Investigating the Response of Rainfed Chickpea (Cicer arietinum L.) to Different Nitrogen Fertilizer Levels in Cold Regions

IntroductionChickpea (Cicer arietinum L.), a key legume of the Fabaceae family, is cultivated as a winter or spring crop, primarily under rainfed conditions, in western and northwestern of Iran. To position chickpea as a viable alternative in cereal-based rotations and enhance its contribution to sustainable agriculture, agronomic practices—especially nutrient management—must be thoroughly optimized and studied.  Nitrogen (N), the most critical macronutrient for plant growth and yield enhancement, is required in greater quantities than any other element. While chickpea fulfills much of its nitrogen demand via biological nitrogen fixation (BNF), a minimal "starter" N dose is essential to ensure robust seedling establishment and meet early growth requirements before effective rhizobial symbiosis is achieved. Precise determination of starter N for rainfed chickpea is critical, as excessive application increases costs, risks environmental contamination, and suppresses yields, whereas insufficient doses compromise productivity. Materials and MethodsA randomized complete block design (RCBD) with three replications was employed during the 2022–2023 growing season to evaluate the N requirements of rainfed chickpea (cv. Ana) under cold rainfed conditions in Bukan (autumn sowing) and Maragheh (autumn and spring sowing). Treatments included five urea levels: 0, 25, 50, 75, and 100 kg ha⁻¹. Following reduced tillage (using a combination tillage implement), chickpea seeds were sown at 40 seeds m⁻². Autumn sowing occurred in October 2023, and spring sowing in late February 2024, using an ASKE 3-shank 11-row planter with 53–17 cm row spacing. Experimental plots measured 10 × 30 m. Urea was applied via subsurface banding (5–6 cm below seeds) at sowing. Measured parameters included root traits (length, volume, dry weight, nodule weight), plant height, 100-seed weight, biological yield, grain yield, harvest index, and rainwater use efficiency. Results and DiscussionEnvironmental effects significantly influenced root length, nodule weight, root volume, dry weight, plant height, biological yield, grain yield, rainwater use efficiency (p ≤ 0.01), and 100-seed weight (p ≤ 0.05). Nitrogen levels significantly affected all parameters (p ≤ 0.01), except 100-seed weight (p ≤ 0.05). Autumn-sown Bukan exhibited superior root traits, plant height, and yields compared to Maragheh. Root length, volume, and dry weight declined with increasing N, likely due to restricted root growth from urea banding at higher doses. Enhanced nodulation in Bukan may reflect its soil’s native rhizobia populations, as Maragheh lacks prior legume cultivation. Elevated soil fertility and favorable climatic conditions in Bukan further supported root development. Excessive mineral N inhibited rhizobial symbiosis, reducing nodule weight. Plant height and 100-grain weight peaked at 50 kg ha⁻¹ N in Bukan (57 cm, 6 cm above control). In autumn-sown Bukan, grain yield plateaued at 25 kg ha⁻¹ N, while Maragheh achieved maxima at 75 kg ha⁻¹ (autumn) and 50 kg ha⁻¹ (spring). The highest harvest index (48%) occurred at 50 kg ha⁻¹ N in spring-sown Maragheh. Starter N bolstered early growth and photosynthetic capacity, enhancing yields. Moderate N optimized harvest index by favoring grain over biomass allocation. Rainwater use efficiency (RUE) was higher in autumn-sown chickpea in Bukan compared to both autumn- and spring-sown crops in Maragheh. Furthermore, RUE initially increased with rising nitrogen application rates but declined at elevated doses. Regression analysis of grain yield against urea application rates revealed maximum achievable yields of 1535 and 940 kg ha⁻¹ for autumn-sown rainfed chickpea in Bukan and Maragheh, respectively. To attain these yields, starter urea doses of 62 and 61 kg ha⁻¹ were required for Bukan and Maragheh. For spring-sown chickpea in Maragheh, the peak grain yield was 835 kg ha⁻¹, achievable with a urea input of 43 kg ha⁻¹. The minimal urea requirements for maximizing net economic returns were 60 kg ha⁻¹ for autumn-sown chickpea in both regions, whereas spring-sown systems in Maragheh required 42 kg ha⁻¹. Simple correlation analyses between grain yield and yield components demonstrated that grain yield showed the strongest positive correlations with biological yield, plant height, and root dry weight, in descending order of magnitude. Similarly, rainwater use efficiency exhibited the highest positive correlations with grain yield, biological yield, and plant height, respectively. The robust correlations between grain yield and these traits suggest that these parameters likely exert direct influences on grain yield. ConclusionsExcessive nitrogen fertilization leads to salinity, toxicity, and disruption of symbiotic relationships, offsetting potential yield benefits. A minimal starter dose of 42 kg ha⁻¹ urea is sufficient for spring-sown chickpea, while 60 kg ha⁻¹ urea maximizes economic returns in autumn-sown systems. Precision in N management is critical to balance productivity, sustainability, and environmental stewardship in rainfed chickpea cultivation.