Yongqing Luo

The role of citric acid on the phytoremediation of heavy metal contaminated soil.

Adsorption and hydroponics experiments were conducted to study the role of citric acid on the phytoremediation of heavy metal contaminated soil. The results show that addition of citric acid decreased the adsorption of both lead and cadmium, such an effect was bigger for cadmium than for lead. The decrease in the adsorption of Pb and Cd was mainly due to a decrease of pH in the presence of citric acid. The presence of citric acid could alleviate the toxicity of Pb and Cd to radish, and stimulate their transportation from root to shoot. The studies of heavy metal forms using sequential extraction demonstrated that lead was mainly existed as FHAC (a lower bioavailable form) in the root, while F(HCl) was the dominant form in the leaf. The addition of citric acid to the soil changed the concentration and relative abundance of all the forms. The detoxifying effect of citric acid to Pb in shoots might result from the transformation of higher toxic forms into lower toxic forms. Cadmium was mainly present as F(NaCl), therefore, it had higher toxicity than lead. The addition of citric acid increased the abundance of F(H2O) + F(NaCl), indicating that citric acid treatment could transform cadmium into more transportable forms.

Rapid degradation of butachlor in wheat rhizosphere soil

The degradative characteristics of butachlor in non-rhizosphere, wheat rhizosphere, and inoculated rhizosphere soils were measured. The rate constants for the degradation of butachlor in non-rhizosphere, rhizosphere, and inoculated rhizosphere soils were measured to be 0.0385, 0.0902, 0.1091 at 1 mg/kg, 0.0348, 0.0629, 0.2355 at 10 mg/kg, and 0.0299, 0.0386, 0.0642 at 100 mg/kg, respectively. The corresponding half-lives for butachlor in the soils were calculated to be 18.0, 7.7, 6.3 days at 1 mg/kg, 19.9, 11.0, 2.9 days at 10 mg/kg, and 23.2, 18.0, 10.8 days at 100 mg/kg, respectively. The experimental results show that the degradation of butachlor can be enhanced greatly in wheat rhizosphere, and especially in the rhizosphere inoculated with the bacterial community designated HD which is capable of degrading butachlor. It could be concluded that rhizosphere soil inoculated with microorganisms-degrading target herbicides is a useful pathway to achieve rapid degradation of the herbicides in soil.

Physiological mechanism of plant roots exposed to cadmium

Physiological experiments on plant roots exposed to cadmium were conducted on carrot and radish using a liquid culture and a pot experiment with a series of cadmium applications. Activities of four enzymes (catalase, peroxidase, polyphenol oxidase, superoxide dismutase), and concentrations of free proline and malonaldehyde in the roots of both plants were investigated. Results showed that the germination rate and growth of roots of both plants were inhibited at the concentration of 20 mg Cd/l, and the inhibition was increased with the increasing concentrations of cadmium, both in the liquid culture and in the pot experiment; activities of the four enzymes declined similarly in both species. The concentration of proline in roots reached the maximum when the application of cadmium was at the level of 20 mg/l in the liquid culture (or 20 mg/kg in soil), and then it declined slowly with the increasing concentration of cadmium. However, the reverse trend was observed for the concentration of malonaldehyde. All of bio-indicators measured here was quite sensitive to the addition of cadmium.

Chemical behavior of Cd in rice rhizosphere.

Chemical behavior of Cd in rice rhizosphere as affected or not by Pb was investigated. The NH4OAc extractable Cd in the rhizosphere was distinctly lower than that in bulk soil. The depletion of Cd in the rhizosphere could not be simply attributed to Cd uptake by rice. The observed phenomena could be attributed to the decreasing pH in the rhizosphere and the complexing capabilities of soluble exudates for Cd. Extractable Cd increased in both the rhizosphere and bulk soil after the addition of Pb, which might be caused by the replacement of Pb for Cd. The extractable Cd in the non-rhizosphere varied with the distance from the root surface, especially within 0-1 mm, which was greatly affected by the combined effects of mass flow, activation and fixation, and had the lowest extractable Cd. Pb addition affected the distribution of extractable Cd in the non-rhizosphere, implying that the affinity of Pb for organic matter was greater than that of Cd. The difference of Cd species between rhizosphere and bulk soil demonstrated that the transformation of exchangeable Cd (EXC-Cd) to OM-Cd (bound to organic matter) and FMO-Cd (bound to iron and manganese oxide) occurred in the rice rhizosphere due to the exudations from the rice root, the activity of microorganisms on the root surface and the activation of Fe and Mn oxides. The interaction between Pb and Cd resulted in the content of EXC-Cd being higher in the presence of Pb, whereas the OM-Cd content was lower in the presence of Pb.

Soil solution dynamics of Cu and Zn in a Cu- and Zn-polluted soil as influenced by γ-irradiation and Cu-Zn interaction

A pot experiment was conducted to study soil solution dynamics of Cu and Zn in a Cu/Zn-polluted soil as influenced by gamma-irradiation and Cu-Zn interaction. A slightly acid sandy loam was amended with Cu and Zn (as nitrates) either singly or in combination (100 mg Cu and 150 mg Zn kg(-1) soil) and was then gamma-irradiated (10 kGy). Unamended and unirradiated controls were included, and spring barley (Hordeum vulgare L. cv. Forrester) was grown for 50 days. Soil solution samples obtained using soil moisture samplers immediately before transplantation and every ten days thereafter were used directly for determination of Cu, Zn, pH and absorbance at 360 nm (A360). Cu and Zn concentrations in the solution of metal-polluted soil changed with time and were affected by gamma-irradiation and metal interaction. gamma-Irradiation raised soil solution Cu substantially but generally decreased soil solution Zn. These trends were consistent with increased dissolved organic matter (A360) and solution pH after gamma-irradiation. Combined addition of Cu and Zn usually gave higher soil solution concentrations of Cu or Zn compared with single addition of Cu or Zn in gamma-irradiated and non-irradiated soils, indicating an interaction between Cu and Zn. Cu would have been organically complexed and consequently maintained a relatively high concentration in the soil solution under higher pH conditions. Zn tends to occur mainly as free ion forms in the soil solution and is therefore sensitive to changes in pH. The extent to which gamma-irradiation and metal interaction affected solubility and bioavailability of Cu and Zn was a function of time during plant growth. Studies on soil solution metal dynamics provide very useful information for understanding metal mobility and bioavailability.

Influence of [S, S]-EDDS on Phytoextraction of Copper and Zinc by Elsholtzia Splendens From Metal-Contaminated Soil

Two pot experiments were conducted to investigate the time course effects of the (S, S)-N, N′-ethylenediamine disuccinic acid (EDDS) addition to contaminated soil on the uptake of Cu and Zn by the Cu accumulator Elsholtzia splendens and on plant Cu and Zn concentrations at different growth stages. EDDS increased the amounts of Cu and Zn soluble in the soil, taken up by plants, concentrated in the xylem sap, and translocated from roots to stems and leaves. The increase in soil-soluble metals, especially Cu, resulted in a corresponding increase in metal concentrations in the xylem sap and leaves. The addition of EDDS to the soil increased plant Cu and Zn concentrations, especially in the leaves, and changed the proportions of Cu and Zn taken up by different plant parts. The proportions of Cu and Zn taken up by the roots were higher than by the leaves of control plants, but EDDS-treated plants showed the opposite trend. EDDS exerted greater effects at the end of the vegetative growth stage than at the start of the flowering or reproductive stages.

Artificial root exudates and soil organic carbon mineralization in a degraded sandy grassland in northern China

Plant root exudates contain various organic and inorganic components that include glucose, citric and oxalic acid. These components affect rhizosphere microbial and microfaunal activities, but the mechanisms are not fully known. Studies concerned from degraded grassland ecosystems with low soil carbon (C) contents are rare, in spite of the global distribution of grasslands in need of restoration. All these have a high potential for carbon sequestration, with a reduced carbon content due to overutilization. An exudate component that rapidly decomposes will increase soil respiration and CO2 emission, while a component that reduces decomposition of native soil carbon can reduce CO2 emission and actually help sequestering carbon in soil. Therefore, to investigate root exudate effects on rhizosphere activity, citric acid, glucose and oxalic acid (0.6 g C/kg dry soil) were added to soils from three biotopes (grassland, fixed dune and mobile dune) located in Naiman, Horqin Sandy Land, Inner Mongolia, China) and subjected to a 24-day incubation experiment together with a control. The soils were also analyzed for general soil properties. The results show that total respiration without exudate addition was highest in grassland soil, intermediate in fixed dune and lowest in mobile dune soil. However, the proportion of native soil carbon mineralized was highest in mobile dune soil, reflecting the low C/N ratio found there. The exudate effects on CO2-C emissions and other variables differed somewhat between biotopes, but total respiration (including that from the added substrates) was significantly increased in all combinations compared with the control, except for oxalic acid addition to mobile dune soil, which reduced CO2-C emissions from native soil carbon. A small but statistically significant increase in pH by the exudate additions in grassland and fixed dune soil was observed, but there was a major decrease from acid additions to mobile dune soil. In contrast, electrical conductivity decreased in grassland and fixed dune soil and increased in mobile dune. Thus, discrete components of root exudates affected soil environmental conditions differently, and responses to root exudates in soils with low carbon contents can differ from those in normal soils. The results indicate a potential for, e.g., acid root exudates to decrease decomposition rate of soil organic matter in low carbon soils, which is of interest for both soil restoration and carbon sequestration.

Effects of grazing exclusion on carbon sequestration and the associated vegetation and soil characteristics at a semi-arid desertified sandy site in Inner Mongolia, northern China

Li, Y., Zhao, X., Chen, Y., Luo, Y. and Wang, S. 2012. Effects of grazing exclusion on carbon sequestration and the associated vegetation and soil characteristics at a semi-arid desertified sandy site in Inner Mongolia, northern China. Can. J. Soil Sci. 92: 807-819. Chinas Horqin Sandy Land is a region vulnerable to disturbance that has been subjected to serious desertification, mainly because of overgrazing. We investigated whether the establishment of grazing exclosures in areas with active sand dunes would benefit vegetation recovery and improve soil quality. The results showed that along the age sequence of grazing exclosure for 8, 13, and 26 yr, plant cover, species number, and above-ground biomass increased, the soils water-holding capacity, fine particle content, organic C, total N, total P, available N and K, and electrical conductivity also increased, and the soil coarse sand content, pH, and bulk density in the top 20 cm of the soil decreased. However, the greatest improvements compared with the continuous grazing sites occurred in the 13- and 26-yr exclosures. Based on the area of heavily and severely desertified land in the study region and the results of the present study, the amount of C sequestered in the top 20 cm of the soil could reach 7.8 Mt after 26 yr of grazing exclusion. Our results confirm that grazing exclusion is a positive way to restore desertified ecosystems and has a high potential for sequestering soil C and improving soil quality in the semi-arid Horqin Sandy Land.

Soil organic carbon and total nitrogen storage under different land uses in the Naiman Banner, a semiarid degraded region of northern China

Abstract. Li, Y., Han, J., Wang, S., Brandle, J., Lian, J., Luo, Y. and Zhang, F. 2014. Soil organic carbon and total nitrogen storage under different land uses in the Naiman Banner, a semiarid degraded region of northern China. Can. J. Soil Sci. 94: 9-20. Accurate investigation of soil organic carbon (SOC) and total nitrogen (TN) storage at a regional level is important for detecting changes in the C and N sequestration and emission potentials induced by land-use and cover type changes. In a degraded semiarid region of northern Chinas Horqin Sandy Land, we selected 208 locations and calculated SOC and TN storage to a depth of 100 cm for the main land-use and cover types. The productive cropland on former grassland had the highest level of SOC and TN storage (6613 g C m-2 and 709 g N m-2). The corresponding storage values were 3758 g C m-2 and 402 g N m-2 in degraded grassland, 3449 g C m-2 and 373 g N m-2 in afforested dunes, 2674 g C m-2 and 320 g N m-2 in unproductive cropland on former dunes, and 1109 g C m-2 and 129 g N m-2 in sand dunes (from mobile to fixed). The average soil bulk density was highest in sand dunes, with a value of 1.59 g cm-3, and lowest in productive cropland on former grassland, with a value of 1.39 g cm-3. The conversion of severely degraded sandy land into other land-use and cover types therefore has considerable potential to partially offset the SOC and TN loss during the past century that has resulted from desertification in the Horqin Sandy Land.

Carbon sequestration in the total and light fraction soil organic matter along a chronosequence in grazing exclosures in a semiarid degraded sandy site in China

Horqin Sandy Land is a fragile, seriously desertified region located in Inner Mongolia of China. Over- grazing is one of the primary drivers of desertification in this region. We investigated whether the establishment of grazing exclosures in areas with active sand dunes enhances soil carbon (C) sequestration and benefits soil re- covery. The results showed that soil organic C storage was 1.4, 1.9, and 3.5 times, and light fraction C storage was 2.3, 3.2, and 4.4 times in the 100-cm topsoil after 7, 12, and 25 years of grazing exclusion, respectively, compared to the case in active sand dunes. The light fraction of soil played an important role in soil C sequestration, although it might not change rapidly to provide an early indication of how soil C is increasing in response to grazing exclusion. The results indicated that soils could potentially sequester up to 13.8 Mt C in 25 years if active sand dunes in the study area were to be protected by exclosures. This corresponds to 12.8% of the estimated carbon loss (107.53 Mt) that has been associated with desertification over the past century in the Horqin Sandy Land. Our results suggested that exclosures have the capacity to increase soil C sequestration; however, decades will be required for soil C to recover to historical grassland levels observed prior to desertification.