Dissolved biochar fractions and solid biochar particles inhibit soil acidification induced by nitrification through different mechanisms.

Biochar can inhibit soil acidification by decreasing the H+ input from nitrification and improving soil pH buffering capacity (pHBC). However, biochar is a complex material and the roles of its different components in inhibiting soil acidification induced by nitrification remain unclear. To address this knowledge gap, dissolved biochar fractions (DBC) and solid biochar particles (SBC) were separated and mixed thoroughly with an amended Ultisol. Following a urea addition, the soils were subjected to an incubation study. The results showed that both the DBC and SBC inhibited soil acidification by nitrification. The DBC inhibited soil acidification by decreasing the H+ input from nitrification, while SBC enhanced the soil pHBC. The DBC from peanut straw biochar (PBC) and rice straw biochar (RBC) decreased the H+ release by 16 % and 18 % at the end of incubation. The decrease in H+ release was attributed to the inhibition of soil nitrification and net mineralization caused by the toxicity of the phenols in DBC to soil bacteria. The abundance of ammonia-oxidizing bacteria (AOB) and total bacteria decreased by >60 % in the treatments with DBC. The opposite effects were observed in the treatments with SBC. Soil pHBC increased by 7 % and 19 % after the application of solid RBC and PBC particles, respectively. The abundance of carboxyl on the surface of SBC was mainly responsible for the increase in soil pHBC. Generally, the mixed application of DBC and SBC was more effective at inhibiting soil acidification than their individual applications. The negative impacts of dissolved biochar components on soil microorganisms need to be closely monitored.

Adsorption of Cd(II) by two variable-charge soils in the presence of pectin.

Batch experiments were conducted to investigate cadmium(II) (Cd(II)) adsorption by two variable-charge soils (an Oxisol and an Ultisol) as influenced by the presence of pectin. When pectin dosage was less than 30 g kg(-1), the increase in Cd(II) adsorption with the increasing dose of pectin was greater than that when the pectin dosage was >30 g kg(-1). Although both Langmuir and Freundlich equations fitted the adsorption isotherms of Cd(II) and electrostatic adsorption data of Cd(II) by the two soils well, the Langmuir equation showed a better fit. The increase in the maximum total adsorption of Cd(II) induced by pectin was almost equal in both the soils, whereas the increase in the maximum electrostatic adsorption of Cd(II) was greater in the Oxisol than in the Ultisol because the former contained greater amounts of free Fe/Al oxides than the latter, which, in turn, led to a greater increase in the negative charge on the Oxisol. Therefore, the presence of pectin induced the increase in Cd(II) adsorption by the variable-charge soils mainly through the electrostatic mechanism. Pectin increased the adsorption of Cd(II) by the variable-charge soils and thus decreased the activity and mobility of Cd(II) in these soils.

Effect of pectin on adsorption of Cu(II) by two variable-charge soils from southern China.

The influence of pectin on Cu(II) adsorption by two variable-charge soils (an Oxisol and an Ultisol) was investigated. Pectin increased the adsorption, and the extent of adsorption increased linearly with the dose of pectin, being greater in the Oxisol than that in the Ultisol because the adsorption of pectin by the Oxisol was greater. Both Langmuir and Freundlich equations fitted the adsorption isotherms of Cu(II) for both soils well. The fitting parameters of both equations indicated that pectin increased not only the adsorption capacity of the soils for Cu(II) but also the adsorption strength of Cu(II). The effect of pectin decreased with rising pH in the pH range 3.5-6.0, although the extent of electrostatic adsorption of Cu(II) by both soils was markedly greater over the pH range. Fourier-transformed infrared spectroscopy analysis and zeta potential measurement of soil colloids indicated that adsorption of pectin by the soils made the negative charge on both soils more negative, which was responsible for the increase in the electrostatic adsorption of Cu(II) induced by the addition of pectin. In conclusion, pectin-enhanced adsorption of Cu(II) especially at low pH would be beneficial to the soils as it would decrease the activity and mobility of Cu(II) in acidic variable-charge soils.

[Determination of trace selenium in plants by hydride generation atomic fluorescence spectrometry with program temperature-controlled graphite digestion].

Discussed several methods of pretreatment for the determination of selenium were discussed, and a program temperature-controlled graphite digestion method was developed to digest 5 kinds of representative standard plant samples of citrus leaves, tea, cabbage leaves, shrubs and rice. The effect of the pretreatment method of digestion solution, digestion temperature and digestion time on the extraction of selenium was investigated in detail. The instrumental working parameters were optimized. For the reaction conditions of hydride generation atomic fluorescence spectrometry (HG-AFS), the effect of the concentration of KBH4 and HCl on the determination of selenium was emphasized. Not only the effect of the concentration of carrier flow HCl was considered, but also the effect of the concentration of sample HCl on the determination of selenium was studied. The best method for determination of trace selenium in plant samples by atomic fluorescence spectrometry with program temperature-controlled graphite digestion was established. Results indicated that the recovery of the method of selenium was 87.1% - 106.2%, the detection limit was 0.018 microg x L(-1) and the relative standard deviation (RSD) was less than 6.0%. In the range of 0-10 microg x L(-1) (low standard) and 0-100 microg x L(-1) (high standard) fluorescence was linearly related to the concentration of selenium, the coefficient of r was 0.9999 and r was 0.9997. Therefore, this method has wide linear range, high sensitivity, low detection limit and good stability, which was very suitable for the determination of trace selenium of plant. And the method was of easy and safe operation, strong practicability, low cost, and low toxicity of chemicals used, so it can be used as a routine analysis method in general laboratory.