Abscisic acid (ABA) plays a central role in the plant response to water deficit by inducing stomatal closure to conserve water when the soil dries. Exogenous ABA was applied at 45 days after sowing (DAS) as a soil drench, the physiological and seed yield response of soybean to exogenous ABA were examined as the soil was drying. Three experiments were conducted using the drought-tolerant soybean cultivar Jindou 19, grown in pots at the Yuzhong Experimental Station of Lanzhou University, China. In experiment 1, plants were exposed to progressive soil drying and leaf ABA concentration, leaf photosynthesis rate, leaf relative water content (RWC) and osmotic adjustment (OA) were measured. In experiment 2, plants were under progressive soil drying and lethal leaf water potential was measured. In experiment 3, flower production and abortion, and grain yield were measured in plants under well-watered (WW), moderate (MWD) and severe water deficits (SWD). Exogenous ABA application increased ABA accumulation in leaves and reduced the rate of soil drying. It also increased leaf photosynthetic rate, stomatal conductance and transpiration rate at 7–10 days after withholding water. The intrinsic and instantaneous water use efficiency (WUE) was consistently higher with exogenous ABA than without ABA as the soil dried. Exogenous ABA increased OA when the leaf relative water content (RWC) decreased at eight days after withholding water, lowering the lethal leaf water potential by 0.4 MPa. Exogenous ABA reduced water use, increased WUE for grain yield under WW and MWD, and had no effect on flower number, flower abortion or grain yield in any water treatment. We concluded that (1) exogenous ABA induced OA, improved leaf photosynthetic rate, leaf water relations and desiccant tolerance, but did not benefit grain yield in soybean under water deficits; (2) exogenous ABA improved the WUE at the leaf level as soil drying and WUE for grain yield under moderate water deficit.

Karst terrain is the typical area covered with a high background of heavy metals under geochemical anomaly. This research explored the accumulation of geochemical elements and soil sources in karst terrain from rock and soil exposed in carbonate areas. The comprehensive ecological risk and enrichment of heavy metals from parent rock weathered to soil was investigated in 11 formations in the carbonate and clastic areas of the Weining and Hezhang counties in northwest Guizhou. The single factor pollution index, geoaccumulation index, and the potential risk coefficient were used to assess the environmental risk. The results revealed that the heavy metals in an overall geologically high background level of soil in northwest Guizhou is at a slight risk level. However, except for Cd, the heavy metals did not exceed the standard pollution reference. Moreover, the UNMIX model and Cd and Pb isotopes were used to analyze the source of heavy metals, comprising of cadmium (Cd), arsenic (As), lead (Pb), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn), and the geochemical elements of silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), and calcium (Ca). The study showed that most elements in the soil carbonate area exceed the national standard, and the heavy metals in the soil showed a strong enrichment, while the major elements Si and Mg display strong loss. Heavy metal concentrations in soil in the carbonate area were higher than in the clastic area. Geological sources and atmospheric deposition were the main contributors to heavy metal concentrations in both carbonate and clastic areas, and their concentrations differ according to soils developing in different formations.

The high geological background of heavy metal cadmium (Cd) in geochemical anomaly areas in Southwest China and the anthropogenic pollution superposition effect in some typical areas due to mining exploitation have attracted special attention for several decades. The accumulation and migration of Cd in the farmland soil-crop system was worth discussing. In this study, the representative yellow soil and limestone soil in Guizhou Province, as well as three types of pepper (Capsicum annuum L.) were selected to investigate Cd accumulation and migration regulation from soil to plants using pot tests at different Cd concentration levels. For red cluster pepper, line pepper and hybrid pepper, the accumulation capacity of Cd in various parts was similar as follows: Cd_root > Cd_stem ≈ Cd_leaf > Cd_fruit. The differences in the Cd concentration between pepper varieties were as follows: Cd in line pepper roots was higher than that in red cluster pepper and hybrid pepper, but for leaves and fruits, the Cd concentration of red cluster pepper was higher than the others. A higher accumulation and lower transport capacity of Cd in yellow soil as well as a lower accumulation and higher transport capacity of Cd in limestone soil were achieved based on the results of enrichment coefficients and transport coefficients in yellow soil. The red pepper Cd concentration was higher than that of the other two types. The accumulation and transformation of Cd for peppers in yellow soil is more significant (p < 0.05), which results in a higher risk of migration through the food chain. GRAPHICAL ABSTRACT

Guizhou Province is covered by a large area of carbonate rocks where, with a higher background of heavy metals under the geochemical anomaly, more than 3.6 × 10⁵ ha of heavy metal–contaminated soil in the northwest area is related to historical indigenous zinc smelting. To explore the superposition effect of industrial source atmospheric deposition on soil, two watersheds were selected for study: 1) Maoshui reservoir watershed (MS), where there is a zinc smelting plant, and 2) Haishe lake watershed (HS), which was the control. We collected atmospheric depositions and soil for 3 years and analyzed Cadmium (Cd), lead (Pb), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn) content. The results show that the heavy metals in the atmospheric deposition of the pollution watershed in MS were much higher than those in the control site, HS. The deposition fluxes of Cd, Pb, Cr, Ni, and Zn in MS were 27.8, 602, 145, 43.9, and 2,225 mg·m⁻²·a⁻¹, respectively, and were 1.37–2.01 times higher than in HS. Soil heavy metals in MS were 1.01–5.69 times higher than in HS. The elevated concentrations were found focused from northeast to southwest around the plant but was distributed uniformly in HS. The average concentration of Cd, Pb, and Zn in the soil was 6.54, 67.4, and 264 mg·kg⁻¹, respectively, in HS, which represents a high geochemical background even without pollution. After 13 years of deposition by prediction, the contribution of the atmospheric deposition on the soil in the zinc-smelting area was lowest, at 5.10%, for Ni, and highest, at 17.9%, for Cd. Principal component analysis of atmospheric deposition and soil heavy metals reflected that the pollution sources in MS were more diversification than those in HS. Zinc smelting atmospheric deposition showed superposition effects on the accumulation of heavy metals in soil under the geochemical anomaly in this region.

Exogenous sources and the superposition effect of HMs in agricultural soils made the idenfication of sources complicated in a karst area. Here, a typical watershed, a research unit of the karst area, was chosen as the study area. The smaller-scale study of watersheds allowed us to obtain more precise results and to guide local pollution control. In this study, sources of HMs in agricultural soil were traced by a CMB model. Superposition effects were studied by spatial analysis of HMs and enrichment factor (EF) and chemical fraction analysis. The average concentrations of Cd, Pb, Cr, Cu, Ni and Zn in surface soils were 8.71, 333, 154, 51.7, 61.5 and 676 mg∙kg⁻¹, respectively, which exceeded their corresponding background values. The main sources of Cd, Pb and Zn in agricultural soil were rock weathering, atmospheric deposition and livestock manure, and their contributions were 47.7%, 31.0% and 21.2% for Cd; 7.63%, 78.7% and 13.4% for Pb; and 17.0%, 52.3% and 28.1% for Zn. Cr mainly derived from atmospheric deposition (73.8%) and rock weathering (20.0%). Cu and Ni mainly came from livestock manure (81.3%) and weathering (87.5%), respectively, whereas contributions of pesticides and fertilizers were relatively limited (no more than 1.04%). Cd, Pb, Zn and Cu were easily enriched in surface soils near the surrounding pollution sources, whereas Cr and Ni were easily enriched in the high-terrain area, where there was less of an impact of anthropogenic activities. The superposition of exogenous sources caused accumulation of Cd, Pb and Zn in topsoil, contaminated the subsoil through leaching and improved bioavailability of Cd and Pb, causing high ecological risk for agricultural production. Therefore, Cd and Pb should be paid more attention in future pollution control.

The karst terrain of Guizhou in southwestern China is ecologically fragile, but has undergone severe heavy metal contamination. To assess such contamination, the spatial distribution of cadmium (Cd) within soils was studied in a lead (Pb)-zinc (Zn) smelting area, coal mining area, Pb-Zn mining area, sewage irrigation area, and an uncontaminated area. Cd concentrations were highest in topsoil, with the highest value of 23.36 mg/kg in the Pb-Zn mining area and lowest value of 0.46 mg/kg in the uncontaminated area. Cd content decreased from 0 to 0.8 m depth, then sharply increased, reflecting Cd precipitation within the contaminated soil profiles. Migration of Cd within the soil was affected by organic content in the Pb-Zn smelting area (R2 = 0.99**), coal mining area (r = 0.72*), and Pb-Zn mining area (r = 0.73*). In contrast, Cd accumulated within a clay horizon in the uncontaminated area, where the correlation between Cd and specific surface area was 0.78**; Cd concentrations reached 2.11 mg/kg within this horizon. Reducible, oxidizable, and acid-exchangeable fractions accounted for 60-80% of total Cd in soils having pH values of 5.05-6.86. This indicates that Cd could easily transfer from soil to food or water, leading to human health and environmental risks.