Food demand and soil sustainability have become urgent issues recently because of the global climate changes. This study aims to evaluate the application of a biochar produced by rice hull, on changes of physiochemical characteristics and erosion potential of a degraded slopeland soil. Rice hull biochar pyrolized at 400°C was incorporated into the soil at rates of 2.5%, 5%, and 10% (w/w) and was incubated for 168 d in this study. The results indicated that biochar application reduced the Bd by 12% to 25% and the PR by 57% to 92% after incubation, compared with the control. Besides, porosity and aggregate size increased by 16% to 22% and by 0.59 to 0.94 mm, respectively. The results presented that available water contents significantly increased in the amended soils by 18% to 89% because of the obvious increase of micropores. The water conductivity of the biochar-amended soils was only found in 10% biochar treatment, which might result from significant increase of macropores and reduction of soil strength (Bd and PR). During a simulated rainfall event, soil loss contents significantly decreased by 35% to 90% in the biochar-amended soils. In conclusion, biochar application could availably raise soil quality and physical properties for tilth increasing in the degraded mudstone soil.

Background: Loss of nutrients and organic carbon (OC) through leaching or erosion may degrade soil and water quality, which in turn could lead to food insecurity. Adding biochar to soil can effectively improve soil stability, therefore, evaluating the effects of biochar on OC and nutrient retention and leaching is critical. Methods: We conducted a 42-day column leaching experiment by using sandy loam soil samples mixed with 2% of biochar pyrolyzed from Honduran mahogany (Swietenia macrophylla) wood sawdust at 300 °C (WB300) and 600 °C (WB600) and a control sample. Leaching was achieved by flushing the soil column on day 4 and every week during the 42-day experiment and adding a water volume for each flushing equivalent to the field water capacity. Results: Biochar application increased the final soil pH and OC, NH₄⁺-N, NO₃⁻-N, available P concentrations but not exchangeable K concentrations. In particular, WB600 exhibited superior performance in alleviating soil acidification; WB300 engendered high NO₃⁻-N concentrations. Biochar application effectively retained water in soil and inhibited the leaching of the aforementioned nutrients and dissolved OC. WB300 reduced NH₄⁺-N and K leaching by 30%, and WB600 reduced P leaching by 68%. Conclusions: Biochar application can improve nutrient retention and reduce the leaching potential of soils and connected water bodies.

Pristine biochar typically exhibits limited capacity for heavy metal adsorption due to its inadequate pore development and insufficient surface functionality. This study introduces an innovative chemical strategy to enhance the surface of sawdust biochar with sulfur-based functional groups (C=S, C-S, S-S, S^2-, S-H, -SO₃^2-, -SO₄^2-) to significantly improve cadmium (Cd) adsorption. Sulfur-doping using H₂SO₄, Na₂S, and Na₂S₂O₃ markedly increased the sulfur content from 0.11% (pristine) to 2.81% (H₂SO₄), 0.57% (Na₂S), and 13.27% (Na₂S₂O₃). Characterization techniques such as SEM-EDS, FTIR, and XPS confirmed the successful incorporation of sulfur moieties and additional oxygen-containing groups, improving surface functionality. The Cd adsorption capacity of S-modified biochar increased by 4.8-9.0 times compared to pristine biochar, with peak values of 39.38, 20.84, and 34.14 mg g^-1 for H₂SO₄, Na₂S, and Na₂S₂O₃-modified biochar, respectively. The equilibrium time was significantly reduced from 4 h (pristine) to 5-10 min (S-modified). The enhanced Cd adsorption was attributed to the synergistic interplay of electrostatic attraction, cadmium-π electron interactions, complexation, and ion exchange mechanisms, facilitated by the presence of oxygen and sulfur functional groups. Density Functional Theory (DFT) calculations showed that sulfur doping modulated the electronic properties of the biochar-Cd systems, narrowing the band gap and enhancing the Cd-O bonds, thereby improving the Cd adsorption performance. Additionally, the binding energies of the S-modified biochar-Cd complex were found to be more stable compared to those before Cd adsorption. This study demonstrates that both oxygen and sulfur-functionalized sawdust biochar is an effective and eco-friendly adsorbent for Cd removal, highlighting the significance of tailored surface modifications to augment biochar's reactivity and affinity towards specific contaminants. The developed material offers a sustainable and scalable solution for Cd removal from aqueous environments, contributing to advanced water treatment technologies and environmental remediation strategies.