Introduction
Chickpea (Cicer arietinum L.) serves as a vital source of dietary protein and plays a crucial role in sustainable crop rotations within the Iranian agricultural system. Its productivity in dryland conditions is predominantly constrained by suboptimal sowing dates and inadequate nutrient management, leading to significant yield gaps. In recent years, bio-fertilizers have emerged as a promising ecological strategy to improve crop growth, soil health, and resource use efficiency. However, the performance of these microbial inoculants is highly context-dependent and influenced by agronomic practices, particularly sowing date. Despite this, the interactions between sowing dates and specific bio-fertilizer formulations under the variable climatic conditions of Iran's rain-fed ecosystems remain poorly understood. Therefore, this two-year field study was conducted to evaluate the efficacy of various commercial bio-fertilizers applied as seed treatments on the growth, yield, and water productivity of rain-fed chickpea, with a focus on comparing autumn and spring sowing dates.
 
Materials and Methods
A two-year field study (2023-2025) was conducted in Maragheh, East Azerbaijan province, using a split-plot arrangement within a randomized complete block design with three replications. The main plots were assigned to two sowing dates (autumn and spring), and the sub-plots to five seed treatments: Non-inoculated control, Probio96, Bioguard, MycoSupre Plus, and Humic acid. Following land preparation with a combined tiller, chickpea seeds (cv. Ana) were inoculated and sown at a density of 40 seeds.m⁻² on plots measuring 30×5 m in rotation with wheat, using a three-axis ASKE seeder with a 17.5-53 cm row arrangement. Weed control was performed manually. At flowering, plant density, root volume, dry root weight, root nodule weight, and plant height were measured. At physiological maturity, whole plots were hand-harvested to determine 100-seed weight, biological yield, grain yield, harvest index, and rainwater productivity.
 
Results and Discussion
Combined analysis of variance revealed that the main effect of sowing date was significant (p≤0.01) for all studied traits except plant establishment and 100-grain weight. Autumn sowing consistently and significantly enhanced root system development, resulting in higher root dry weight (2.97 g.plant⁻¹ in Year 1) and root nodule weight compared to spring sowing across both years. This superior root architecture under autumn sowing translated directly into improved agronomic performance, culminating in significantly higher mean grain yield (1084 kg.ha⁻¹) and rainwater productivity (2.84 kg.mm⁻¹). The main effect of year was also highly significant (p≤0.01) for key traits, reflecting the substantial impact of inter-annual environmental variability. The performance of bio-fertilizer treatments was profoundly influenced by these year-to-year fluctuations and their interactions with the sowing date. Among them, Bioguard, a consortium of Trichoderma and Bacillus velezensis, achieved the peak performance under optimal conditions (autumn sowing in the favorable Year 1), yielding the highest grain yield (1084 kg.ha⁻¹) and biological yield (2337 kg.ha⁻¹). However, its efficacy was highly unstable, showing the most drastic decline in the stressful Year 2 under spring sowing, where it produced the lowest yield (405 kg.ha⁻¹), even underperforming the control. This highlights its high sensitivity to environmental stress. In contrast, MycoSupre Plus, a complex formulation containing mycorrhizal fungi, Azotobacter, and Pseudomonas, demonstrated greater stability across contrasting years. While it did not reach the absolute peak yield of Bioguard in the optimal year, it provided consistent and reliable improvements in root volume, plant height, and final yield in both autumn and spring sowings, establishing it as a lower-risk option. Probio96, based on Bacillus velezensis, had a limited impact on root biomass but registered the highest harvest index (46.8%), indicating a superior efficiency in partitioning photosynthetic assimilates towards grain production rather than vegetative growth. Significant two-way (Year × Sowing Date, Sowing Date × Bio-fertilizer) and three-way (Year × Sowing Date × Bio-fertilizer) interactions were observed for most traits, including root volume, grain yield, and rainwater productivity. These interactions underscore the complex interplay between management practices and the environment. For instance, in spring sowing under stressful conditions, the untreated control sometimes showed better plant establishment than some bio-fertilizers, suggesting that the inoculated microbes could pose an additional osmotic stress on germinating seeds when environmental conditions were already adverse.
 
Conclusions
The choice of bio-fertilizer should be tailored to specific environmental conditions, including seasonal climate and sowing date. Integrating autumn sowing with bio-fertilizers that possess stable mechanisms of action, particularly MycoSupre Plus, is recommended as an effective strategy for achieving both high yield and production stability in rain-fed chickpea systems.