Natural colloids are widely distributed in soil and groundwater. Due to their specific characteristics, colloids can actively involve various transport contaminants, resulting in a complicated fate and the transport of heavy metals to the environment. This study investigated the effects of soil colloids on the adsorption and transport of Ni2+ in saturated porous media under different conditions, including pH, ion strength (IS), and humic acid (HA), because these indexes are non-negligible in the fates of various organic or inorganic matters in the subsurface environment. The results indicate that Ni2+ adsorption by soil colloids slightly increased from 17% to 25% with the increase of pH from 5.5 to 7.5 at the IS of 30 mmol·L−1, whilst it significantly reduced from 55% to 17% with the increase of IS from 0 to 30 mmol·L−1 at a pH of 5.5. Both Langmuir and Freundlich models can fit the adsorption isotherms of Ni2+ on soil colloids and quartz sand. According to the column experiment, the presence of soil colloids increased the initial penetration rate, but could not increase the final transport efficiency of Ni2+ in the effluent. The presence of soil colloids has weakened the effect of IS on Ni2+ transport in the sand column. Moreover, this experiment implies that HA remarkably decreased the Ni2+ transport efficiency from 71.3% to 58.0% in the presence of soil colloids and that there was no significant difference in the HA effect on the Ni2+ transport in the absence of soil colloids.
Global agricultural intensification leads to a decline in soil quality; however, the extent to which long-term rice cultivation adversely impacts soil, based on chemical and microbial perspectives, remains unclear. The present study was conducted on a seed multiplication farm in Wuchang, Heilongjiang Province, China, to quantify changes in the nutrient properties and microbial profiles of meadow soil in cultivated (rhizosphere and bulk soil) and uncultivated paddy plots from spring to winter. A non-parametric method was used to compare carbon metabolism characteristics among the three groups of soil samples. Principal component analysis was used to distinguish soil chemical properties and carbon source utilization profiles among the soil samples across different seasons. Under rice cultivation, pH, organic matter, total nitrogen, and alkali-hydrolyzed nitrogen concentrations were generally higher in rhizosphere soils than in bulk or uncultivated soils. However, microbial biomass in cultivated soils was consistently lower than in uncultivated soils. There was a discernible difference in carbon substrate preference between summer and other seasons in the three sample groups. In conclusion, agricultural activities in rice cultivation could reshape soil microbial communities in the long term. Notably, specific cultivation activity may induce distinct soil microbial responses, which are more sensitive than chemical responses.
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