Effect of soil C/N ratio on nitrogen utilization and growth of apple

Soil C/N ratio is a critical soil nitrogen cycle factor. Two-year old apple trees (Borkh. cv. 'Fuji'/ Malus hupehensis) were used to study the effect of soil C/N ratio 6.21 (CK), 10, 15, 20, 25, 30, 35 and 40 on nitrogen utilization/loss and apple growth using the ¹⁵N trace technique. The results showed that with increasing soil C/N ratio, apple shoot length and fresh weight initially increased and then decreased. The highest apple shoot length and fresh weight were under the treatments of 15, 20 and 25 C/N ratio. Although no significant differences were noted among the three treatments, the shoot length and fresh weight of apple tree under these three C/N ratios treatments were significantly higher than those under the other treatments. Statistical analysis revealed significant difference in nitrogen utilization rate among different treatments. The highest nitrogen utilization rate was in soils with C/N ratio of 25, which value was 22.87%, not significantly different from that in soils with C/N ratio of 20. However, the nitrogen utilization under both the two treatments was significantly higher than those under the other treatments. Soil C/N ratio of 40 showed the lowest nitrogen utilization rate (15.43%), which was even lower than that of CK (16.65%). The proportion of plant absorbed fertilizer nitrogen was much higher under treatments of soil C/N ratio range of 15~25. But proportion of plant absorbed soil nitrogen was much higher under too low (<15) or high (>25) soil C/N ratio. Soil residual nitrogen increased gradually with increasing soil C/N ratio. Soil residual nitrogen under C/N ratio of 40 was 1.32 times that of CK. With increasing soil C/N ratio, fertilizer nitrogen loss initially dropped and then increased. Fertilizer nitrogen loss was minimum (49.87%) under soil C/N ratio of 25 and the maximum (61.54%) under CK. Therefore regarding nitrogen utilization and apple growth, soil C/N ratio range of 15~25 was favorable. It increased soil nitrogen fixation efficiency, reduced nitrogen loss, and enhanced nitrogen utilization rate.