Study On Distribution Characteristics Of Nitrogenand Phosphorus In Soil Profile And Their Regulatingfactors In Apple Orchards Of China’s Loess Plateau

China’s Loess Plateau occupies 24.4%of the world’s apple harvested area and produces27.0%of the world’s apple production,while apple yield on the Loess Plateau is lower than the national and global average.Economic benefits drive long-term over-fertilization in apple orchards of China’s Loess Plateau.However,studies on the distribution patterns of NO₃^--N and Olsen-P in deep soil profiles and their driving forces in apple production systems are scarcely documented.In this study,we elevated the characteristics of NO₃^--N and Olsen-P in 6-m soil profiles in apple orchards,and identified their driving forces,and ultimately provide data support for optimizing nitrogen and phosphorus management in apple production system on the Loess Plateau.The main results are as follows:（1）Similar distribution patterns were observed for soil moisture in the 0-600 cm profiles between farmland and 8-yr apple orchard,and soil moisture contents within the depths of 0-100 cm,100-260 cm and 260-600 cm were characterized with increase,decrease and stable trends,respectively.The amounts of soil water storage within the 0-600cm soil profiles in farmland were equivalent to that in soils of 8-year-old apple orchard.By contrast,soil moisture content in 17-yr and 25-yr apple orchards continuously decreased in the 100-600 cm layer,and their observations in the 260-600 cm layer were significantly lower than those in farmland and 8-yr apple orchards.Consequently,the amounts of soil water storage in 17-yr and 25-yr apple orchards decreased by 140 mm and 150 mm,respectively,when compared with farmland.Regression analysis revealed that the soil moisture in the 300-600 cm layer rather than the 0-300 cm layer was affected by stand age of apple tree.Changes in soil water storage of 0-600 cm soil depth was primarily associated with the 300-600 cm layer because of their similar significant relations to stand age.（2）In the rainfed apple-planting region,no significant difference was observed for NO₃^--N across the 0-600 cm soil profile between 8-yr apple orchard and farmland,and soil NO₃^--N in 25-yr orchards were appreciably higher in 80-540 cm soil layer than that in farmland,and reached the maximum value of 214 mg kg^-1in the 120-cm soil depth.By contrast,soil NO₃^--N of 25-yr orchards in irrigated region exceeded the value of 100 mg kg^-1across the 0-800 cm soil profile,and the peak value of 262 mg kg^-1was observed in the380-cm depth.Notably,the 140-600 cm layer of 25-yr orchards exhibited significantly higher NO₃^--N values in irrigated region than that in rainfed region.Regression analysis showed that residual soil nitrate in the 0-600 cm soil profile and the residual ratio of NO₃^--N significantly increased with the accumulative amount of applied N fertilizer,which could be well-fitted by a quadratic model,indicating that nitrate is the major fate of N fertilizers applied in rainfed apple orchards.In addition,the non-significant differences in NO₃^--N among farmland,8-yr,17-yr and 25-yr apple orchards implied that 6-m soil profile is sufficient for evaluating dynamics of NO₃^--N in rainfed apple production systems.（3）Land-use types including farmland,8-yr,17-yr and 25-yr apple orchards significantly affected Olsen-P at 0-40 cm soil,with the concentration generally increasing with age of apple orchard.By contrast,Olsen-P at 40-600 cm was not significantly different among the cropping systems.In both farmland and apple orchards,the concentration of Olsen-P decreased at 40-100 cm,compared with that in the 0-40 cm layer,increased with depth from 100 to 400 cm and then stabilized from 400 to 600 cm.Residual soil Olsen-P in the 6-m profile significantly increased with stand age of apple tree,while the significant differences in residual soil Olsen-P were only observed in the 0-100 cm depth.Regression analysis revealed that the vertical distribution of soil Olsen-P was regulated by P fertilization at 0–100 cm and by crop roots at 100–600 cm,irrespective of cropping system.The change point in soil Olsen-P against P leaching was 71.2 mg kg^-1,but in farmland and 8-yr,17-yr,and 25-yr apple orchards,the averaged-depth contents of Olsen-P were all below the critical value,indicating no P leaching risk.Notably,the 80-100 cm soil depth of irrigated 25-yr apple orchards presented a peak value of 14.5 mg kg^-1,which was appreciably higher than that of rainfed 25-yr orchards.Overall,long-term apple cultivation could lead to soil desiccation in the 0-600 cm profile,NO₃^--N accumulated in deep soil layers,and excessive surplus of Olsen-P in topsoil.Irrigation potentially enhanced the loss risks for NO₃^--N and Olsen-P leaching into deep soil depth.