[Effects of different fulvic acid application rates on fruit yield, quality and fate of N-urea in 'Red General' apple].

To explore the effects of different fulvic acid application rates on N-urea absorption, utilization, residue amount, loss and fruit yield and quality, fifteen-year-old 'Red General'/ Malus micromalus Makino was selected as experiment material by using the N-labeled tracer technique. Five treatments were established: single urea (CK), combination of urea and fulvic acid with varied rates (NF, NF, NF and NF represented 75, 150, 300 and 450 kg·hm fulvic acid application rates, respectively). The main results were as follows: the N derived from fertilizer (Ndff) in root, one-year-old branch and leaf were displayed with an order of NF>NF>NF>NF>CK at the fruit maturity stage, and significant difference was observed in Ndff value among the five treatments. The total N content and N absorption amount were highest in NF, followed by NF, and lowest in CK. Compared with CK, the N utilization rate in NF, NF, NF and NF was increased by 14.2%, 33.5%, 64.2% and 50.0%, and the N loss rate was decreased by 9.1%, 18.5%, 37.1% and 28.7% respectively. The residue amount of N-urea varied across different treatments, and that in 0-60 cm soil layer under combination of urea and fulvic acid was significantly higher than CK, with the highest value occurred in NF, while it was lower than CK in 60-100 cm soil layer. In comparison with CK treatment, the largest increment of yield per plant and net income appeared in NF, with an enhancement of 15.8% and 20.2%, respectively. In addition, the fruit hardness, soluble solid and sugar-acid ratio reached the highest value in NF. Considering the responses of fruit yield and N utilization efficiency, the suitable fulvic acid application rate was 326.41-350.61 kg·hm.