Adsorption and Desorption of Arsenate in Different Soils and Gold Mining Substrates of Minas Gerais State,Brazil

Background  Arsenic (As) availability in natural environment is related to the element’s adsorption and desorption processes in soils. Total As is better related to available As in temperate soils than in tropical soils. In tropical soils, total As is not very significant in terms of availability, therefore justifying the necessity for studies into As dynamics. Knowledge of As dynamics in soil as well as development of new analytical methodologies involving tropical soils are insufficient and necessary for future mitigation projects. Objective  The objectives of this study were: (1) To adjust methodologies which may assist in understanding arsenate dynamics in tropical soils and substrates; (2) To evaluate the adsorption and desorption of arsenate in soils and substrate samples, and to find a minimum value of arsenate available in soil which is lethal to sorghum plants. Material and Methods  Samples of three soils from Minas Gerais State (YL, RYL, and CS) and two sulfide substrates of gold mining (B1 and B2) were used in the assays. All the material was physically and chemically characterized. Remaining As (As-rem) and remaining P (P-rem) of each material, along with MAC_P and MAC_As (using the Langmuir isotherms), were obtained. After agitation to obtain MAC_P and MAC_As, arsenate was extracted by anionic resin and Mehlich-III to evaluate arsenate desorption of the material retained on the filter paper. Subsequently, arsenate desorption curves for the different materials were obtained, and arsenate availability was determined through a bioassay with sorghum plants. Samples of soils and substrate B1 were incubated with six levels of As doses. Plants were grown under greenhouse conditions for 30 days. The plants were then harvested, dried and weighed. Available As in the soils and substrate was determined by Mehlich-III. Results and Discussions  As-rem level decreased from YL (sandy) to RYL (clayey) soil samples, which always showed lower values than P-rem. Among the soils and substrates evaluated, RYL showed the highest MAC_As and MAC_P, followed by CS, YL and Bl. The results were in accordance with the values observed for As-rem and P-rem and confirm the idea that the ability of the assayed materials to remove As from the soil/substrate solution is higher than the ability to remove P. On the other hand, the binding energy (a) between soil/substrate and As is weaker than the binding energy of P. Given the fact that the studied soils present a real ability to remove As from the solution, only a small part of As would be unavailable considering MAC_As as a reference. As-Mehlich-III values were higher than As-resin for substrate Bl. Mehlich-III seemed to be more appropriate to extract labile forms of arsenate in substrate B1 as well as in the soils. Available As by Mehlich-III (26.9 mg/dm³) was considered a reference of As LCL to sorghum plants. CC50 was sensitive to the buffering capacity of each soil, showing values varying from 1.34 mg/dm³ As (clay soil with lower As-rem) to 12.31 mg/dm³ As (sandy soil with higher As-rem). Conclusions  The adaptation of the As-rem and MAC_As methodologies was satisfactory and of great value in the study of adsorption, desorption and As availability for soils and mining substrate. Mehlich-III was also satisfactory to estimate available As and was sensitive to soil buffering capacity. Nevertheless, resin can also be used as an alternative. MAC_As varied among soils and was higher than MAC_p. However, As showed higher lability than P. Using Mehlich-III, we determined the value corresponding to CC50 that showed a good reference of toxicity to available As. Outlook  The environmental implications of the As behavior are quite serious. Beside the fact that arsenate is removed very fast from the soil solution, an anthropogenic input of the element, being part of the soil quantity factor, may remain in a reversible form for a long time. As may therefore return to the soil solution and becomes available to plants, animals and the entire environment. Considering that CC50 is the maximum contents of available As the environment can tolerate to allow some vegetal biomass production, the maximum capacity of As immobilization in each soil is reduced when compared to the soils’ MAC_As values. Therefore, the maximum and safe values of reference to be used in the evaluation of incidental discharge of the element in soils must be reduced.     

Long‐term fertilization and manuring effects on physically separated soil organic‐matter pools under continuous wheat cropping at a rainfed semiarid site in China

An essential prerequisite for a sustainable soil use is to maintain a satisfactory soil organic‐matter (OM) level. This might be achieved by sound fertilization management, though impacts of fertilization on OM have been rarely investigated with the aid of physical fractionation techniques in semiarid regions. This study aimed at examining changes in organic C (OC) and N concentrations of physically separated soil OM pools after 26 y of fertilization at a site of the semiarid Loess Plateau in China. To separate sensitive OM pools, total macro‐OM (> 0.05 mm) was obtained from bulk soil by wet‐sieving and then separated into light macro‐OM (< 1.8 g cm^–3) and heavy macro‐OM (> 1.8 g cm^–3) subfractions; bulk soil was also differentiated into light OM (< 1.8 g cm^–3) and mineral‐associated OM (> 1.8 g cm^–3). Farmyard manure increased concentrations of total macro‐OC and N by 19% and 25%, and those of light fraction OC and N by 36% and 46%, compared to no manuring; both light OC and N concentrations but only total macro‐OC concentration responded positively to mineral fertilizations compared to no mineral fertilization. This demonstrated that the light‐fraction OM was more sensitive to organic or inorganic fertilization than the total macro‐OM. Mineral‐associated OC and N concentrations also increased by manuring or mineral fertilizations, indicating an increase of stable OM relative to no fertilization treatment, however, their shares on bulk soil OC and N decreased. Mineral fertilizations improved soil OM quality by decreasing C : N ratio in the light OM fraction whereas manuring led to a decline of the C : N ratio in the total macro‐OM fraction, with respect to nil treatment. Further fractionation of the total macro‐OM according to density clarified that across treatments about 3/4 of total macro‐OM was associated with minerals. Thus, by simultaneously applying particle‐size and density separation procedures, we clearly demonstrated that the macro‐OM differed from the light OM fraction not only in its chemical composition but also in associations with minerals. The proportion of the 0.5–0.25 mm water‐stable aggregates of soil was higher under organic or inorganic fertilizations than under no manure or no mineral fertilization, and increases in OC and N concentrations of water‐stable aggregates as affected by fertilization were greater for 1–0.5 and 0.5–0.25 mm classes than for the other classes. Results indicate that OM stocks in different soil pools can be increased and the loose aggregation of these strongly eroded loess soils can be improved by organic or inorganic fertilization.     

红壤小流域不同利用方式水土流失和有机碳流失特征研究

研究了红壤小流域不同利用方式的试验区水土流失和有机碳流失的特征 ,结果表明 :径流、泥沙及有机碳流失主要集中在 5月、6月及 8月份 ,其间径流流失量占全年流失量的 68.8%～ 73 .1 %,泥沙流失量和有机碳流失量占全年流失量的 90 %以上。径流量和泥沙流失量的大小顺序均为无保护性措施、侵蚀严重的试验区 5>粗放经营利用的试验区 1 >保护性经营利用试验区 2 >保护性经营利用试验区 4>恢复保护性植被试验区 3 ;流失的泥沙主要为推移质 ,有机碳流失量的大小顺序为粗放经营利用的试验区 1 >保护性经营利用的试验区 2 >保护性经营利用的试验区 4>无保护措施、侵蚀严重的试验区 5>恢复保护性植被的试验区 3 ,径流流失的有机碳和推移质流失的有机碳基本接近。从保护资源角度来看 ,恢复保护性植被试验区 3的利用方式控制水土流失和有机碳的效果最好。从农林利用角度来看 ,以有保护性经营利用综合性措施 (等高梯田、植被篱笆、农林间作等措施 )的试验区 2和试验区 4利用方式 ,既有利于水土保持 ,又有利于防止土壤肥力下降     

Relationship between soil organic matter and micropores in a long-term experiment at Ultuna,Sweden

Our study showed that long‒term addition of organic matter to a fine textured soil (36.5% clay, 41% silt, 22.5% sand) resulted in an increase of both macro‒ and microporosity in the top soil layer. In terms of changes of the absolute pore volume, macropores were of main importance. However, in relative terms, the increase of microporosity was comparable to that of macroporosity (75% and 90%). Changes in porosity upon different organic matter levels had a marginal effect on the water storage capacity. Micropores with diameters in the range of 1—30 μm were highly significantly correlated to soil organic matter characteristics showing that there is a non‒uniform distribution in relation to pores. Mechanisms leading to disproportionally high concentrations of soil organic matter in relation to micropores are discussed.     

CO2 emissions from soils of different depths of a permafrost peatland,Northeast China: response to simulated freezing–thawing cycles

Soil freezing–thawing cycle (FTC) is an important factor controlling C dynamics in mid–high latitude regions, especially in the permafrost regions impacted by global warming. Nonetheless, the response of C cycling in the deeper active layers of permafrost regions to FTC remains far from certain. We aimed to characterize the emission of CO₂ from soils of multiple depths as impacted by FTC and its relationship with active organic C (OC) and enzyme activities. We collected soil samples from three soil layers (0–15, 15–30, and 30–45 cm) of an undisturbed peatland in the Da Xing'anling Mountains, NE China, and then subjected them to various freezing (10 to –10°C) and thawing (–10 to 10°C) cycles. Soil CO₂ emissions, two active OC fractions, and activities of three enzymes were monitored during incubation periods. At the thawing stage of the first FTC, CO₂ emission rates in the three soil layers presented transient peaks being ≈ 1.6–1.7 times higher than those of the unfrozen control sample. Although FTC did not change the overall patterns of decreasing CO₂ emission along the soil profile, FTC significantly reduced the amount of CO₂ emission when compared with the unfrozen control sample, possibly because the small CO₂ emission at the freezing stage neutralized the peak of CO₂ emission at the thawing stage. This study suggests that in the active layer of permafrost peatlands, CO₂ emission during FTCs may be lower than the emission under higher temperatures, but experiment with more temperature gradients should be encouraged to verify this conclusion in the future. Meanwhile, FTC significantly increased water extracted OC release from the three soil layers, ≈ 1.2–1.6 times higher compared to the unfrozen control sample, indicating that soil carbon loss in the form of leachate may increase during freezing–thawing periods. Additionally, the CO₂ emissions impacted by FTCs were significantly correlated with active OC fractions and enzyme activities, which indicated that active OC and enzymes were sensitive to FTCs, and surviving microbes and enzymes might use the increased liable substrates and induce the CO₂ emission during freezing–thawing periods.     

Carbon sequestration and relationship between carbon addition and storage under rainfed soybean–wheat rotation in a sandy loam soil of the Indian Himalayas

Soil organic matter (SOM) contributes to the productivity and physical properties of soils. Although crop productivity is sustained mainly through the application of organic manure in the Indian Himalayas, no information is available on the effects of long-term manure addition along with mineral fertilizers on C sequestration and the contribution of total C input towards soil organic C (SOC) storage. We analyzed results of a long-term experiment, initiated in 1973 on a sandy loam soil under rainfed conditions to determine the influence of different combinations of NPK fertilizer and fertilizer + farmyard manure (FYM) at 10 Mg ha⁻¹ on SOC content and its changes in the 0–45 cm soil depth. Concentration of SOC increased 40 and 70% in the NPK + FYM-treated plots as compared to NPK (43.1 Mg C ha⁻¹) and unfertilized control plots (35.5 Mg C ha⁻¹), respectively. Average annual contribution of C input from soybean (Glycine max (L.) Merr.) was 29% and that from wheat (Triticum aestivum L. Emend. Flori and Paol) was 24% of the harvestable above-ground biomass yield. Annual gross C input and annual rate of total SOC enrichment were 4852 and 900 kg C ha⁻¹, respectively, for the plots under NPK + FYM. It was estimated that 19% of the gross C input contributed towards the increase in SOC content. C loss from native SOM during 30 years averaged 61 kg C ha⁻¹ yr⁻¹. The estimated quantity of biomass C required to maintain equilibrium SOM content was 321 kg ha⁻¹ yr⁻¹. The total annual C input by the soybean–wheat rotation in the plots under unfertilized control was 890 kg ha⁻¹ yr⁻¹. Thus, increase in SOC concentration under long-term (30 years) rainfed soybean–wheat cropping was due to the fact that annual C input by the system was higher than the required amount to maintaining equilibrium SOM content.