To improve our ability to predict SOC mineralization response to residue and N additions in soils with different inherent and dynamic organic matter properties, a 330-day incubation was conducted using samples from two long-term experiments (clay loam Mollisols in Iowa [IAsoil] and silt loam Ultisols in Maryland [MDsoil]) comparing conventional grain systems (Conv) amended with inorganic fertilizers with 3 yr (Med) and longer (Long), more diverse cropping systems amended with manure. A double exponential model was used to estimate the size (C_a, C_s) and decay rates (k_a, k_s) of active and slow C pools which we compared with total particulate organic matter (POM) and occluded-POM (OPOM). The high-SOC IAsoil containing highly active smectite clays maintained smaller labile pools and higher decay rates than the low-SOC MDsoil containing semi-active kaolinitic clays. Net SOC loss was greater (2.6 g kg⁻¹; 8.6%) from the IAsoil than the MDsoil (0.9 g kg⁻¹, 6.3%); fractions and coefficients suggest losses were principally from IAsoil’s resistant pool. Cropping history did not alter SOC pool size or decay rates in IAsoil where rotation-based differences in OPOM-C were small. In MDsoil, use of diversified rotations and manure increased k_a by 32% and k_s by 46% compared to Conv; differences mirrored in POM- and OPOM-C contents. Residue addition prompted greater increases in C_a (340% vs 230%) and C_s (38% vs 21%) and decreases in k_a (58% vs 9%) in IAsoil than MDsoil. Reduced losses of SOC from residue-amended MDsoil were associated with increased OPOM-C. Nitrogen addition dampened CO₂-C release. Clay type and C saturation dominated the IAsoil’s response to external inputs and made labile and stable fractions more vulnerable to decay. Trends in OPOM suggest aggregate protection influences C turnover in the low active MDsoil. Clay charge and OPOM-C contents were better predictors of soil C dynamics than clay or POM-C contents.

Heavy-metal pollution has become a serious problem in the vicinity of mining sites. In this study, we investigated the pollution degree and spatial distribution of heavy metals and determined their sources in soil near a typical Pb/Zn smelter in an arid area of northwest China. The pollution degree, spatial distribution and source identification of heavy metals were evaluated and analyzed by the geo-accumulation index (I_geo), enrichment factor (EF), ArcGIS-based kriging interpolation models and multivariate statistical techniques. The results showed that the average concentrations of Pb, Zn, Cd and Cu were higher than their background contents of Shaanxi Province. The results of chemical speciation fractions show that the non-residual fraction (Acid soluble + Reducible + Oxidizable) of Pb, Zn and Cd were greater than the residual fractions of the same elements, while the results of Cr, Cu and Mn were just contrary. The I_geo values demonstrated that the issue of soils contaminated by Pb, Zn and Cd in this area is very serious, where Cd pollution was the most significant. The EF values decrease in the following order: Cd > Zn > Pb > Cu > Cr, and indicating Cd was extremely severe enrichment. The spatial distribution indicated that Pb, Zn, Cd and Cu mainly originated from smelting activities, which were heavily accumulated in topsoils and decreased with increasing soil depth in the profile distribution. The indexes of I_geo and EF showed that Pb, Zn, Cd and Cu were the most important contaminants, which had the same sources, deriving from anthropogenic metal smelting activities. These results can serve as a basis for designing effective strategies to prevent, control and reduce further heavy-metal contamination of the soils. Meanwhile, the relevant study methods, study hypotheses and methodologies may provide certain reference significance for the study of soil pollution from similar heavy metals.

Soil heavy metal pollution from mining activities is potentially harmful to human health through the food chain. In this study, a total of 43 soil samples were collected from a depth of 0–20 cm from fields close to a Pb and Zn smelter. The samples were used to: 1) analyze the pollution level of heavy metals (Pb, Zn, Cr, and Cu) and spatial distribution pattern; 2) evaluate the degree of accumulation and enrichment, potential ecological risk, and human health risk; and 3) perform source apportionment in Fengxiang County, Shaanxi Province of China. The results showed that the concentration ranged from 43.67 to 189.55, 131.43 to 239.53, 74.77 to 112.25, and 24.69 to 37.71 mg·kg⁻¹ for Pb, Zn, Cr, and Cu, respectively, and the mean concentration for Pb, Zn, Cr, and Cu was 129.46, 192.85, 91.98 and 31.67 mg·kg⁻¹, respectively. The concentrations were greater than the Shaanxi Province background values, while they were lower than the second-level limits of Environmental Quality Standard for Soils of China (EQSS). The spatial distribution of heavy metal contents showed a banded in soil except Cu. The spatial distribution pattern and pollution assessment indexes (I_geo, EF) indicated that the investigated metals had been accumulated in the study areas, and implied significant influences from anthropogenic activities, local meteorological situation, and soil properties. The ecological risk assessment showed that the risks were relatively low (RI<150). Compared with the exposure risk for adults, that for children was significantly greater. The ingestion of heavy metals in the soils by humans was the main exposure pathway compared with the dermal exposure. There may be a risk of noncarcinogenic adverse health effects (HQ < 1, 0.377 ≤ HI≤1.553) on children, but the adults were unlikely to experience obvious adverse health effects (HQ < 1, HI < 1). The carcinogenic risk of Cr for adults and children was at an unacceptable level. The carcinogenic and noncarcinogenic risks were in the order of children > adults. The correlation analysis showed that Pb, Cr, and Cu have identical anthropogenic and natural sources, while Zn has another identical source. This study could provide a basis for the sustainable management of this region by reducing metal inputs and to protect soils from long-term heavy metal accumulation.