Agricultural crop influences availability of nickel in the rhizosphere; a study on base cation saturations, Ni dosages and crop succession

Nickel availability in the soil is altered mainly by pH, which can indirectly be changed by base saturation (BS). The objective was to evaluate the effect of consecutive plants growth on Ni availability and enzymatic activities in the rhizosphere. It was evaluated the interaction between BS ratios (50% and 70%) and Ni application rates (0, 0.1, 0.5, 1.0, and 10.0 mg dm⁻³ of soil) on the availability of Ni in the rhizosphere. Two agricultural crops were grown in succession (soybean and sunflower). The effects of Ni rates and BS ratios on the activity of the urease and dehydrogenase in the soil were evaluated. Ni distribution in the bulk soil fractions was also investigated by Ni sequential extraction. The objective was to evaluate the effect of consecutive plants growth on Ni availability and enzymatic activities in the rhizosphere. It was observed the Ni availability was higher in the rhizosphere than bulk soil. The plant species have a large influence on the enzymatic activities in the rhizosphere. Urease activity was higher in the rhizosphere with BS70 under soybean growth, whereas for sunflower, BS50 lead to the highest urease activity, in all rates. The dehydrogenase activity was greater with BS70, regardless of plant species. The extractants Mehlich-1, Mehlich-3, and diethylenetriamine penta acetic acid (DTPA) showed comparable ability to estimate the available Ni fraction in the rhizosphere with a significant correlation with Ni accumulation in the grains. The Ni sequential extraction results showed the predominant effect of pH on the availability of Ni in bulk soil. Ni bound to organic matter (OM) was the fraction that showed the greatest BS influence. In soil under BS50, up to 20% of total Ni in the soil was bound to OM. Increasing Ni application rates to the soil increased the exchangeable fraction of Ni in the bulk soil up to 36-fold, but did not cause toxicity for either crops. Collectively, the results showed that nickel availability in rhizosphere affected urease and dehydrogenase activities, Ni uptake, and plant growth-related responses to different extents. Our work suggests that the rhizosphere should be evaluated for Ni monitoring in Ni-supplied agricultural areas.