Soil contamination by potentially toxic metals (PTMs) has become a public concern in China. However, the distribution and controlling factors of soil PTM fractions remain largely unknown, limiting our ability to assess their health risks and thus to make sound controlling polices. Here, we investigate the fraction distribution of cadmium (Cd) and copper (Cu) in Chinese soils and their relationships with soil pH, based on a national meta-analysis of 163 published literatures. Exchangeable Cd in southern China accounted for 19.50 ± 14.97% of total Cd, significantly (p < 0.01) higher than the corresponding 13.42 ± 6.95% in northern China. Potentially available fractions constituted about 60% of total Cd at the national scale. By contrast, about half of soil Cu existed in unavailable residual fraction. Phytoavailable (i.e., exchangeable) fraction accounted for only 2.71 ± 1.65% and 2.54 ± 1.58% of total Cu in northern and southern China, respectively. Percentages of exchangeable Cd and Cu were negatively correlated (p < 0.01) with soil pH, while potentially available fractions increased significantly (p < 0.05) with soil pH. Our results provide the first national assessment of Cd and Cu fraction distribution and their responses to soil pH variations, highlighting the necessity to consider their fraction distribution and soil properties when assessing the health risks of soil PTM contamination in China.

Nitrous oxide (N₂O) is a potent greenhouse gas with intensive emissions from acidic soil. This study explored the impact of the disruption of the microbial balance from microbial inhibitors (streptomycin and cycloheximide) on soil’s N₂O emission and nitrogen (N) dynamics. Under all the conditions examined, biotic processes accounted for 96–98% of total N₂O emissions. High concentrations of streptomycin (6 and 10 mg g⁻¹) reduced N₂O emissions from 2.24 μg kg⁻¹ h⁻¹ to 1.93 μg kg⁻¹ h⁻¹ and 2.12 μg kg⁻¹ h⁻¹, respectively, whereas lower concentrations (2 and 4.5 mg g⁻¹) increased emissions from 2.24 μg kg⁻¹ h⁻¹ to 2.95 μg kg⁻¹ h⁻¹ and 3.27 μg kg⁻¹ h⁻¹, respectively. Lower cycloheximide (2 and 4.5 mg g⁻¹) significantly enhanced N₂O emissions, reaching 9.15 μg kg⁻¹ h⁻¹ and 5.68 μg kg⁻¹ h⁻¹, respectively, whereas higher dosages (6 mg g⁻¹ and 10 mg g⁻¹) inhibited N₂O emissions, reducing them to 5.55 μg kg⁻¹ h⁻¹ and 4.84 μg kg⁻¹ h⁻¹, respectively. Carbon dioxide (CO₂) emissions generally decreased with increasing inhibitor dosages but significantly increased at 2 mg g⁻¹ and 4.5 mg g⁻¹ streptomycin. The inhibitors also altered soil N and carbon (C) dynamics, increasing ammonium (NH₄⁺-N), dissolved organic nitrogen (DON), and dissolved organic carbon (DOC) levels. Pearson correlation analysis indicated that N₂O emission was negatively correlated with cycloheximide dosage (R = −0.68, p < 0.001), NH₄⁺-N (R = −0.31, p < 0.001) and DOC content (R = −0.57, p < 0.05). These findings highlight the consequences of microbial disruption on N₂O emission and the complex microbial interactions in acidic soils. High concentrations of microbial inhibitors effectively reduce N₂O emissions by suppressing key microbial groups in nitrification and denitrification. Conversely, lower concentrations may prompt compensatory responses from surviving microorganisms, resulting in increased N₂O production. Future research should focus on sustainable management strategies to mitigate N₂O emissions while preserving the soil’s microbial community.