Impact of Heavy Metal Pollution on Soil Fertility of Paddy Fields around an Abandoned Tungsten Mining Area

Soil heavy metal contamination caused by mining activities threatens the health and sustainable use of farmland ecosystems, and may also endanger human health through food chain. Soil fertility, a core component of soil ecological functioning, may be impaired by heavy metal contamination, yet the underlying mechanisms remain unclear. In this study, red soil paddy fields around an abandoned tungsten mine in Jiangxi Province were taken as the research object, and soil samples were collected from 26 sites. The soil heavy metal content (cadmium, copper, arsenic, lead, chromium, mercury, zinc, nickel), soil physicochemical properties (pH, organic matter, total nitrogen, total phosphorus, total potassium, alkali-hydrolyzable nitrogen, available phosphorus, available potassium, clay content, cation exchange capacity) and microbial properties (urease, sucrase, acid phosphatase, microbial biomass carbon, microbial entropy) were analyzed. The Nemerow comprehensive index and soil fertility comprehensive index were used to evaluate the characteristics of soil heavy metal pollution and soil fertility quality level, respectively. The structural equation model was used to analyze the effects of heavy metal pollution on soil fertility level and crop yield. The results show that the heavy metal pollution in farmland soil in the study area was generally at a moderate to light pollution level, with exceedance rates of 88.46%, 57.69%, and 7.69% for Cd, Cu, and As, respectively, which were the main pollutants. Mining-related transport through water and agricultural irrigation were identified as the main causes of heavy metal pollution in farmland soil. Correlation analysis show that the Nemerow comprehensive pollution index was significantly negatively correlated (correlation coefficients ranging from -0.55 to -0.39, P<0.05) with soil nutrients (organic matter, total nitrogen, alkali-hydrolyzable nitrogen, available potassium), clay content and microbial properties (sucrase, acid phosphatase, microbial biomass carbon, microbial entropy); at the same time, the Nemerow comprehensive pollution index was also significantly negatively correlated (a correlation coefficient of -0.54, P<0.05) with the comprehensive soil fertility index. At the same time, heavy metal pollution also significantly reduced the comprehensive soil fertility index. The results of the structural equation model (χ²/df=1.01,GFI=0.94,RMSEA=0.02) show that heavy metal pollution could directly affect the soil fertility level by reducing soil nutrient content, and also could indirectly lower soil fertility by reducing soil clay content and destroying aggregate structure, and by reducing microbial biomass and enzyme activities, thereby reducing crop yields. It is recommended to implement an in situ remediation strategy centered on "biochar-humic acid-microorganisms", by applying biochar-humic acid compound preparations, and applying heavy metal-tolerant functional microbial inoculants and other measures, which can not only reduce the availability of heavy metals, but also increase soil organic matter (carbon) sequestration, improve soil structure and restore microbial metabolic functions. In addition, combined with exogenous control measures mainly based on "interception-buffering", the goal of heavy metal pollution control and soil fertility improvement in farmlands around mining areas can be achieved in a coordinated manner. The study elucidates the key impact mechanisms of heavy metal pollution on the fertility level of farmland soil, and provides data support and scientific reference for the ecological restoration and sustainable utilization of farmland contaminated by heavy metals in mining areas.