Effects of Soil C/N Ratio on Apple Growth and Nitrogen Utilization, Residue and Loss

Soil C/N ratio is an important influencing factor in soil nitrogen cycling. Two-year old apple trees (Borkh. cv. ‘Fuji’/ Malus hupehensis) were used to understand the effect of soil C/N ratio [6.52 (CK), 10, 15, 20, 25, 30, 35 and 40] on apple growth and nitrogen utilization and loss by using 15N trace technique. The results showed that, with the increasing of soil C/N ratio, apple shoot length and fresh weight increased at first, and then decreased; the higher apple shoot length and fresh weight appeared in C/N=15, 20 and 25 treatments, and there were no significant differences among these three treatments, but significantly higher than the other treatments. Statistical analysis revealed that there was significant difference in nitrogen utilization rate between the different treatments, the highest N utilization rate was occurred in soil C/N=25 treatment which value was 22.87%, and there was no significant difference between soil C/N=25 and C/N=20 treatments, but both the two treatments were significantly higher than the other treatments; Soil C/N=40 had the lowest N utilization rate which value was 15.43%, and this value was less than CK (16.65%). The proportion of plant absorption nitrogen from fertilizer was much higher when the value of soil C/N ratio in the range of 15-25, but the percentage of plant absorption nitrogen from soil was much higher when the soil C/N ratio was too low (<15) or high (<25). Amount of residual nitrogen in soil increased gradually with the soil C/N ratio increasing, the amount of residual nitrogen in C/N=40 treatment was 1.32 times than that in CK. With the increasing of soil C/N ratio, fertilizer nitrogen loss decreased at first, and then increased, fertilizer nitrogen loss was the minimum in C/N=25 treatments (49.87%) and the maximum were occurred in CK (61.54%). Therefore, regarding the apple growth and nitrogen balance situation, the value of soil C/N ratio in the range of 15-25 would be favorable for apple growth and could increase effectively nitrogen fixed by soil, reduce nitrogen loss, and improve the nitrogen utilization ratio.