Effects of soil temperature and moisture on biological nitrogen fixation in soybean crop

Abstract

Soil temperature and humidity are the chief determinants for good nodule formation at the time of sowing and emergence. The aim of this study was to estimate the soil temperature and humidity range, at which the Bradyrhizobium spp., and Azospirillum brasilense may have the highest effect on root enhancement and development of nodulation in soybean. Two experiments were conducted, the first of which was done in the seed laboratory performing the treatments listed as non-inoculated; inoculation with Bradyrhizobium; root enhancers; inoculation with Bradyrhizobium + root enhancers; co-inoculation with Bradyrhizobium + Azospirillum brasilense; co-inoculation with Bradyrhizobium + Azospirillum brasilense + root enhancers, and assessed at temperatures of 15, 20, 25, 30 and 35°C. The experimental design was completely randomized with distribution in a factorial. The second experiment was executed in a greenhouse, employing the same treatments affected in the seed laboratory experiment, but with the addition of non-inoculated control with mineral nitrogen. Tests were done at the soil moisture levels of 25, 50, 75 and 100% water retention capacity, forming a factorial 7 x 4 (only second experiment). All co-inoculated treatments induced the soybean root system to improve, in terms of length, volume, surface area and root diameter, exhibiting superiority to the uninoculated control in the 20 to 30°C temperature range. The co-inoculation raised nodulation in the soybean crop, when soil water retention capacity was at the range of 56 - 96%, achieving higher means compared to the standard inoculation. The root planter added no improvement to either the root system or nodulation in soybean.