Nutrient Surplus And Environmental Effects In Major Apple Producing Areas On The Loess Plateau

Since the early 1980s,significant changes in land-use patterns have occurred in the Loess Plateau,primarily due to crop structure adjustment.Because of their great economic worth,large cereal sections have been transformed into cash crops（e.g.,fruit trees and vegetables）.At present,the Loess Plateau has become China’s greatest apple-producing region,with apple area and yield leading worldwide.Excess fertilizer（especially nitrogen fertilizers）inputs resulted in a substantial surplus of agricultural nutrients as the area of apple orchards increased.The Loess Plateau’s large-scale apple development began in the irrigated region to the south（represented by Liquan County）,with a narrow vadose zone.It is essential to investigate how cropland nutrients move and accumulation after a change from cereals to orchards and how that changes groundwater.As a result,we chose Liquan County,a major apple producing areas in the south of the Loess Plateau,China,to examine cropland nutrient balance changes from 1981 to 2019,and then investigated the fertilizer application status of apple orchards in different regions and collected soil profile samples in different regions to study the spatial variation of soil nitrate accumulation in different regions apple orchards.The groundwater quality in various parts of the study area was regularly monitored to assess the impact of apple industry development on groundwater quality.The following are the primary conclusions:（1）From 1981 to 2019,the nutrient budget of Liquan County was estimated using the balance technique.According to the findings,the area of apple orchards has expanded 15times in the last 40 years,from 2347 ha in 1981 to 37857 ha in 2019.In addition,the proportion of cultivated land has gone from 5.7%to 67.2%.Since 1995,the acreage of apple orchards has outgrown cereal land.Between 1981 and 2019,the nitrogen,phosphorus,and potassium fertilizer（calculated by N,P₂O₅,and K₂O,respectively）intake in the Liquan County cropland soil rose dramatically,resulting in an annual surplus of 1310,588,and 845 t,respectively.The biggest contributor is fertilizer nutrient inputs.The contribution ratios were79.1%,85.4%,and 65.3%,respectively,while nutrient utilization efficiency（output/input）decreased by 0.96%,1.60%,and 5.08%.According to the findings,converting land use from cereal to high-input apple orchards resulted in a considerable nutrient surplus in county cropland,and consideration should be given to potential environmental pollution risk.（2）Fertilization investigation and soil profile samples analysis of apple orchards in the Liquan County revealed differences in nutrient input in southern,central,and northern apple orchards,with nitrogen（N）application rates of 1060±473,955±471,and 786±293 kg N ha^-1,phosphorus（P₂O₅）application rates of 544±302,479±318,and 372±255 kg P₂O₅ ha^-1,and potassium（K₂O）application rates were 663±356,557±339,and 490±297 kg K₂O ha^-1,respectively.The nutrient intake of an irrigated apple orchard was substantially greater than that of a rainfed orchards.The mean nitrate accumulation in the 0–800 cm soil profile of southern,central,and northern apple orchards was 16723,11554,and 6075 kg N ha^-1,respectively,substantially greater than that of cereal(443 kg N ha^-1).In the last 40 years,the predominat fate of surplus N has been nitrate accumulation in the soil profile.It is estimated that nitrate storage in 0–8 m soil layers of cropland in the Liquan County was 517.8 Gg N,with more than 73%accumulating in 2–8 m layers,indicating that long-term irrigation in irrigated apple orchards caused nitrate migration to the deep vadose zone.Consequently,it makes crops challenging to use effectively and poses a significant threat to groundwater quality.（3）The analysis of nitrate concentrations in groundwater in the Liquan County（4 times from July 2020 to October 2021）revealed that there were differences in groundwater chemical parameters in different regions,with the average contents of EC,Na⁺,Ca²⁺,Mg²⁺,Cl^-,SO₄^2-,HCO₃^-,and NO₃^-being all in the south>central>north.Besides a significant positive correlation between nitrate content in groundwater samples and EC value,with an average content of 16.7 mg N L^-1,the nitrate content of 52.0%of groundwater samples in the study area exceeded the WHO drinking water standard(11.3 mg N L^-1).The southern area consisted of all shallow wells with an exceeding standard rate of 59.3%,the central area had a rate of 53.3%,and the northern area consisted of all deep wells with no surpassing phenomenon.The isotopic tracing results demonstrate the nitrate source and chemical and organic fertilizer application in the shallow groundwater of an orchard in the southern region.The finding shows that the development of the apple industry in the Liquan County had a significant impact on groundwater quality,with the southern part of the county being a groundwater nitrate vulnerability region.In conclusion,the transition in land use from cereal lands to apple orchards in the Liquan County over the last 40 years has resulted in a substantial surplus of nutrients,with a large quantity of N surplus accumulated in the soil profile as nitrate.Irrigation and rainfall accelerated nitrate movement and leaching into the soil,threatening groundwater quality.As a result,it is critical to restrict fertilizers and irrigation water input in the Liquan County to reduce soil N accumulation and loss and ensure apple production’s healthy and sustainable development in this region.