Enzymatic activity and culturable bacteria diversity in rhizosphere of amaranth, as indicators of crop phenological changes

Background: Amaranth is a plant of interest in farming due to its ability to adapt into arid and semi-arid climates. Biological activity by microorganisms in rhizosphere determines plant performance and quality. Hypothesis: The enzymatic activity is different in two types of soil, rhizosheath soil (adhered by roots) and loose soil (non-adhered by roots), in four cropping periods of amaranth. Species study: Amaranthus hypochondriacus L. Methods: Parameters of soil organic matter and several enzyme activities in the amaranth rhizosphere were assessed. Two types of soil, rhizosheath soil and loose soil, and four cropping periods were compared. Thirty-seven culturable bacterial isolates obtained from rhizosheath soil were molecularly identified. Results: Rhizosheath soil had higher content of carbon and total nitrogen compared with loose soil; however, potential enzyme activity in both soil types was similar. Dehydrogenase and acid phosphatase activities were very sensitive to the crops phenological stages. Acid and alkaline phosphatases, cellulase and protease activities correlated to changes in soil moisture. The greatest diversity of culturable bacteria was found during the flowering stage. Conclusions: In the rhizosphere of A. hypochondriacus grown in a pumiceous sandy soil, enzymatic activities in the rhizosheath and loose soils were similar, which must be considered a unique rhizosphere environment. Dehydrogenase and acid phosphatase activities were highly sensitive to changes in the crop phenology. The behavior of phosphatases and dehydrogenase activities suggests an increased dynamic soil organic matter (SOM) during the post-harvest period. In the amaranth rhizosphere, native culturable bacteria are involved in the breakdown of SOM.