Nutrient availability and microbial activity in a potato field: Impacts of Azospirillum inoculation and nitrogen levels under semi-arid conditions

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Inoculation with Azospirillum spp. is commonly used to improve rhizosphere functioning. Potato growth and productivity may be strongly affected by Azospirillum inoculation and nitrogen application under semi-arid conditions. A field experiment was conducted in an agricultural field in the Al-Azawiya area of Babil Governorate, approximately 40 km north of Al-Hilla, during the spring growing season of 2025. The experiment aimed to evaluate the effects of nitrogen-fixing Azospirillum spp. under different nitrogen levels on soil nutrient availability, soil microbial activity, and the growth and yield of potato (Solanum tuberosum L.). The Dutch cultivar Burren was grown in silty loam soil. The study included two factors. The first factor was Azospirillum inoculation applied at three levels (0, 10, and 20 mL per tuber and soil), coded as a0, a1, and a2, respectively. The second factor was nitrogen fertilization applied at four levels (0, 100, 200, and 300 kg N ha^-1), coded as n0, n1, n2, and n3, respectively. The experiment was arranged in a randomized complete block design (RCBD) with three replications, resulting in 36 experimental units. Soil samples were collected to assess biological parameters, including Azospirillum population density after emergence and before harvest. The interaction treatment a2n3 recorded the highest bacterial counts at both stages, reaching 2.436 and 2.69 × 10^{9sup> CFU g-1 dry soil, respectively. Azospirillum spp. inoculation, nitrogen level, and their interaction significantly increased microbial respiration. The highest value was observed under a2n3, reaching 7.40 mg CO₂ g-1 soil. Furthermore, Azospirillum counts varied over time and were higher at harvest than after emergence. Nitrogen fertilization at the n3 level significantly increased soil nutrient concentrations, which reached 48.171 mg N kg-1 soil, 26.65 mg P kg-1 soil, and 216.28 mg K kg-1 soil. Inoculation with Azospirillum spp. combined with 300 kg N ha-1 significantly improved most potato growth traits. The highest total tuber yield was obtained under the a2n3 treatment, reaching 47.48 Mg ha-1. We conclude that Azospirillum inoculation rate and nitrogen level are important factors in improving soil biological properties and potato yield under semi-arid conditions, with the magnitude of the response depending on inoculum density and nitrogen application rate.}