Self-functionalization of soil-aged biochar surfaces increases nitrate retention

Application of compost amended with biochar on the distribution of antibiotic resistance genes in a soil-cucumber system-from the perspective of high-dose fertilization

The transfer of antibiotic resistance genes (ARGs) from soils to vegetables negatively impacts human health. This study explored the effects of the high-dose (18.73 t/ha) application of traditional compost (TC) and composts produced through the co-composting of traditional materials with large-sized (5-10 mm) biochar-amended compost (LBTC) or small-sized (< 0.074 mm) biochar-amended compost (SBTC) on the distribution of ARGs in a soil-cucumber system were explored. Results indicated that the SBTC group had the highest soil nitrogen, phosphorus, and potassium contents, followed by the LBTC, TC, and control treatment groups. These findings aligned with the quality and weight of harvested cucumbers. Bacterial community diversity decreased in compost-fertilized soils. Compared with their preexperimental values in soils, the total absolute abundances of ARGs and mobile genetic elements (MGEs) increased by 23.88 and 6.66 times, respectively, in the control treatment group; by 5.59 and 5.23 times, respectively, in the TC group; by 5.50 and 1.81 times, respectively, in the LBTC group; and by 5.49 and 0.47 times, respectively, in the SBTC group. Compared with those in the control treatment group, the absolute abundance of ermB, ermT, gyrA, qnrS, tetC, and intI1 decreased by 6-100% in the soil of the SBTC group. Compost application to soils significantly decreased ARG abundance in cucumbers; SBTC had the most significant effect and reduced the number of host bacteria at the phylum level from four to three. Nutrient levels in soils were important factors influencing the migration of ARGs from soils to cucumbers. In summary, when compared to other composts, the high-dose (18.73 t/ha) application of SBTC is more effective at reducing the risk of the accumulation and transfer of ARGs in the soil-cucumber system.

Iron-modified biochars and their aging reduce soil cadmium mobility and inhibit rice cadmium uptake by promoting soil iron redox cycling

Iron (Fe) modified biochar has been widely used for cadmium (Cd) contaminated soil remediation. However, the accompanying anions introduced during the modification process potentially affect the behavior of Cd in soil. In this study, we investigated the distinct Cd immobilization mechanisms by Fe2(SO4)3 modified biochar (FSBC) and Fe(NO3)3 modified biochar (FNBC) in a two-year pot experiment. Results showed that both FSBC and FNBC significantly reduced Cd concentrations in rice grains by 23%-42% and 30%-37% compared to pristine biochar (BC). Specifically, NFBC promoted the formation of amorphous Fe oxides by enhancing the NO3--reducing Fe(II) oxidation process, which significantly increased Fe/Mn oxide-bound Cd and decreased soil CaCl2-extractable Cd. For FSBC, the introduction of SO42- significantly promoted the formation of Fe plaques by enhancing the Fe(III) reduction process, which blocked the Cd transfer from the soil to the rice roots. More importantly, after two years of biochar application, an organo-mineral complex layer is formed on the biochar surface, which immobilized a large amount of Cd. The Cd immobilization on the surface of aged biochar could be due to the fixation by the secondary Fe oxides within the organo-mineral layer and the complexation by the surface functional groups. The result of laser ablation inductively coupled plasma mass spectrometry showed that the Cd content on aged FNBC and FSBC was 5.9 and 2.6 times higher than on aged BC. This might be attributed to the Fe-modified biochar's higher electron exchange capability (EEC), which promoted the development of organo-mineral complexes. Notably, the EEC of biochar was maintained during its aging process, which may keep the biochar surface active and facilitate continual Cd immobilization. This study revealed the complex mechanisms of soil Cd immobilization with Fe-modified biochar, providing new insights into sustainable biochar environmental remediation.

Granulation compared to co-application of biochar plus mineral fertilizer and its impacts on crop growth and nutrient leaching

Mechanized biochar field application remains challenging due to biochar's poor flowability and bulk density. Granulation of biochar with fertilizer provides a product ready for application with well-established machinery. However, it's unknown whether granulated biochar-based fertilizers (gBBF) are as effective as co-application of non-granulated biochar with fertilizer. Here, we compared a gBBF with a mineral compound fertilizer (control), and with a non-granulated biochar that was co-applied at a rate of 1.1 t ha-1 with the fertilizer in a white cabbage greenhouse pot trial. Half the pots received heavy rain simulation treatments to investigate nutrient leaching. Crop yields were not significantly increased by biochar without leaching compared to the control. With leaching, cabbage yield increased with gBBF and biochar-co-application by 14% (p > 0.05) and 34% (p < 0.05), respectively. Nitrogen leaching was reduced by 26-35% with both biochar amendments. Biochar significantly reduced potassium, magnesium, and sulfur leaching. Most nitrogen associated with gBBF was released during the trial and the granulated biochar regained its microporosity. Enriching fertilizers with biochar by granulation or co-application can improve crop yields and decrease nutrient leaching. While the gBBF yielded less biomass compared to biochar co-application, improved mechanized field application after granulation could facilitate the implementation of biochar application in agriculture.

Aging properties and cadmium remediation mechanism of biochar in sediment from phosphorus-rich water

Cadmium (Cd) is the main heavy metal pollutant in sediments from East China. The biochar-sediment nexus can provide carbon sequestration and pollution control. In this work, an in situ study was conducted to investigate the long-term effects and control mechanism of biochar and the effect of biochar aging on Cd stabilization in overlying water-pore water-sediment. The Cd2+ concentration in the overlying water was positively correlated with total nitrogen (0.960, P < 0.05), total organic carbon (0.983, P < 0.05), and total phosphorus (0.993, P < 0.01) in pore water. Biochar stabilized Cd2+ by increasing the pH and oxidation-reduction potential of the sediment environment and promoting the formation of Cd1.25Ca0.75(P2O7) on the biochar surface in sediment from phosphorus-rich water. These changes were closely related to the Brunauer-Emmett-Teller surface area and average pore size of the biochar. Within 60 days, the biochar in the sediment underwent aging, which was closely related to the preparation temperature of the biochar. The organic composition of biochar prepared at a low temperature (≤ 300 °C) and the surface structure of biochar prepared at a high temperature (≥ 500 °C) were altered. The biochar parameter changes were in the order of pore volume > Brunauer-Emmett-Teller surface area > pore size. Our results show that biochar modification can enhance the remediation capacity of biochar, but may be unfavorable to biochar anti-aging. This knowledge will support policymakers and researchers when exploring long-term biochar use in contamination control and strengthen future research.