[Transfer characteristics of cadmium in soil-vegetable-insect food chain].

Taking two kinds of vegetables (Brassica rapa and Amaranthus mangostanus) and one insect species (Prodenia litura) as test materials, a greenhouse pot experiment was conducted to study the transfer characteristics of cadmium (Cd) in soil-vegetable-insect food chain and the distribution patters of different Cd chemical forms in the organs of the two vegetables. With the increasing concentration of applied Cd in soil, the biomass of the two vegetables decreased significantly, while the Cd concentration in the vegetables had a significant increase. The Cd concentration in the vegetable organs decreased in the order of stem > root > leaf for A. mangostanus, and of stem > leaf > root for B. rapa. The Cd concentration in P. litura larvae also increased with the increasing concentration of Cd in soil, and the maximum Cd concentration in the P. litura larvae on B. rapa and A. mangostanus was 36.7 and 46.3 mg x kg(-1), respectively. In the feces of the larvae on B. rapa and A. mangostanus, the Cd concentration was up to 190 and 229.8 mg x kg(-1), respectively, suggesting that the most part of Cd absorbed by P. litura larvae was excreted out of their bodies via feces. In the organs of the two vegetables, NaCl-extractable Cd was the dominant Cd form (> 70%), followed by d-H2O- and ethanol-extractable Cd, while the HAc-extractable Cd (insoluble cadmium phosphate), HCl-extractable Cd (insoluble cadmium oxalate), and residual Cd only had a very low concentration. Such a present pattern of different Cd forms in vegetable organs could be conducive to the Cd transfer in the food chain. P. litura could ease Cd poison by excreting large amount of absorbed Cd via feces, and effectively restrict the transfer of Cd to next trophic level. Since B. rapa and A. mangostanus could accumulate large amount of Cd in their biomass, the two vegetables were suggested not to be planted in highly Cd-contaminated soil.

[Effects and mechanisms of plant roots on slope reinforcement and soil erosion resistance: a research review].

Plant roots play an important role in resisting the shallow landslip and topsoil erosion of slopes by raising soil shear strength. Among the models in interpreting the mechanisms of slope reinforcement by plant roots, Wu-Waldron model is a widely accepted one. In this model, the reinforced soil strength by plant roots is positively proportional to average root tensile strength and root area ratio, the two most important factors in evaluating slope reinforcement effect of plant roots. It was found that soil erosion resistance increased with the number of plant roots, though no consistent quantitative functional relationship was observed between them. The increase of soil erosion resistance by plant roots was mainly through the actions of fiber roots less than 1 mm in diameter, while fiber roots enhanced the soil stability to resist water dispersion via increasing the number and diameter of soil water-stable aggregates. Fine roots could also improve soil permeability effectively to decrease runoff and weaken soil erosion.

Effect of fertilizer and water content on N2O emission from three plantation soils in south China.

The effects of fertilizers and water content on N2O emission were studied using the three most typical plantation soils. Soil incubations were performed and fertilization and water content treatments were designed. At 25% of saturated water content(SWC), N2O emissions from the soil treated with urea, KNO3, (NH4)2 SO4 and KH2 PO4 were compared at application rates of 0, 100, 200, 300 and 500 kg/hm2. At 80% of SWC, similar experiments were carried out but at only one application rate(500 kg/hm2). N2O emissions at various water contents(20%, 35%, 50%, 65%, 80% and 100% of SWC) were studied. At low water content(25% of SWC), neither nitrogen nor phosphorus(or potassium) fertilizers led to a high level of N2O emission, which generally ranged from 2.03 to 29.02 microg/(m2 x h). However, at high water content(80% SWC), the fertilizers resulted in much greater N2O emission irregardless of soil tested. The highest N2O emission rates after 24 h of water addition were 1233 microg/(m2 x h) for S. superba soil, 1507 microg/(m2 x h) for P. elliottii soil and 1869 microg/ (m2 x h) for A. mangium soil respectively. N2O emission from soils treated with urea, (NH4)2 SO4 and KH2 PO4 immediately dropped to a low level but steadily increased to a very high level for the soil treated with KNO3. High NO3- content was a basis of high level of N2O emission. N2O emission rates from soils peaked shortly after flooding, rapidly dropping to a very low level in soil from non-legume plantations, but lasting for a relatively long period in soil from legume plantations. When soil water content increased equaling to or higher than 65%, the accumulated N2O emission over a period of 13 d ranged from 20.21-29.78 mg/m2 for S. superba, 30.57-70.12 mg/m2 for P. elliottii and 300.89-430.51 mg/m2 for A. mangium. The critical water content was 50% of SWC, above which a high level of N2O emission could be expected, and below which very little N2O emissions were detected. The results suggest that, at low water content (< 50% of SWC), the fertilization practice is safe with regard to N2O emissions, but at high water content (> 50% of SWC), nitrogen fertilizer in the form of nitrate could yield a 100-fold increase in N2O emissions. Legume plantations like A. mangium should be avoided in low lands which could easily suffer from flooding or poor drainage.

Accumulation of heavy metals in four grasses grown on lead and zinc mine tailings.

A field experiment was conducted to compare the growth and metal accumulation of Vetiveria zizanioides, Paspalum notatum, Cynodon dactylon and Imperata cylindraca var. major on the tailings, amended with 10 cm domestic refuse + complex NPK fertilizer(Treatment A), 10 cm domestic refuse(Treatment B) and complex NPK fertilizer (Treatment C) respectively, and without any amendment used as control (Treatment D). The results indicated that V. zizanioides was a typical heavy metal excluder, because the concentrations in shoots of the plants were the lowest among the four plants tested. The most of metal accumulated in V. zizanioides distributed in its root, and transportation of metal in this plant from root to shoot was restricted. Therefore, V. zizanioides was more suitable for phytostabilization of toxic mined lands than P. notatum and C. dactylon, which accumulated a relatively high level of metals in their shoots and roots. It was also found that I. cylindraca var. major accumulated lower amounts of Pb, Zn and Cu than C. dactylon and P. notatum, and could also be considered for phytostalilisaton of tailings. Although the metal(Pb, Zn and Cu) concentrations in shoots and roots of V. zizanioides were the lowest, the total amounts of heavy metals accumulated in shoots of V. zizanioides were the highest among the four tested plants due to the highest dry weight yield of it. The results indicated that V. zizanioides was the best choice among the four species used for phytoremediation (for both phytostabilization and phytoextraction) of metal contaminated soils.