Agronomic efficiency of nitrogen and response of quinoa (Chenopodium quinoa Willd.) cultivars to inoculation with Azotobacter at varying nitrogen application levels

Introduction
The provision of nitrogen to plants poses significant challenges due to the element's mobility within the soil. In this context, the effective use of biological resources for nitrogen biological fixation can significantly enhance nitrogen productivity (Aasfar et al., 2021). Employing plant growth-promoting bacteria (PGPB), is recognized as a beneficial agronomic strategy to improve nitrogen absorption and optimize fertilizer use efficiency, thereby improving plant nutrition (Consentino et al., 2022).
Quinoa (Chenopodium quinoa Willd.), known for its extensive genetic diversity and adaptability to various climates, as well as its high efficiency in utilizing water resources, presents an ideal option for cultivation in limited water availability conditions and saline soils (Lavini et al., 2014).
The objective of this research was to assess the response of two quinoa cultivars, Titicaca and Redcarina, to inoculation with Azotobacter chroococcum at varying nitrogen application levels and to analyze the agronomic efficiency of nitrogen fertilization.
Materials and Methods
This study was conducted during the 2023-2024 growing season at the Shahid Bahonar University Research Farm, Kerman, Iran. The research was carried out in a split-factorial design based on a randomized complete block design with three replications. The main factor examined was the application of nitrogen fertilizer treatments. Additionally, seed inoculation with Azotobacter and quinoa cultivars were investigated as factorial treatments in subplots. Urea fertilizer was applied at three levels: control (no chemical fertilizer application), 50% of the plant's nutritional requirement, and 100% of the plant's nutritional requirements. The impact of biofertilizer inoculation was assessed at two levels: no inoculation (control) and inoculation with Azotobacter chroococcum. The cultivars evaluated in this study were Titicaca and Redcarina. At the end of the growth period, various traits were measured, including plant height, number of panicles per plant, number of seeds per panicle, total seeds per plant, thousand-seed weight, grain yield per m2, as well as the bacterial inoculation efficiency, agronomic efficiency of nitrogen fertilization, and nitrogen use efficiency.
Results & Discussion
The findings indicated that the Redcarina cultivar exhibited greater plant height and a higher number of panicles per plant compared to the Titicaca. The maximum seed number per plant was achieved through bacterial inoculation under 100% fertilization conditions, which did not significantly differ from the results obtained with 50% fertilization. Furthermore, the Redcarina cultivar produced 34% and 33% more seeds per plant than the Titicaca cultivar under no fertilization and 50% fertilization conditions, respectively. Conversely, under both no fertilization and 50% fertilization conditions, Redcarina demonstrated a lower thousand-seed weight by 21% and 22%, respectively, compared to the Titicaca cultivar. Inoculation with Azotobacter resulted in a 15% increase in grain yield and nitrogen use efficiency (NUE) relative to no-inoculation. Additionally, the application of the 100% fertilization level led to a 29.3% enhancement in grain yield and a 25.7% increase in NUE compared to the control, although no significant difference was observed when compared to the 50% fertilization. The investigation of the agronomic efficiency of nitrogen fertilization (AEN) revealed that the examined cultivars exhibited varying responses to bacterial inoculation across different fertilizer levels. Notably, both cultivars achieved the highest AEN at a 50% fertilization rate when bacterial inoculation was applied. However, in the case of the Redcarina cultivar, an increase in fertilizer application to 100% resulted in enhanced AEN compared to no-inoculation, suggesting that Redcarina demonstrates a superior response to nitrogen availability. Additionally, the findings regarding the bacterial inoculation effect (BIE) indicated that the BIE in the Redcarina cultivar was improved with higher levels of fertilizer application. Conversely, the Titicaca cultivar reached its highest efficiency at a 50% nitrogen fertilizer application, indicating its optimal performance under limited nitrogen supply conditions. This characteristic contributed to a decline in the bacterial inoculation efficiency in Titicaca cultivar as fertilizer application levels increased.
Conclusion
Redcarina cultivar exhibited a superior response to nitrogen availability, as elevating the fertilizer level to 100% enhances the agronomic efficiency of nitrogen fertilization. Conversely, the Titicaca cultivar demonstrates the highest agronomic efficiency of nitrogen under limited-nitrogen supply conditions, since raising the fertilizer level to 100% did not improve its agronomic efficiency. Consequently, the cultivars examined can be strategically utilized to optimize the potential of quinoa, taking into account varying levels of agricultural land fertility.