Effects of nitrogen fertilizer levels on soil enzyme activity and fruit quality of sand-covered apple orchard in Loess Plateau of Eastern Gansu

Abstract: Large-scale application of nitrogen fertilizer has made a great contribution to improving crop yield. However, excessive nitrogen application is a common and serious problem in orchards, not only reduces nitrogen use efficiencies of trees, but also causes potential environmental problems. Accordingly, reasonable nitrogen application is an important strategy for increasing yield and maintaining a sustainable agroecosystem. The objective of this research was to elucidate the variation regularity of soil enzyme activity and fruit quality in apple orchard, and their correlations were analyzed, so as to provide a scientific basis for reasonable nitrogen application, enhancing nitrogen utilization rate, and reducing the environmental pollution. In this study, a field experiment was conducted with 'Nagafu 2', and five nitrogen fertilizer treatments (Control: CK, 165 kg/hm2: N1, 330 kg/hm2: N2, 495 kg/hm2: N3, and 660kg/hm2: N4) was established in 2015. The soil total nitrogen content, soil enzyme activity, and fruit quality were measured in 2017. The results showed that the soil total nitrogen content was enhanced with increasing nitrogen application in the same soil depth, and yet it decreased with increasing of soil depth. It suggested that the effect of sand-covered on fertilizer conservation was obvious and the utilization of nitrogen fertilizer was improved. In soils at depth of 0-20 cm and >20-40 cm, the maximal activities of soil urease and phosphatase occurred under N2 treatment, while the maximal activity of invertase occurred under N3 treatment. The soil catalase activity was higher with increasing of nitrogen application in >40-60 cm soils. Compared with the control, the fruit horizontal diameter, soluble solid, titratable acidity, vitamin C, and total sugar content increased significantly when the nitrogen application rate was 330-495 kg/hm2 (P<0.05). With the increasing of nitrogen application, fruit yield decreased first and then increased. Excessive fertilization could significantly increase fruit yield (P<0.05). The soil urease, invertase, and phosphatase activities, as well as most quality index of apple were suppressed when nitrogen fertilizer application exceeded 495 kg/hm2. Significant (P<0.05) or remarkably significant (P<0.01) correlation was found between activities of soil catalase (except > 20-40 cm soil depth) and invertase and contents of soluble solid, titratable acidity. It is noteworthy that three kinds of soil enzyme activies have a significant negative correlation with soluble solid (P<0.05). Urease activity was significantly correlated with fruit yield (P<0.05), but was no significantly correlated with invertase activity (P>0.05). Therefore, invertase activity could be used as an indicator of soil fertility in sand-covered apple orchard. All of these results demonstrated that the optimal nitrogen fertilizer application could significantly (P<0.05) enhance soil enzyme activities and nitrogen utilization efficiency, and then improved fruit quality and yield in sand-covered apple orchard. According to the relationship between soil enzyme activity and apple fruit quality, the optimal nitrogen fertilizer application should be 330-495 kg/hm2. It could create a good soil biochemical environment for stable and high yield of apple orchards, and could also further improve the health of agricultural ecosystem.