Effects of bagged controlled- release fertilizer on grape fine root growth and nitrogen absorption

Abstract: 【Objective】With the continuous promotion of the“double reduction of pesticide and fertilizer”action, slow-release fertilizer has been paid attention to in fruit production because of its high nutrient utilization rate and labor-saving. Slow-release fertilizer can improve nutrient utilization rate. From the perspective of nutrient release and soil nutrient supply, the nutrient release of such fertilizer is slow and the change range of soil nutrient concentration is small after using slow-release fertilizer, compared with using the traditional chemical fertilizer. According to the nutrient demand characteristics of fruit trees, controlled-release fertilizer bagged with paper and plastic materials reduces the cost and achieves good results. It has been popularized and applied in Shandong, Shanxi and other fruit production provinces. Root system is the nutrient absorption organ of fruit trees, especially the new root system. The mechanisms involved in improvement of nitrogen use efficiency via the usage of bagged controlled-release fertilizer were studied from the perspective of grape fine root growth and development with an aim to provide reference for rational fertilization in grape production.【Methods】In field conditions, seven-year old trees of grape Summer black (Vitis vinifera) were taken for the experiment. The experiment used a randomized complete design with three treatments, bagged controlled- release fertilization (BCRF), broadcasting application fertilization (SA) and no fertilization (CK). For root observation,there were 5 replicates in each treatment, while for 15N tracing, there were 3 single-tree biological replicates. Root observation tubes were installed at an angle of 45° 40 cm from the vine trunk in the east direction. Fine root growth dynamics were determined using the minirhizotron technique in three consecutive years. Root images were taken using a CI-600 Root Imager and analyzed using a ROOTSNAP image analysis software. Effects of bagged controlled- release fertilizer on nitrogen utilization of grape were studied with 15N isotope tracer method. Soil samples were taken using a stainless steel soil sampling auger, and the samples were taken to the laboratory for testing. Fine roots were washed from the soil samples and analyzed for root activity.【Results】The soil ammonium and nitrate nitrogen concentration in BCRF was maintained at a high concentration and with little variation. The number of fine roots increased more in BCRF than SA and the control. The number of fine roots was 603, 413 and 326 in BCRF, SA and control, respectively. Fine roots birth showed a typical double sigmoid model with two peaks. The first in spring, the second in autumn, and the autumn peak was higher than the spring peak. Fine roots in SA and the control became pigmented at day 46 and 63, respectively, while those in BCRF at day 71. The median lifespan of fine roots was 238, 196, 140 days in BCRF, SA and control, respectively. The turnover rate of the fine roots in BCRF was markedly lower than in SA and control in 2018. It was 0.61, 1.99 and 1.51, respectively. The turnover rate of the fine roots in BCRF was also markedly lower than in SA and control in 2019, and was 0.35, 0.57 and 0.50, respectively. In 2020, fine root turnover rate in BCRF, SA and control was 0.30, 0.62 and 0.47, respectively. At the end of August, the root activity in BCRF was 54.36 mg·g^-1 ·h^-1 , which was significantly higher than that in SA and control. At the end of December, the root activity in all treatments was low and had no difference. The biomass of current year organs in BCRF was significantly higher than that in SA and control. There was no difference in biomass of perennial organs among treatments. The nitrogen derived from BCRF was significantly higher than that from SA. The nitrogen utilization rate reached 16.69% in BCRF, significantly higher than 8.38% in SA.【Conclusion】The soil Nmin concentration was maintained at a high level with little variation in the growing season with bagged controlled-release fertilizer treatment, which significantly increased the number of fine roots, prolonged their lifespan, reduced the annual fine root turnover rate, and improved the root activity of grape in growing season. The bagged controlled-release fertilizer treatment significantly improved the nitrogen utilization rate of grape by affecting the supply of soil nutrients and changing the growth and development of roots.