黄土高原旱地苹果园土壤硝态氮剖面特征及氮肥优化管理OA

[Objectives]A comprehensive understanding of soil nitrogen(N)characteristics at a deep-profile scale under long-term excessive N-fertilization is crucial for developing targeted N-fertilization regimes in apple production systems.[Methods]This study was conducted in rainfed apple-producing regions of the Loess Plateau.The objectives were:1)to clarify the vertical characteristics of soil nitrate nitrogen(NO3--N)in the 6-m profile by integrating literature data with field soil sampling and measurement;2)to identify appropriate N application rates and N reduction potential for different apple-planting regions of the Loess Plateau using regression analysis;and 3)to predict the accumulation trend of residual soil NO3--N in the 6-m profile(RSN6-m)under different scenarios.[Results]Across the eastern,southern,western,northern,and central regions of the Loess Plateau,RSN6-m was mainly regulated by fertilizer N application.Both NO3--N concentrations and RSN6-m showed an increasing trend with apple tree age.The stabilized depth of soil NO3--N—defined as the soil depth at which NO3--N contents no longer differ significantly between farmland and orchards—deepened with increasing tree age and reached a maximum of 540 cm,indicating that a 6-m profile is robust for evaluating vertical NO3--N patterns in rainfed apple orchards on the Loess Plateau.For 25-year-old apple orchards,RSN6-m in the northern,eastern,southern,western,and central apple-planting regions reached 4230,7210,6975,12740,and 8760 kg/hm2,respectively.Nitrate accumulation in the 1-4 m soil layer exceeded 50%of RSN6-m,with this proportion increasing with tree age but decreasing with increasing rainfall,indicating that longer apple-planting duration and higher rainfall accelerate NO3--N leaching into soils deeper than 1 m.A significant positive relationship was observed between average NO3--N contents in the 0-40 cm and 0-600 cm soil layers,indicating that soil NO3--N in the 0-40 cm layer can serve as a reliable indicator of orchard soil N supply.The recommended nitrogen application rates for the young-tree stage(0-4 years),initial-fruiting to early full-fruiting stage(5-10 years),early full-fruiting to full-fruiting stage(11-17 years),and full-fruiting to senescent stage(≥18 years)were 135,270-390,390-420,and 420 kg/hm2,respectively.Integrated analysis indicated that apple orchards aged 17-30 years possess a reduction potential of 2-8 years without N fertilizer application while maintaining yield.Under both orchard reconstruction and tree-retention scenarios,conventional farmer N application rates exacerbated RSN6-m accumulation,whereas recommended N-fertilization regimes potentially maintained RSN6-m at current levels.Combining residual soil NO3--N reuse with recommended N application rate can reduce RSN6-m and maintain it at the level observed in 17-year-old orchards.Groundwater NO3--N contents in two typical rainfed apple-planting regions of Luochuan and Liquan Counties were lower than the World Health Organization(WHO)limit of 11.9 mg/L,indicating a relatively low groundwater NO3--pollution risk despite a surplus of NO3--N being observed.[Conclusions]Long-term excessive N application caused a massive surplus of soil NO3--N,subsequently resulting in soil acidification,NO3--pollution risk of groundwater,and greenhouse gas emissions,ultimately leading to increasingly severe problems of soil degradation and environmental pollution in the rainfed production system on the Loess Plateau.Future research should prioritize strategies for the effective reuse of residual soil NO3--N in deep profiles of apple orchards on the Loess Plateau.