To assess heavy metal pollution and ecological risk, a total of 28 surface paddy soil samples were collected and analyzed around a famous copper smelter in Guixi, China. The results showed that all sites were heavily contaminated by both Cu and Cd, compared with soil background values, whose average concentrations exceeded the standard by 5.7 and 12.3 times, respectively, posing a slight ecological risk related to Cu (potential ecological risk index 320). The risks were also demonstrated through the speciation analyses of Cu (CaCl₂-Cu 2.63%, acid-soluble Cu 8.67%, and residual Cu 74.17%, on average) and Cd (CaCl₂-Cd 47.30%, acid-soluble Cd 45.02%, and residual Cd 28.87%, on average) in the surface paddy soil, including the use of a CaCl₂ extraction procedure and the BCR (Community Bureau of Reference) sequential extraction scheme. Several soil properties (residual carbon, cation exchange capacity, and soil texture) were significantly correlated with soil Cd but made a small contribution to their variability with a poor linear fit because of external Cd input to the soil, while soil total potassium largely influenced the soil Cu species except for residual Cu. Therefore, an effective Cu pollution regulation strategy through soil potassium control is suggested for this smelter soil.

To identify the effects of straw return on different organic carbon pools in surface paddy soils (0–20 cm), a total of 33 soil samples under different annual straw return rates (SRr) was collected, and then the samples were analyzed based on a 100-day incubation. The data from acid hydrolysis-incubation experiments were fitted to a three-pool first-order kinetics model that divided soil organic carbon (SOC) into active (C_a), slow (C_s) and resistant (C_r) pools. The results showed that the mean pool sizes of C_a, C_s, and C_r were 0.27, 10.26, and 13.46 g·kg⁻¹, representing a mean of 1.35%, 41.91%, and 56.74% of the total SOC (TOC), respectively. The SOC pools in the surface paddy soils in Dongxiang had a small C_a pool but had longer mean residence times of the C_a and C_s pools than those in other regions in China. The three carbon pools were less affected by the paddy soil type but showed obvious spatial variations. The SRr contributed a strong positive effect on the variability of C_s and C_r, especially on C_s variability, while it had very little effect on C_a variability. Soil available nitrogen dominated the variability in TOC and C_r compared to the other soil properties. Therefore, the C_s pool is more sensitive than the other carbon pools to long-term straw return.