Migration and leaching characteristics of base cation: indicating environmental effects on soil alkalinity in a karst area.

In karst areas, rock dissolution often results in the development of underground networks, which act as subterranean pathways for rapid water and nutrient (and possibly soil) loss during precipitation events. Loss of soluble nutrients degrades surface soils and decreases net primary productivity, so it is important to establish flow pathways and quantify nutrient loss during rainfall events of different magnitudes. We conducted a simulated rainfall experiment in karst and nonkarst areas to compare the concentration of nutrients in surface and subsurface flow water and effects on soil alkalinity in three lithologic soil formations under five different rainfall intensity treatments. Compared with the nonkarst area, the runoff in subsurface flows and the proportion of nutrient loss in the subsurface flow are larger in the karst area and less affected by rain intensity. The maximum loss loads of calcium (Ca2+) and magnesium (Mg2+) ions were 32.9 and 19.8 kg ha-1, respectively. With the estimate of base cation loss loads in the China southern karst area under the rainfall intensity of 45 mm h-1, more than 80% of the base cation loss load occurred in the limestone karst area. Although the alkalinity leaching value in nonkarst was similar to that in the karst area under simulated rainfall conditions, its impact on the ecological environment was quite different.

[Concentration Variations and Flux Estimation of Dissolved Carbon in Karst Spring of a Typical Karst Area].

Dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) are two important indices for studying karstification, carbon sinks, and the carbon cycle. In order to further understand the migration characteristics of DIC and DOC in karst springs in small watersheds and improve the estimation accuracy of carbon flux under conditions of discrete and limited water quality monitoring data, the concentration variations of DIC and DOC were studied in karst outlet springs of Chenqi small watershed in Puding County, Guizhou Province, China. The flux estimation regression equations of DIC and DOC were established by the LOADEST model, and the carbon sink intensity in Chenqi karst spring basin was estimated. The results showed that the concentrations of DIC and DOC were 16.47-42.31 mg·L-1 and 0.87-6.89 mg·L-1, which displayed exponential decrease and increase with increased instantaneous runoff, respectively. Based on the regression equations constructed by the LOADEST model, the daily flux load of DIC was mainly affected by runoff, whereas that of DOC was affected by both time and runoff. The estimated total fluxes of DIC and DOC were 9490.01 kg·a-1 (95% confidence interval of 11293.58-7972.33 kg·a-1) and 1704.87 kg·a-1 (95% confidence interval of 1895.24-1553.24 kg·a-1), respectively. The carbon sink intensity of the Chenqi karst spring basin was 3.40 g·(m2·a)-1[95% confidence interval of 2.85-4.05 g·(m2·a)-1]. The LOADEST model fully utilized discrete and limited water quality data to improve flux estimation accuracy from the monthly average to the daily average. Therefore, it is an effective tool to estimate the fluxes of DIC and DOC in karst springs under low frequency water quality monitoring conditions.

Nitrogen loss from karst area in China in recent 50 years: An in-situ simulated rainfall experiment's assessment.

Karst topography covers more than 1/3 of the People's Republic of China in area. The porous, fissured, and soluble nature of the underlying karst bedrock (primarily dolomite and limestone) leads to the formation of underground drainage systems. Karst conduit networks dominate this system, and rainfall takes a crucial role on water cycle at China karst area. Nitrogen loss from the karst system is of particular concern, with regard to nutrient use efficiency as well as water quality, as much of the karst system, including steeply sloping terrain, is used for intensive agriculture. We use simulated rainfall experiments to determine the relationship between rainfall and nitrogen loss at typical karst slope land and then estimate nitrogen loss from the karst soil. The results show that both surface runoff and subsurface runoff have a significant linear correlation with rainfall at all studied sites. Subsurface runoff is larger than surface runoff at two karst sites, while the opposite is true at the non-karst site. Exponential function satisfactorily described the correlation between rainfall and nitrogen concentrations in runoff. Nitrates accounted for 60%-95% of the dissolved nitrogen loss (DN, an index of N-loss in this research). The estimated annual N-loss load varies between 1.05 and 1.67 Tg N/year in the whole karst regions of China from 1961 to 2014. Approximately, 90% of the N-loss load occurred during the wet season, and 90% of that passed through the subsurface. Understanding the processes and estimating N-loss is highly valuable in determining long-term soil security and sustainability in karst regions.

Adsorption and desorption characteristics of endosulfan in two typical agricultural soils in Southwest China.

Endosulfan is an organochlorine pesticide widely used in Southwest China. In this paper, the adsorption and desorption characteristics of endosulfan in two typical agricultural soils (latosol and lateritic red soil) in this area were studied. The results showed that Langmuir isothermal equation could well describe the adsorption thermodynamic characteristics of endosulfan in latosol and lateritic red soil, and the maximum adsorption capacities of α-endosulfan were 0.186 and 0.209 mg/g, while those of ß-endosulfan were 0.140 and 0.148 mg/g, respectively. Endosulfan adsorption in the two soils was an exothermic physicochemical process, but dominated by physical process. The adsorption kinetic characteristics of endosulfan in the two soils could be well described by second-order kinetic equation, and the initial rate constants were 0.228 and 0.325 mg/(g min) for α-endosulfan, while those were 0.119 and 0.125 mg/(g min) for ß-endosulfan, respectively. The adsorbed endosulfan in the two soils was difficult to be desorbed into the liquid phase, and showed weak desorption hysteresis. These results implied that endosulfan could be firmly adsorbed by the two soils, and their adsorption and desorption abilities may be related to the contents of soil clay and organic matter.

Microbial degradation of endosulfan in contaminated soil with the elution of surfactants.

In this work, an endosulfan-degrading strain was isolated from the aged soil contaminated by endosulfan, and identified as Ochrobactrum sp. EB-4 by 16S rDNA sequence analysis. The microbial degradation characteristics of endosulfan in three eluents (Tween 80 + SDS, Tween 80 + Na2SiO3, Tween 80 + SDS + Na2SiO3) were investigated. The results showed that the degradation percents of α-, ß-endosulfan in the three eluents were 86.83 %∼92.91 % and 88.90 %∼93.94 % in 15 days, respectively. The degradation process can be well described by the first-order kinetic model, and the half-times of α-endosulfan in eluent 1∼eluent 3 were 3.83, 5.29, and 4.53 days, while those of ß-endosulfan were 3.35, 4.50 and 3.79 days, respectively. The endosulfan diol and endosulfan sulfate as main metabolites were detected, and the former can be further degraded by this strain, which revealed that the simultaneously happened hydrolysis and oxidation reactions were the main degradation processes, and dominated by hydrolysis reaction. After 5 days of washing with the eluents, 56.00∼84.33 % of α-endosulfan, and 46.49∼68.56 % of ß-endosulfan in soil were eluted, respectively, and can be entirely biodegraded in 12 days, which indicated that the microbial degradation was the rate-determining step.

[Elution of endosulfan from contaminated soil by surfactants and their combination].

The elution efficiency and dynamic characteristic of endosulfan from the contaminated soil were studied using non-ionic surfactants and anionic/non-ionic surfactants in the presence or absence of Na2SiO3 with batch experiment and parallel desorption experiment. The results showed that the elution percents of alpha-, beta-endosulfan followed a decreasing order of Tween 80/SDS, Tween 80 and Triton X-100 in the absence of Na2SiO3, while that by Triton X-100/SDS at low concentration (100-500 mg x L(-1)) and high concentration (800-1000 mg x L(-1)) were lower and higher than those by the corresponding concentrations of Triton X-100, respectively. The elution efficiencies for four elution modes obviously increased in the presence of Na2SiO3, and the elution percents of alpha-, beta-endosulfan followed a decreasing order of Tween 80/SDS, Tween 80, Triton X-100/SDS and Triton X-100, and that of alpha-endosulfan were as 1.17-2.73, 1.87-4.02, 1.85-6.56 and 1.87-2.85 times as that in the absence of Na2SiO3. The elution process of endosulfan could be described by a 4-parameter biphasic first-order kinetic model, and obviously showed a rapid elution phase and a slow elution phase. Both of the elution percent and elution rate of beta-endosulfan were lower than those of alpha-endosulfan, which indicated that beta-endosulfan was difficult to be eluted from soil. The addition of Na2SiO3 could increase the rate constants of rapid elution and slow elution, and decrease the slow elution percent. Compared with other elution modes, Tween 80/SDS in the presence of Na2SiO3 could elute endosulfan from soil more effectively and rapidly, and shown as a fine mixed eluent for endosulfan.