The role of extracellular polysaccharide produced by Bradyrhizobium strain in root growth, improvement of soil aggregate stability and reduction of soil detachment capacity

Abstract

It is well demonstrated that the production of extracellular polysaccharide by bacteria, as biological binding agent, can increase soil aggregate stability. Also, plant roots have significant effects on the soil erosion rates, since they can strongly change the soil detachment capacity (Dc). This study quantifies Dc at different flow rates in soils treated with biofertilizer under four species (Alnus subcordata, Brachythecium plumose, Gleditsia caspica and Sambucus ebulus species) in the Saravan forest park of Iran and develops regression equations for predicting Dc. Undisturbed samples collected from soils with the four tree species and subjected to five slopes (from 4.2 to 38.3%) and five water discharges (from 0.28 to 0.71 L m−1 s−1) using a hydraulic flume. The results showed that Dc was significantly lower in soils with Alnus subcordata species compared to Brachythecium plumose, Gleditsia caspica and Sambucus ebulus species, as the consequence of the changes in the root characteristics, mean weight diameter (MWD) and macronutrients, so that Dc was negatively correlated with root weight density, root length and root biomass. A linear regression model was found for MWD as a function of extracellular polysaccharide concentration (R2 = 0.74). The unit stream power had high accuracy for predicting Dc for all of the studied species. The lowest value of rill erodibility (Kr) was obtained in soils with Alnus subcordata species using regression relationship between the Dc and the shear stress of the soil. This experiment helped to show the importance of microorganisms in reducing the soil detachment rates and provided a contribution in understanding the choice of appropriate species for soil conservation.