Calcium-based polymers for suppression of soil acidification by improving acid-buffering capacity and inhibiting nitrification.

Soil acidification is a major threat to agricultural sustainability in tropical and subtropical regions. Biodegradable and environmentally friendly materials, such as calcium lignosulfonate (CaLS), calcium poly(aspartic acid) (PASP-Ca), and calcium poly γ-glutamic acid (γ-PGA-Ca), are known to effectively ameliorate soil acidity. However, their effectiveness in inhibiting soil acidification has not been studied. This study aimed to evaluate the effect of CaLS, PASP-Ca, and γ-PGA-Ca on the resistance of soil toward acidification as directly and indirectly (i.e., via nitrification) caused by the application of HNO3 and urea, respectively. For comparison, Ca(OH)2 and lignin were used as the inorganic and organic controls, respectively. Among the materials, γ-PGA-Ca drove the substantial improvements in the pH buffering capacity (pHBC) of the soil and exhibited the greatest potential in inhibiting HNO3-induced soil acidification via protonation of carboxyl, complexing with Al3+, and cation exchange processes. Under acidification induced by urea, CaLS was the optimal one in inhibiting acidification and increasing exchangeable acidity during incubation. Furthermore, the sharp reduction in the population sizes of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) confirmed the inhibition of nitrification via CaLS application. Therefore, compared to improving soil pHBC, CaLS may play a more important role in suppressing indirect acidification. Overall, γ-PGA-Ca was superior to PASP-Ca and CaLS in enhancing the soil pHBC and the its resistance to acidification induced by HNO3 addition, whereas CaLS was the best at suppressing urea-driven soil acidification by inhibiting nitrification. In conclusion, these results provide a reference for inhibiting soil re-acidification in intensive agricultural systems.

Immobilizing arsenic in soil via amine metal complex: a case study using iron-ethylenediamine.

Fe-based nanomaterials have been extensively investigated for their application in mitigating arsenic (As) pollution in groundwater, sediment, and soils. Here, an iron-ethylenediamine (Fe-EDA) complex was synthesized and characterized using Fourier transform-infrared spectroscopy and X-ray photoelectron spectroscopy before its use as an amendment to ameliorate As-polluted soils. Column leaching tests at three Fe-EDA application rates (1%, 3%, and 5%) were conducted, and their results were compared with those acquired after using nano zerovalent iron (nZVI) and Fe3O4, to assess their efficiency to amend As-contaminated paddy soils. After leaching, stabilization efficiency and soil chemical characteristics were determined. Additionally, As fractions were measured using inductively coupled plasma-mass spectroscopy by employing a sequential extraction procedure to evaluate the performance of the treatments and understand the underlying their mechanisms. Compared with the control treatment, the Fe-EDA treatment reduced As release by more than 35.33% in the 2nd leaching cycle, whereas nZVI and Fe3O4 decreased the As release by 11.84% and 24.60%, respectively. Moreover, the optimal addition of the Fe-EDA chelate was 5%, which stabilized more than 50% As in the soil from the 7th to 11th leaching cycles. After sequential extraction, the Fe-Mn oxide binding fraction, which was originally 12.65%, increased to 21.5%, 18.23%, and 21.71% after the application of nZVI, Fe3O4, and Fe-EDA amendments, respectively. Furthermore, our treatments promoted the binding of the As fraction with crystalline Fe (III) (oxyhydr)oxide (F3); however, other fractions did not increase considerably, suggesting that the Fe-EDA complex could effectively stabilize As through electrostatic attraction between the arsenate anion and EDA, as well as As-O-Fe bond formation via a coordinating reaction. Overall, Fe-EDA was found to be a potent amendment for mitigating As-polluted soil.