Effects of excessive nitrogen application on orchard soil and tree mineral nutrients and fruit quality of ponkan

ObjectiveTo supply reference for applying nitrogen fertilizer scientifically in citrus production,this study revealed the effects of excessive nitrogen application on soil of ponkan orchards,mineral nutrients in ponkan tree and fruit quality.MethodUsing eight-year-old Ota ponkan(Citrus reticulata Blanco cv.Ponkan)trees as materials,they were treated by excessive nitrogen application continuously.The pokan seedlings treated by ordinary fertilization were as control(CK).Variation of mineral nutrients in soil and pokan seedlings,chlorophyll contents in leaf and fruit quality were monitored.ResultThe result showed that soil pH in orchard with excessive nitrogen application was greatly lower than that optimum value(5.50-6.50),and other mineral nutrients were also affected at different extents.At fruit mature stage (195 d after flowering),the organic matter content in soil with excessive nitrogen application was 15.42 g/kg,significant-ly higher than that in CK soil 9.50 g/kg(P<0.05,the same below).Available nitrogen in treatment soil was 323.92 mg/Kg, higher than optimum value(100.00-200.00 mg/kg)and significantly higher than CK(91.75 mg/kg).With excessive nitro-gen application treatment,leaf nitrogen content was 3.12% at fruit mature stage,significantly higher than 2.62% in CK;other mineral nutrients in ponkan leaves were also changed at various levels. Meanwhile,total chlorophyll in leaf in-creased significantly,and it rose by 73.3% compared to CK 165 d after full bloom.Nitrogen in fruit was also increased, the pokan had rough peel and large-size fruits after the fruit grew,and sucrose content in fruit also increased slightly. Cor-relation analysis suggested that there were significant positive correlations between soil and leaf nitrogen contents and fruit peel thickness,as well as between fruit nitrogen content and citrate content.ConclusionExcessive nitrogen application breaks the balance of other nutrients in soil,trees and fruits in ponkan orchard.It also leads to chlorophylls content in-crease,photosynthate accumulation and ponkan fruit with rough peel and larget-size fruit.Therefore,nitrogen fertilizer should be used properly and scientifically in ponkan production.