Features Of Soil Phosphorus Distribution And Phosphorus Fractions Of Arable Loess In Guanzhong Plain Under Winter Wheat Summer Maize Cropping System

In general,the soil available phosphorus?P?content is very low,which limits plant growth and development.Application of P fertilizer is the standard measure to meet crop requirements,but P resource is limited.Therefore understanding P cycling characteristics in soil-plant systems is of great significance.This study investigated the contribution of inorganic P fractions to soil available P under equivalent soil organic matter content,the changes of soil Olsen-P contents,and the distribution of soil P in the past 30 years in Guanzhong Plain,Shaanxi province.The main results achieved were followed:1)In the past 30 years,the soil Olsen-P content in the farmland of Guanzhong Plain has increased.The mean value of soil Olsen-P content in Baoji,Xianyang and Weinan Countries was 26.09 mg kg^-1,27.50 mg kg^-1 and 21.53 mg kg^-1,respectively.The proportion of soil Olsen-P level both increased,with the largest increase of15-30 mg kg^-1 and a decrease of<10 mg kg^-1.At present,the soil Olsen-P content is higher than the agronomic threshold,which can meet the phosphorus supply during high yield of crops in the large areas of Guanzhong Plain.Meanwhile,the cumulative distribution probability of soil Olsen-P content higher than 37 mg kg^-1 both in Baoji and Xianyang countries is more than20%,and the water-soluble P content higher than 2 mg kg^-1 both in Baoji and Xianyang countries is more than 22%.There is a high risk of water environmental pollution.2)The calcium bonded phosphorus fraction was a dominant form in cultivated soil.They were accounting for 66.67%of the total amount of inorganic phosphorus.Soil Olsen-P was increased significantly and linearly with the increasing content of Ca₂-P,Ca₈-P,Ca₁₀-P,Al-P,Fe-P and O-P;while the phosphorus activation coefficients,defined as the ratio of soil Olsen P to total P,were correlated significantly positively and linearly to the content of Ca₂-P,Ca₈-P,Al-P,Fe-P and O-P.The direct effects of the forms of soil inorganic phosphorus on available phosphorus?direct path coefficient?were compared as follows:Ca₂–P?0.974?>Al-P?0.186?>Ca₈-P?0.182?>Fe-P?0.150?>Ca₁₀-P?0.007?>O-P?-0.074?.The contribution of inorganic phosphorus to phosphorus activation coefficient?PAC?was Ca₂-P?0.768?>Al-P?0.082?>Ca₈-P?0.071?>Fe-P?-0.018?>Ca₁₀-P?-0.055?>O-P?-0.388?,which was consistent with the contribution of soil P component to available phosphorus.Results of the stepwise regression analysis showed that Ca₂-P and Ca₈-P were the major two contributors to Olsen P,but the Ca₂-P contributed the most to PAC.