Control Factors Of Environmental Behaviors Of Phosphorus In Fluvo-aquic Soil:The Role Of Calcium,Iron And Organic Matters

Excessive fertilization has brought high risk of phosphorus（P）leaching problems in North China Plain,where the most typical cropland soil is Fluvo-aquic soil.Previous studies have reported the significant effects of Fe minerals,calcium carbonate（CaCO₃）,and OM on the immobilization of P in soils.However,the main factors controlling P environmental behavior and their acting time-sequence in Fluvo-aquic soil after long-term P fertilizer application and the flux of P leaching in soil profile have not been well-recognized.A clear understanding is essential for effective P management.In this study,farmland investigation and laboratory experiments were combined to study above questions.Correlation analysis between Ca,Fe,OM,and P and its fractions in field soils were studied based on the investigation data.Preliminary results showed that P associated with Ca was found to be the dominant P fraction in the fluvo-aquic soil.However,no significant correlation between P and Ca was observed in our study.Nevertheless,the concentrations of P and its fractions in the sampled soils were strongly correlated with Fe and TOC.In the laboratory incubation experiments,the effects of Ca,Fe,and OM on P immobilization were examined by amending the soil samples with calcium carbonate（CaCO₃）,goethite（Gt）,and manure,respectively.The different impact of P input（low and high P input）under the action of the above three factors was also investigated.Preferential adsorption of P onto Gt and the gradual formation of P-Ca were observed.While the addition of manure caused P release and inhibited P adsorption in the incubation experiments,no similar observation was made in the farmland investigation.Our results suggest that Fe minerals,CaCO₃,and OM（NSM）have different effects on the environmental behavior of P in fluvo-aquic soils over time.During the initial stage of application,Fe minerals control P concentration in pore water through adsorption,thereby facilitating the subsequent formation of P-Ca.Organic fertilizer is an important source of P and its application inhibits P adsorption.In the long-term,however,OM may facilitate P immobilization.Although excess application of P fertilizers in fluvo-aquic soil will lead to gradual formation of insoluble P-Ca,the risk of P loss remains.Phosphorus adsorption to Fe minerals takes time to happen,which can keep the P concentration in the soil solution relatively high.The risk of P loss was found to be more prominent following the application of organic fertilizers.Batch adsorption experiments were conducted to investigate the adsorption behavior of P on ferrihydrite and its colloid.Ferrihydrite colloid is regarded as a carrier to adsorb P and thus impact the distribution and transport of P in soil.Adsorption quality of P on ferrihydrite and its colloid were measured,separately.We also investigated the effect of ironic strength on the distribution of P among solution,ferrihydrite and its colloid.Results showed that adsorption quality of P was increased with the increasing ironic strength.Ferrihydrite colloid showed a lower ironic strength stability,so the proportion of P adsorbed on Fh colloid in low ironic strength was significantly larger than high condition.That means ironic strength indirectly impact P mobilization.Column transport experiments were conducted in glass columns,which were wet-packed with the soil aggregates.Swine manure and Gt have been applied to soil to investigate the effects of iron and OM on P leaching,respectively.After pretreatment,PO₄³⁺solution was introduced and the effluent solutions were continuously collected for analyzing P concentration.The CDE（Convection-Dispersion Equation）model was used to calculate the retarding factors of all soil columns.Results showed that more retention of P in Gt-amended soils.That means iron will immobilize soil P and offer lasting benefits in improving soil P retention.Swine manure can be a considerable P source,which inhibited P retention and increased P immobilization,thus soil applied swine manure is assumed to observe P leaching easily.