Effect of Nitrogen Fertilizer and Irrigation Levels on Yield and Some Physiological Traits of Wheat under Drip Irrigation

Wheat is globally one of the most critical cereals. It is necessary to increase its yield to cope with the increasing population through management improvement or breeding due to decreased arable lands. Soil moisture before planting and rainfall during the growing season are the two primary water supply sources for rainfed wheat production. However, the non-uniform distribution of rainfall during the growing season leads to drought, affecting crop water consumption and natural wheat growth. Irrigation is the primary way to meet the plant's water requirement for growth, development, and high yield. Since water availability is limited in Iran and on the other hand, different cultivars have different sensitivities to drought stress at various stages of growth, so reducing different degrees of water consumption may have unequal effects on crop yield. This type of management, known as deficit-irrigation, often increases water use efficiency. Drip irrigation provides optimized use of water and nutrients during the growing season. In addition to water consumption, the balanced application of fertilizers is an influential factor in increasing agricultural production, and nitrogen is the most critical fertilizer recommended to improve wheat yield. Nitrogen can increase wheat yield by increasing the number of spikes per square meter, the number of grains per spike, and 1000-grain weight. This study aims to determine the optimum water consumption and nitrogen fertilizer under the drip irrigation system according to wheat physiological traits.

This experiment was performed as split plot based on a randomized complete block design with three replications in the Research Farm of the University of Kurdistan located in the Dehgolan plain in 2018-19 cropping year. Factors were in various irrigation levels (60, 80, 100, and 120% of crop water requirement) as the main plots and nitrogen fertilizer treatments (fertilizer application of 50, 75, 100, and 125% of plant nitrogen requirement based on soil test) as subplots. Sampling was done in all three replications to calculate soil weight moisture and determine the irrigation water requirement in each irrigation stage. The soil moisture balance method was used to determine the crop's water requirement according to the volume percentage of moisture in the control plot (treatment of providing 100% water requirement). Based on the soil test results, the optimal nitrogen application in the control treatment was considered equivalent to 200 kg.ha-1 of urea. Other experimental treatments were calculated based on the control treatment. In this experiment, traits such as biological yield, grain yield, harvest index, water use efficiency (WUE), chlorophyll content (chlorophyll a, chlorophyll b, and total chlorophyll), remobilization, grain protein content, protein percent, and agronomic nitrogen use efficiency (ANUE) were evaluated. Data were analyzed using SAS statistical software, and the means were compared using Duncan's multiple range test at 5% probability.

The results showed that the effects of different irrigation and nitrogen levels were significant on biological yield, grain yield (P <0.01), and grain protein percent (p≤0.05). The interaction effect of irrigation and nitrogen was significant on WUE (p≤0.05), chlorophyll content, remobilization, and ANUE (P <0.01). In comparing different irrigation levels, The highest and lowest biological yields were obtained in the treatments of 120% water requirement (15976 kg.ha-1) and 60% water requirement (12975 kg.ha-1), respectively. Among different nitrogen treatments, the highest and lowest biological yields were observed in 125% fertilizer requirement (15141 kg.ha-1) and 50% fertilizer requirement (12640 kg.ha-1), respectively. The highest and lowest yields were observed in the treatments of supply of 120% (6498 kg.ha-1) and 60% (4933 kg.ha-1) of water requirement, respectively. The rate of yield increase in 120% water requirement treatment was 9, 18, and 24%, compared to 100, 80, and 60% of water requirement treatments, respectively. However, the highest WUE was obtained in 60% crop water requirement treatment and providing 100% of nitrogen consumption (3.08 kg.m-3). In water deficit conditions, providing 100% of the plant's nitrogen requirement keeps WUE in high level. If the amount of nitrogen is reduced, WUE was also decreased. The highest and lowest ANUE were observed in 100% water requirement treatment fertilized with 75% of nitrogen requirement (79 kg.kg-1) and 120% water requirement treatment fertilized with 100% nitrogen requirement (9 kg.kg-1), respectively. With decreasing water consumption, the rate of stem remobilization increased to the highest level, which was observed in the treatment of 60% of water requirement fertilized with 100% nitrogen requirement. The highest chlorophyll concentration was related to the 100% crop water requirement treatment fertilized with 100% of nitrogen requirementirement.