inorganic Olsen-P_i in S1 and S2 under the additions of organic P compounds harboring monophosphate group were comparable to that under orthophosphate addition;however,it was higher under orthophosphate addition than under the addition of organic P compounds in S3 and S5.In summary,the combination application of rational low-rate organic fertilizer or 7.5 t·hm^-2low-amount application of swine manure alone can promote the P transformation and P resupply by improving soil chemical properties and the activities of microorganisms and related enzymes,achieving sustainable and efficient utilization of soil P.However,applying a high amount of swine manure(15-45 t·hm^-2)was not sustainable,which not only reduced P adsorption and significantly increased the P loss risk in soil but also was not conducive to the maintenance of functional microbial vitality related to nutrient cycling,weakening the enhancement effect of organic fertilizer.