Study Of The Characteristics Of Soluble Phosphorus Diffused From Soils And Its Plant Availability Using The DGT Technique

Due to the lag of scientific fertilization guidance behind the agricultural production,over fertilization is prevalent in many areas.Thus,a large amount of phosphorus（P）has been accumulated in soils.Accurate evaluation of P availability in soils is the basis for scientific management of soil P.Diffusive gradients in thin films（DGT）technology can reflect the bioavailability of some elements in soils more accurately than traditional methods.If this method can accurately evaluate available P in soils,the technology of soil P management can be further improved.In this study,80 representative soil samples from the Loess Plateau were used to evaluate the status of soil P pool and its availability under the condition of imbalanced fertilizer input for long period.The relationship between DGT P and different P pools was studied.The effects of different conditions（air drying process,diffusive gel thickness and deployment time）on DGT P were investigated.A pot experiment was conducted to compare the accuracy of DGT technology with traditional Olsen P method in predicting plant availability of soil P.The main results and conclusions were as follows:The distribution of soil P was imbalanced in the Loess Plateau area,P rich samples are from farmland,and P availabilitywas low in most soils.Activating the unavailable P in soil is the main target of scientific management of P in the Loess Plateau.The proportion of total inorganic P in the soil to total P is in the order of farmland>garden>forest land>grassland.Forestland and grassland soil have no or less supplement of exogenous P,the percentage of Ca₂-P and Ca₈-P with higher activity in soil inorganic P is significantly lower than that of farmland soil,and Ca₁₀-P with lower activity is significantly higher than farmland soil.The average DGT P content in the soil was 41.30μg L^-1.Soil DGT P was significantly correlated with total P,water-soluble P,Colwell P and Olsen P.Path analysis showed that the direct contribution of Ca₂-P to DGT P was the largest,and the direct effect of Fe-P and Al-P on DGT P was negative.Ca₂-P is a direct source of P for soil DGT P.Ca₈-P,Fe-P,Al-P and O-P are effective sources of supplementation.Ca₂-P and Fe-P can regulate soil available P by affecting other forms of P.The effect of soil sample drying process on Olsen P was negligible,while the DGT P results varied widely.The results of DGT P before/after air drying satisfy the equation:DGT P_moist=-0.41+0.80×DGT P_dry.The content of organic P in soil is probably the main factor affecting the difference of soil DGT P before and after air drying.The air-drying process of soil samples converts the easily mineralized organic P into inorganic P in the soil,which is measured as part of inorganic P.At the same deployment time,the larger the thickness of diffusive gels,the larger the C_DGT.Under the same thickness of diffusive gel,the effect of deployment time on soil DGT P results was affected by soil P content.The soil DGT P with higher P content changed gently with deployment time,and the soil DGT P with lower P content decreased sharply at short deployment time（≤6 h）and then stabilized.The DGT P measured at short deployment time is soluble P diffused in the soil solution.As the deployment time increases,DGT P determines the soil P pool gradually close to the exchangeable P pool in the soil（Olsen P）.The pot experiment showed that the traditional method and DGT technique in the soil without fertilization had a poor prediction effect on the P uptake by wheat.There was a moderate correlation relationship between P uptake by wheat root and DGT P in long deployment time（18～36 h）and Olsen P（P<0.05）,and the relationship between DGT P in soil with shorter deployment time（≤12 h）and water-soluble P was not significant.There was a significant or extremely significant correlation between the amount of P in the maize shoot and soil DGT P in the long period（12～36 h）,and the soil available P was not significantly meadured by the traditional method.As the deployment time increases,soil DGT P is getting closer to Olsen P,but not equal to Olsen P.The relationship between P uptake in wheat shoot and Olsen P was the highest,followed by soil DGT P（0.78 mm,24 h）.The relationship between P uptake in wheat shoots and soil Olsen P（R²=0.762）was also better than that of soil DGT P_{0.78 mm,24 h}（R²=0.752）method.However,at 24 h deployment time,the correlations between DGT P（0.78 mm,1.94mm）and root P uptake were higher than that of the traditional Olsen P method.DGT technology predicts plant availability of soil P varies between crop types.In summary,the DGT technology is feasible for evaluating the plant availability of soil P.Compared with the traditional Olsen method,DGT can more accurately simulate the dynamic processes of plant roots absorbing P from soil.It is a promising method for determination of available P in soil,and its accuracy depends on the specific crop type and growth environment.