Nitrification Inhibitor 3,4-Dimethylpyrazole Phosphate Application During the Later Stage of Apple Fruit Expansion Regulates Soil Mineral Nitrogen and Tree Carbon-Nitrogen Nutrition, and Improves Fruit Quality.

In order to solve the problems of nitrogen (N) losses and fruit quality degradation caused by excessive N fertilizer application, different dosages of the nitrification inhibitor, 3,4-dimethylpyrazole phosphate (DMPP) (0, 0.5, 1, 2, and 4 mg kg soil), were applied during the later stage of 'Red Fuji' apple ( Borkh.) fruit expansion in 2017 and 2018. The effects of DMPP on soil N transformation, carbon (C)-N nutrition of tree, and fruit quality were investigated. Results revealed that DMPP decreased the abundance of ammonia-oxidizing bacteria (AOB) gene, increased the retention of NH -N, and decreased NO -N concentration and its vertical migration in soil. DMPP reduced N loss rates and increased N residual and recovery rates compared to the control. C and N double isotope labeling results revealed that DMPP reduced the capacity of N absorption and regulation in fruits, decreased N accumulation in fruits and whole plant, and increased the distribution of C from vegetative organs to fruits. DMPP increased fruit anthocyanin and soluble sugar contents, and had no significant effect on fruit yield. The comprehensive analysis revealed that the application of 1 mg DMPP kg soil during the later stage of fruit expansion effectively reduced losses due to N and alleviated quality degradation caused by excessive N fertilizer application.