Silicon fertilization and Azospirillum brasilense inoculation: effects on agronomic performance, biomass partitioning, and carbon–nitrogen balance of off-season forage sorghum

Introduction Silicon (Si) has been studied as a strategy to improve growth and stress tolerance in several crops, while Azospirillum brasilense has been explored as a plant growth-promoting bacterium capable of reducing the need for nitrogen (N) fertilization. This study aimed to investigate the individual and interactive effects of Si doses and seed inoculation with A. brasilense on the agronomic, morphological, and productive characteristics, biomass partitioning, and carbon (C) and N concentrations of forage sorghum cultivated in the offseason with supplemental irrigation. Methods The experiment was conducted in a randomized complete block design in a 4 × 2 factorial scheme, with five replications. The first factor consisted of Si doses (0, 4, 8, and 12 kg ha − ¹) applied by fertigation; the second factor consisted of seeds inoculated or not with A. brasilense strains Ab-V5 and Ab-V6. Results Silicon increased the per-plant wholeplant, live leaf, stem, and panicle weights, the panicle and stem diameters, and the grain number and threshed panicle weight. For the variables fitted by regression, the estimated maxima ranged from 6.4 to 9.1 kg ha − ¹ Si, whereas panicle weight was evaluated only by Tukey’s test; grain weight showed a weaker response (p = 0.023) and was regarded as exploratory. The means obtained with 4 kg ha⁻¹ did not differ statistically from those of the higher doses. Inoculation did not affect absolute yield variables but reduced the stem proportion and increased the panicle proportion of the plants. The Si × inoculation interaction was significant only for stem dry matter concentration (p = 0.037), a result considered exploratory. Fresh and dry matter yields per hectare and water accumulation in the biomass were not affected by the treatments. Carbon concentration was highest in leaves and grains (43.173% and 43.007%). Nitrogen concentration was influenced by the plant organ × Si interaction: it increased linearly with Si doses in leaves (from 1.45% to 1.75%), varied irregularly in grains, and was not affected in stems and roots. The C:N ratio differed only among plant organs, with the highest value in stems (124.951). Discussion The effects of Si were restricted to per-plant traits and were not detected in forage yield per hectare. Under the conditions of this study, 4 kg ha⁻¹ Si was the lowest dose that produced per-plant responses statistically equivalent to those of the higher doses, whereas inoculation acted mainly on biomass partitioning; an economic analysis is required before the combined practice can be recommended.