Effect of Humic Acid and Cattle Manure on Stomatal Conductance, Photosynthesis, and Growth Variables of White Mulberry Seedlings in the Nursery Condition

Extended Abstract Background: In contemporary times, the increasing population growth in large urban areas, industrial development in these cities, and the need for expanding urban green spaces and green belts have drawn the attention of managers to the use and cultivation of multipurpose tree species more than ever before. In many advanced countries, the use of chemical fertilizers has been discontinued in urban green spaces, parks, agriculture, and horticulture. The major reasons for this include undesirable environmental effects, disruption of soil chemical balance and structure, disruption of the ecology of plant and animal communities, and pollution of freshwater bodies. For this reason, the utilization of various organic fertilizers has been highlighted as one of the important strategies in improving horticultural and agricultural production and reducing the risk of environmental pollution, especially pollution of surface and groundwater resources. Humic acid (HA) is among the main types of organic fertilizers and a type of stabilized organic acid derived from the main compounds of humus, considered one of the most abundant organic compounds in nature. The use of HA for plants can directly and indirectly promote better plant growth, thereby increasing production levels and improving crop quality. Therefore, the present study aimed to assess the possibility of enhancing the growth and viability components of white mulberry seedlings as the most commonly used tree species for planting in the green belt of Mashhad using HA fertilizer and animal manure. Methods: For this research, two-year-old and uniform-sized mulberry seedlings were prepared as bare-root. Then, they were transferred to prepared beds in the green belt nursery of Mashhad for planting in pits with planting distances of 1.5 × 1.5 m in a completely randomized design. In five separate experimental plots, the nursery soil was thoroughly and uniformly mixed with animal manure. Subsequently, HA fertilizer in liquid form (at 50 and 300 mg/l) was mixed with the soil in two plots in addition to the animal manure. In the other two plots, foliar spraying at 50 and 300 mg l-1 on the leaf surfaces of the seedlings was carried out at the beginning of the growing season. Only animal manure treatment was considered for the seedlings in the fifth plot. Ten seedlings were selected for each treatment as replicates for examination. The seedlings were irrigated uniformly throughout a six-month growing period