Xingxiang Wang

Effects of several amendments on rice growth and uptake of copper and cadmium from a contaminated soil

Heavy metals in variable charge soil are highly bioavailable and easy to transfer into plants. Since it is impossible to completely eliminate rice planting on contaminated soils, some remediation and mitigation techniques are necessary to reduce metal bioavailability and uptake by rice. This pot experiment investigated the effects of seven amendments on the growth of rice and uptake of heavy metals from a paddy soil that was contaminated by copper and cadmium. The best results were from the application of limestone that increased grain yield by 12.5-16.5 fold, and decreased Cu and Cd concentrations in grain by 23.0%-50.4%. Application of calcium magnesium phosphate, calcium silicate, pig manure, and peat also increased the grain yield by 0.3-15.3 fold, and effectively decreased the Cu and Cd concentrations in grain. Cd concentration in grain was slightly reduced in the treatments of Chinese milk vetch and zinc sulfate. Concentrations of Cu and Cd in grain and straw were dependent on the available Cu and Cd in the soils, and soil available Cu and Cd were significantly affected by the soil pH.

The Composition of Root Exudates from Two Different Resistant Peanut Cultivars and Their Effects on the Growth of Soil-Borne Pathogen

The high incidence of various soil-borne diseases in the monoculture field of peanut is a major production constraint in the red soil regions of southern China. The peanut root exudates are generally thought to play an important role in regulating soil-borne pathogens. The responses of the soil-borne pathogens, Fusarium oxysporum and F. solani to the peanut root exudates were studied using one susceptible cultivar Ganhua-5 (GH) and one mid-resistant cultivar Quanhua-7 (QH) as the test materials. The components and contents of the amino acids, sugars and phenolic acids in the peanut root exudates were determined. The results demonstrated that the root exudates from both susceptible and mid-resistant cultivars significantly promoted the spore germination, sporulation and mycelial growth of soil-borne pathogens, F. oxysporum, F. solani compared with the control. The extent of the stimulation was depended on the strains of the Fusarium tested, and gradually increased with the increased concentrations of peanut root exudates. HPLC analysis showed that the contents of sugars, alanine, total amino acids in the root exudates of GH were significantly higher than that in QH, whereas the contents of p-hydroxybenzoic acid, benzoic acid, p-coumaric acid and total phenolic acids were significantly lower than that in QH. Results of the study suggested that the differences in the root exudates from the different peanut cultivars were considered to regulate the wilt-resistance mechanism in the rhizosphere of peanut. The results are therefore crucial important to illustrate the mechanism of peanut replanted obstacle, and to develop its control techniques in the red soil regions of southern China.

Prediction Model for Cadmium Transfer from Soil to Carrot (Daucus carota L.) and Its Application To Derive Soil Thresholds for Food Safety

At present, soil quality standards used for agriculture do not fully consider the influence of soil properties on cadmium (Cd) uptake by crops. This study aimed to develop prediction models for Cd transfer from a wide range of Chinese soils to carrot (Daucus carota L.) using soil properties and the total or available soil Cd content. Path analysis showed soil pH and organic carbon (OC) content were the two most significant properties exhibiting direct effects on Cd uptake factor (ratio of Cd concentration in carrot to that in soil). Stepwise multiple linear regression analysis also showed that total soil Cd, pH, and OC were significant variables contributing to carrot Cd concentration, explaining 90% of the variance across the 21 soils. Soil thresholds for carrot (cultivar New Kuroda) cropping based on added or total Cd were then derived from the food safety standard and were presented as continuous or scenario criteria.

Phosphorus in interstitial water induced by redox potential in sediment of Dianchi Lake, China

The sediment redox potential was raised in the laboratory to estimate reduction of internal available phosphorus loads, such as soluble reactive phosphorus (SRP) and total phosphorus (TP), as well as the main elements of sediment extracts in Dianchi Lake. Several strongly reducing substances in sediments, which mainly originated from anaerobic decomposition of primary producer residues, were responsible for the lower redox potential. In a range of -400 to 200 mV raising the redox potential of sediments decreased TP and SRP in interstitial water. Redox potentials exceeding 320 mV caused increases in TP, whereas SRP maintained a relatively constant minimum level. The concentrations of Al, Fe, Ca2+, Mg2+, K+, Na+ and S in interstitial water were also related to the redox potential of sediments, suggesting that the mechanism for redox potential to regulate the concentration of phosphorus in interstitial water was complex.

Optimization of RP-HPLC Analysis of Low Molecular Weight Organic Acids in Soil

Abstract RP‐HPLC analysis for low molecular weight organic acids in soil solution has been optimized. An Atlantis™ C18 column was used for the analyses. An optimal determination for eleven organic acids in soil solution was found at room temperature (25°C) and 220 nm detection wavelength, with a mobile phase of 10 mM KH2PO4–CH3OH (95∶5, pH 2.7), a flow rate of 0.8 mL/min and 10 µL sample size. The detection limits ranged 3.2–619 ng/mL, the coefficients of variation ranged 1.3–4.6%, and the recoveries ranged 95.6–106.3% for soil solution with standard addition on the optimal conditions proposed.

Declined soil suppressiveness to Fusarium oxysporum by rhizosphere microflora of cotton in soil sickness

Cotton yield and quality have been severely compromised by soil sickness throughout the primary cotton-growing regions of China. The aim of this study was to gain insight into the role of rhizosphere microbial community in governing soil sickness of cotton. Plant growth, disease resistance, root exudates, and the composition of the rhizosphere microbial community of cotton were analyzed in two different (4- and 15-year) monocropped soils and in a fallow agricultural soil (control). The monocropped soils significantly influenced cotton growth and root exudates and reduced soil suppressiveness to Fusarium wilt in bioassay experiments. Additionally, pyrosequencing of the whole internal transcribed spacers (ITS1 and ITS2) and 16S rRNA gene amplicons demonstrated clear variations in the microbial composition of cotton rhizosphere between monocropped rhizosphere soils and control soil. Specifically, monocropped soils were characterized by an increase in the abundance of fungal pathogens, including Fusarium oxysporum f. sp. vasinfectum and Verticillium dahliae, relative to the rhizosphere of control soil. Some plant-beneficial and disease-suppressive bacterial taxa, including Xanthomonadaceae, Comamonadaceae, Oxalobacteraceae, and Opitutaceae, were associated with healthy cotton. A significant correlation existed between the presence of certain amino acids (e.g., glutamic acid and alanine) and the above identified taxa, indicating that some constituents in root exudates influenced the microbial compositions of the cotton rhizosphere to manage the disease status of plant in monocropped soils. Collectively, these results suggest that pathogenic fungal build-up and a reduction in the abundance of beneficial rhizobacteria in the rhizosphere contribute to changes in soil suppressiveness to soil-borne pathogens in monocropped soils, resulting in an aggravated level of soil sickness.

Root-induced changes in radial oxygen loss, rhizosphere oxygen profile, and nitrification of two rice cultivars in Chinese red soil regions

AimsTo evaluate the external and internal morphological differences of roots that might influence rice root radial oxygen loss (ROL) and the corresponding rhizosphere nitrification activity, growth characteristics and nitrogen nutrition of rice.MethodsThe root ROL and rhizosphere oxygen profile were determined using a miniaturised Clark-type oxygen microelectrode system, and the rhizosphere nitrification activity was studied with a short-term nitrification activity assay.ResultsThe rice biomass, nitrogen accumulation and nitrogen use efficiency (NUE) of ZH (high yield) were significantly higher than those of HS (low yield). The root biomass, number, diameter and porosity of ZH were also much greater than those of HS. The inner and surface oxygen concentrations of the root of ZH were significantly higher than those of HS. The order of paddy soil oxygen penetration depth was ZH > HS > CK, and the order of the oxygen concentrations detected in the water layer and rhizosphere soil was the same. The rhizosphere nitrification activity and nitrate concentration of ZH were significantly higher than those of HS.ConclusionsMore porous and thicker roots improved the individual root ROL, and more adventitious root numbers enhanced the entire plant ROL and correspondingly improved the rhizosphere nitrification activity, which might influence the growth and nitrogen nutrition of rice.

Effects of soil type and genotype on lead concentration in rootstalk vegetables and the selection of cultivars for food safety

Lead (Pb) contamination of soil poses severe health risks to humans through vegetable consumption. The variations of Pb concentration in different parts of rootstalk vegetables (radish, carrot and potato) were investigated by using twelve cultivars grown in acidic Ferralsols and neutral Cambisols under two Pb treatments (125 mg kg(-1) and 250 mg kg(-1) for Ferralsols; 150 mg kg(-1) and 300 mg kg(-1) for Cambisols) in a pot experiment. The Pb concentration in edible parts was higher in Ferralsols under two Pb treatments, with range from 0.28 to 4.14, 0.42-10.66 mg kg(-1) (fresh weight) respectively, and all of them exceeded the food safety standard (0.1 mg kg(-1)) recommended by the Codex Alimentarius Commission of FAO and WHO. The Pb concentration in edible parts was significantly affected by genotype, soil type and the interaction between these two factors. The variation of Pb concentration in different cultivars was partially governed by Pb absorption and the transfer of Pb from aerial to edible part. The results revealed that caution should be paid to the cultivation of rootstalk vegetables in Pb-contaminated Ferralsols without any agronomic management to reduce Pb availability and plant uptake. For Cambisols with slight to moderate Pb contamination, growing potato cultivar Shandong No.1 and Chongqing No.1 was effective in reducing the risk of Pb entering human food chain. The results suggest the possibility of developing cultivar- and soil-specific planting and monitoring guidelines for the cultivation of rootstalk vegetables on slight to moderate Pb-contaminated soils.

Variation of soil enzyme activities and microbial community structure in peanut monocropping system in subtropical China

The changes in soil microbial community are supposed to be one of key factors for peanut yield decline in long-term continuous monocropping systems. A series of peanut fields, where peanut were continuously monocropped for 3, 6, 10 and 15 years in subtropical China, were selected to investigate the effect of continuous monocropping on peanut yield, soil microbial community structure and enzyme activity. Peanut yield, urease and invertase activities decreased with time, but the activity of polyphenol oxidase decreased in the first several years and then increased. The results of both culture dependent methods and phospholipid fatty acids (PLFA) analysis showed that bacteria (Gram-positive and Gramnegative bacteria) and actinomycetes decreased, while fungal colony-forming units (CFUs) increased with time. Canonical Correspondence Analysis (CCA) showed that with increasing peanut monocropping years, soil microbial structure became more correlative with fungi compositions. The proportion of bacteria in total PLFA decreased from 67.4% to 53.0%, meanwhile the proportion of fungi was increased from 16.9% to 32.8%. Denaturing gradient gel electrophoresis (DGGE) analysis indicated that bacteria diversity decreased and fungi diversity increased with time, and changes in fungi diversity were much greater than those of bacteria.

Plant symbionts: keys to the phytosphere

The exterior and interior of plants, aboveground and belowground, comprise a complex plant micro-ecosystem, known in recent years as the phytosphere. There are three components: the phyllosphere, endosphere, and rhizosphere. Although in comparison with other ecosystems the phytosphere is small, it similarly includes a great variety of functional microbes. Among these are certain microbes that live in symbiotic relationships with plants; these microbes are known as plant symbionts. Recent research has shown that these symbionts have tremendous effects on plant growth, confer resistance to abiotic stresses and pathogens, aid in the accumulation of metabolites, and have crucial relationships with other plant-associated microbes in the phytosphere. We review the ecological effects of plant symbionts on other microbes, and interactions between plant symbionts in the phytosphere. In addition, we discuss internal mechanisms and suggest future hot spots for research.