Effect Of Biochar Application On The Transportation Behavior Of Phosphorus In Soil Of Cold Region And Saturated Porous Media

The black soil area located in the northeast of China is an essential grain production base,and its fertile soil is an extremely valuable non-renewable resource.However,in recent years,the excessive application of phosphorus fertilizer has caused the excessive accumulation of phosphorus in the soil,while the irrigation methods,climatic conditions and other factors result in the serious loss of phosphorus.Some of these phosphorus in the soil occurred downward transportation behaviors,thus caused pollution of groundwater environment.In addition,the frequent occurrence of freeze-thaw events in typical of cold regions in the Northeast of China led to the destruction of soil structure,redistribution of soil water,and further loss of soil phosphorus,reducing soil fertility.In recent agricultural research,biochar,as a new exogenous soil additive with excellent water and fertilizer retention characteristics,has been widely applied.Therefore,the present study will be of ecological importance to investigate the mechanism of phosphorus transportation and regulation in soils and groundwater in cold regions through the application of biochar.In this paper,indoor soil column experiments with different freeze-thaw cycles were conducted on soils from the Xiangyang Experimental Base of Northeast Agricultural University to investigate the effects of different biochar application rates on the physical and chemical properties of soils in cold regions and the transport of phosphorus,and to investigate the effects of five physical and chemical properties on the phosphorus transportation in saturated porous media by using a quartz sand column experiment to simulate the groundwater environment and combining with a two-site model.The effects of five physicochemical properties on the transport of phosphorus in saturated porous media were investigated and fitted.The results of the study are as follows:(1)Biochar application changed the soil p H,organic carbon and organic matter content.The soil p H,organic carbon and organic matter content gradually increased with the increase of biochar application and reached the highest value at 2% biochar addition.Moreover,with the increase of freeze-thaw cycle,soil p H and organic carbon showed a trend of gradually increasing,and soil organic matter showed a trend of first increasing and then decreasing.It peaked around 7-15 days out of a 60-day freeze-thaw cycle.(2)The application of biochar changed the transportation pattern of soil available phosphorus and increased the content of soil available phosphorus.When biochar was not applied,available phosphorus content of the soil at 0-3 cm showed a gradual decrease,and the maximum available phosphorus content appeared at the surface 3-4 cm after 60 days of freeze-thaw cycles.When biochar was applied,soil phosphorus loss was significantly reduced,and all layers of soil available phosphorus content were higher than that of soil without biochar application.Correlation analysis and structural equation modeling showed that soil p H,organic carbon content,organic matter content and available phosphorus content all showed positive correlations with biochar application,and biochar not only directly influenced the content of available phosphorus,but also indirectly influenced available phosphorus through soil organic carbon.Cluster analysis showed that whether biochar was applied or not would cause large differences in soil physicochemical properties,and the differences between 1-2 cm and 19-20 cm soils were obvious with the increase of freeze-thaw cycle.In the comprehensive soil fertility evaluation,the first principal component was the combined reflection of biochar application,soil p H,organic matter,and organic carbon,the second principal component was the effect of soil depth and available phosphorus,and the third principal component was the effect of freeze-thaw cycle.(3)In saturated porous media,the immobilization of phosphorus increased with the increase of biochar addition ratio.The high ionic strength in the solution also increased the fixation of phosphorus on the surface of biochar.p H increase tend to precipitate phosphorus and made it difficult to break through in saturated porous media.Monovalent cations could immobilize more phosphorus on the surface of biochar relative to divalent cations,while quartz sand particle size had no significant effect in the transport of phosphorus.The above research results showed that biochar application had a significant effect on the phosphorus transport process and reduced the loss of available phosphorus from cold zone soils and increased available phosphorus content of soils.In addition,biochar application in saturated porous media also increased phosphorus retention and avoided phosphorus transport to deep groundwater.The results of the study provide a theoretical basis for the efficient use and transport regulation of phosphorus in soils of cold regions of Northeast China,and also provide technical support for the analysis of environmental effects of phosphorus in soil ecosystems.