Effects of type and amount of nitrogen fertilizer on photosynthetic characteristics and yield of jujube under drip irrigation

Abstract: In the main jujube-producing areas, alkaline sandy soils greatly inhibit the storage of water and nitrogen in soils, which would reduce the nitrogen utilization, photosynthesis, product quality and yield of jujubes in the southern Xinjiang of China. The high loss of water and nitrogen fertilizer in large quantities cannot ensure the nutrient supply of plants, while resulting in leaching pollution for groundwater and soils. In order to find a better fertilization method, a field experiment was conducted on the jujubes in the Aksu forest farm from 2015 to 2017. In the experiment, there were three types of nitrogen fertilizer, (conventional urea treatment (U), conventional urea treatment with nitrification inhibitor N-max (NU) and coated urea treatment (CRU)), and five nitrogen supply levels (N0: 0 kg/hm2, N1: 150 kg/hm2, N2: 200 kg/hm2, N3: 250 kg/hm2, and N4: 300 kg/hm2). The leaf Chlorophyll values (Soil and Plant Analyzer Development, SPAD) and Net Photosynthetic Rate (Pn) were measured in the period of flowering and developing fruit, and the soil available nitrogen was measured all the time. The product quality and yield of jujubes were also evaluated after harvesting, as well the contribution rate of nitrogen and the benefit of jujubes. The results showed that the average yield (10022.3 kg/hm2) and the optimal fruit rate (12.1%) of jujubes with the coated urea were significantly higher than those with other nitrogen fertilizer (P<0.05), indicating a better performance on the nitrogen controlled-release effect. Meanwhile, the nitrogen supply capacity, the SPAD and Pn of jujubes significantly increased after the coated-urea treatment, when the same nitrogen level was used in all treatments in the developing fruit period of jujubes (P<0.05). In the treatments that used urea as nitrogen fertilizer (U and NU), the addition of nitrification inhibitor had no effect on the nitrogen utilization, while reducing the proportion of the inferior rate of fruits. The coated urea showed the highest partial nitrogen productivity, followed by the conventional urea with nitrification inhibitor. The content of available nitrogen in soil was the highest at all nitrogen levels treated by U during the flowering period of jujubes, whereas the content of available nitrogen in soil under CRU was always higher than that under other treatments during the fruit growth period of jujubes. The contribution rate of nitrogen yield increased as the increase of nitrogen apply. When the application amount of nitrogen fertilizer was set at the N3 level, the maximum can be achieved in the contribution rate of yield and the output of red jujubes. The production was the highest in the treatment CRU when the nitrogen utilization amount was 250 kg/hm2. Balancing the quality and yield of jujubes, the 250 kg/hm2 nitrogen level of the coated urea can be selected as the optimal combination under the typical drip irrigation for the cultivation mode of jujube orchards in the southern Xinjiang. The findings can provide a promising potential for the nitrogen management of jujube orchards, to improve the quality and efficiency of jujube industry in the areas of southern Xinjiang.