Effects Of Phosphorus Fertilization On Enrichment And Alteration Of Rhizospheric Soil Nitrogen And Phosphorus Forms In A Mustard And Maize Rotation System

The unique physical and chemical properties and biochemical characteristics of rhizosphere soil change the transformation process and bioavailability of nitrogen（N）and phosphorus（P）forms,and affect their absorption and utilization by plants.Soil P content is closely related to the transformation and migration process of rhizosphere nutrient,so it is important to study the effect of P fertilizationss on the transformation of soil N and P in the rhizosphere.The transformation mechanism of N and P in crop rhizosphere soils was mature,but the related research focuses on the interaction between rhizosphere process and the transformation of N and P.However,the change characteristics and transformation relationships of N and P forms in the crop rhizosphere soil was not clear.Therefore,this thesis firstly took the mustard-maize rotation system in the sloping land with purple soil of the Three Gorges Reservoir area as the starting point.The aims were to clarify the effect of P fertilization on the nutrient balance and transformation of N and P forms in the rhizosphere of field plants and to optimize the fertilizations of P by improving crop yield while decreasing the environmental pollution.A field experiment in a purple soil（Purple normal Entisols）with five levels of P(P-0,P-75,P-150,P-225 and P-300;0,75,150,225 and 300 kg P₂O₅ hm^-2 yr^-1)was conducted in Fuling District,Chongqing City.The field experiment contains 15 runoff cells（5.0×20.0 m）.The objectives of this experiment were to study the dynamic changes of N and P forms in the rhizosphere of a mustard-maize cropping system,and to analyze the effect of P fertilizations on relationships between source and sink of soil N and P forms and nutrient balance in their rhizosphere.（1）P fertilizations promoted the accumulation of free N and acid-hydrolyzed amino acid N in the rhizosphere of mustard and the enrichment of ion exchangeable N and acid-hydrolyzed total N in the rhizosphere of maize.The free particulate N（FPN）was an important form of free N in the mustard rhizosphere soil,ranging from 45.9%to 74.4%,respectively.The FPN and free total N in the rhizosphere of mustard under the treatments with high P fertilizations was significant greater than it was in the treatments with low P fertilizations with the increasing fertilization days.The dissolved N in the rhizosphere of maize was the main component of free N（abount 61.5%-88.5%）and the free NO₃^--N in the rhizosphere of maize is the main form（43.1%-73.3%）of dissolved N.P fertilizations increased the free NH₄⁺-N but decreased free NO₃^--N in the rhizosphere of maize.P fertilizations increased significantly ion exchangeable N（IEN）after harvesting the mustard.Carbonate-bound N is the main component of chronic N（84.5%-92.1%）and P fertilizations had little effect on the carbonate-bound N in the rhizosphere of mustard and maize.Compared to the low P fertilizations,high P fertilizations decreased significantly iron and manganese bonded N in the rhizosphere of mustard and maize.Acid-hydrolysis ammonium N（35.5%-60.1%）and acid-hydrolysis amino acid N（25.4%-36.1%）was the main components of acid N.P fertilizations increased significantly acid amino acid N in the rhizosphere of mustard and maize.High P fertilizations decreased the residual N in the rhizosphere of mustard and increased the residual N in the rhizosphere of maize.However,the effect of P fertilizations on the total N in the rhizosphere of mustard and maize was little.（2）P fertilizations drove the ion exchangeable organic N and carbonate bound N from source to sink in the rhizosphere of mustard and the acid-hydrolyzed ammonium N from sink to source in the rhizosphere of maize.Free NH₄⁺was the source in the rhizosphere of mustard and was the sink in the rhizosphere of maize.The ion exchangeable NH₄⁺and ion exchangeable NO₃^-was the source in the rhizosphere of mustard and maize with different P fertilizations.Iron and manganese bound N in the rhizosphere of mustard was the sink under the treatments with high P fertilizations.The acid-hydrolysis ammonium N and acid-hydrolysis amino sugar N was the sink in the rhizosphere of mustard under treatments with≤150 kg P₂O₅ hm^-2 yr^-1 and the acid-hydrolysis amino acid N and unknown acid N was the source in these treatments.The residual N in the rhizosphere of mustard was the source under treatments with≥150 kg P₂O₅ hm^-2 yr^-1 fertilizations.The acid-hydrolysis amino sugar N was the source and unknown acid-hydrolysis N and redicual N was the sink in the maize rhizosphere under the treatments with P after harvesting.（3）P fertilizations promoted P acquisition by mustard/maize and the accumulation of chronic phosphorus in the rhizosphere of mustard/maize.The free particulate P was the main component of free P,ranging from 40.6%to 80.9%。P fertilizations decreased the F-P and NH₄F extracted P and increased ion exchangeable P,resin extracted P（Resin-P）,NaHCO₃ extracted P（NaHCO₃-P）and Olsen P（O-P）in the rhizosphere of mustard.≤225 kg P₂O₅ hm^-2 yr^-1 fertilizations increased NH₄F-P and O-P and decreased the Resin-P in the rhizosphere of maize.The NaOH extracted P was greater in the rhizosphere of mustard than that of the rhizosphere soil of maize.Stable P was the main pool of P in the rhizosphere of mustard and maize,ranging from 55.0%to 65.0%and stable P was least（about 3.00%-10.0%）.Moreover,P fertilizations enhanced significantly the total P in the rhizosphere of mustard and maize and promoted the P acquisition by plant.（4）P fertilizations drove the CH₃COONH₄ extracted organic P from the source to the sink and Olsen P from the sink to the source in the rhizosphere of mustard and maize.The dissolved organic P was the sink in the rhizosphere of mustard with≤150 kg P₂O₅ hm^-2 yr^-1 and was the source in the rhizosphere of mustard with>150 kg P₂O₅ hm^-2 yr^-1.The dissolved P was the sink in the rhizosphere of maize with 150 and 225 kg P₂O₅ hm^-2 yr^-1 and the free particulate P was the source in the rhizosphere of maize with 150 and 225 kg P₂O₅ hm^-2 yr^-1.The ion exchangeable organic P and resin extracted P was the sink in the rhizosphere of mustard and the resin extracted inorganic P was the source and the resin extracted organic P was the sink in the rhizosphere of maize with 0,75 and 300kg P₂O₅ hm^-2 yr^-1.The NaHCO₃ extracted P was the sink in the rhizosphere of mustard with 225 and300 kg P₂O₅ hm^-2 yr^-1.The NaHCO₃ extracted inorganic P was the sink in the rhizosphere of maize with 225 and 0 kg P₂O₅ hm^-2 yr^-1.The CH₃COONH₄ extracted P was the sink and NH₄F extracted P was the source in the rhizosphere of mustard with P fertilizations.The NaOH extracted inorganic P under the treatments with 0 and 150 kg P₂O₅ hm^-2 yr^-1 was the source in the rhizosphere of mustard and was the sink in the rhizosphere of maize.The agglomerates internal P was the sink in the rhizosphere of mustard and maize with 150 and 225 kg P₂O₅ hm^-2 yr^-1.The H₂SO₄ extracted P under treatments with 75,150 and 225 kg P₂O₅ hm^-2 yr^-1 was the source in the rhizosphere of mustard and was the sink in the rhizosphere of maize.The residual P was the source in the rhizosphere of mustard with 150,225 and 300 kg P₂O₅ hm^-2 yr^-1 and the source effect was positively correlated with the amount of P fertilizations.（5）P fertilizations regulated the rhizosphere N and P losses and enrichment by driving the activation of N and P in the rhizosphere of mustard and maize.The labile N in the rhizosphere of mustard was higher under the treatments with 225 and 300 kg P₂O₅ hm^-2 yr^-1 than the other treatments.There was no significant difference in labile N in the rhizosphere of maize between before fertilization and after harvesting among different treatments.The labile P was significantly greater in the rhizosphere of mustard with 150 kg P₂O₅ hm^-2 yr^-1 than the other treatments.P fertilizations increased the labile P in the rhizosphere of maize and the increasing effect was positively correlated with the amount of P fertilizations.P fertilizations increased significantly total P in the rhizosphere of mustard and maize.P fertilizations enhanced significantly the N activation in the rhizosphere of mustard and P activation in the rhizosphere of maize.150 kg P₂O₅ hm^-2 yr^-1 fertilizations enhanced significantly the P activation capacity in the rhizosphere of mustard and N activation capacity in the rhizosphere of maize.P fertilizations enhanced the enrichment of P in the rhizosphere of mustard and maize and increased the loss of N in the rhizosphere of maize.In summary,P fertilizations promoted the accumulation of free N and acid-hydrolyzed amino acid N in the rhizosphere of mustard and the enrichment of ion exchangeable N and acid-hydrolyzed total N in the rhizosphere of maize.P application drove the ion exchangeable organic N and carbonate bound N from the source to the sink in the rhizosphere of mustard and the acid-hydrolyzed ammonium N from the sink to the source in the rhizosphere of maize.Moreover,P fertilizations accelerated the enrichment of chronic P and stable P and induced the the CH₃COONH₄ extracted organic P from the source to the sink and Olsen P from the sink to the source in the rhizosphere of mustard/maize.Lastly,P fertilizations regulated the rhizosphere N and P losses and enrichment by driving the activation of N and P in the rhizosphere of mustard and maize.