Bradyrhizobium sp. inoculation ameliorates oxidative protection in cowpea subjected to long-term composted tannery sludge amendment

• Composted tannery sludge (CTS) affects slightly cowpea-Bradyrhizobium (BR 3267) symbiosis.
• In soil with CTS, N-fix efficiency has increased in cowpea inoculated with BR 3267.
• Leghemoglobin was higher in cowpea inoculated with BR 3267 growing in soil with CTS.

Oxidative stress can strongly affect biological nitrogen fixation (BNF) in nodules; therefore, the antioxidant system in nodules works to reduce the damage caused by the deleterious effects of oxidizing compounds and maintain adequate BNF. In this context, the objective of this work was to evaluate variables related to BNF and oxidative metabolism in the nodules of cowpea inoculated with Bradyrhizobium sp. and grown in soils subjected to different levels of composted tannery sludge amendment. A randomized block design was used with a 4 × 2 factorial scheme, involving four doses of composted tannery sludge (0, 5, 10, and 20 t ha−1) for uninoculated cowpea and cowpea inoculated with Bradyrhizobium sp. The cowpea inoculated with Bradyrhizobium sp. showed a better growth response than the cowpea inoculated with native rhizobia. Increases in the number of nodules, nodule dry mass, nitrogen fixation efficiency and nitrogen content were recorded in the cowpea inoculated with Bradyrhizobium sp. grown in soils with composted tannery sludge. The cowpea nodules colonized by Bradyrhizobium sp. showed lower hydrogen peroxide levels, while leghemoglobin was maintained at the highest levels. The catalase and phenol peroxidase enzymes were positively modulated in the nodules of the cowpea inoculated with Bradyrhizobium sp. We conclude that the presence of composted tannery sludge affects the establishment and development of the symbiosis between rhizobia and cowpea, mainly between native soil rhizobia and cowpea. When cowpea was inoculated with Bradyrhizobium sp., it was concluded that these plants were able to maintain better plant growth and nitrogen capture and lower oxidative stress in their nodules. Thus, inoculation with Bradyrhizobium sp. could be a useful tool for minimizing the deleterious effects of exposing plants to composted tannery sludge and for ensuring plant growth and productivity.