Search

In field conditions, a micro-aerobic layer with 1 cm thickness exists on the surface layer of paddy soil owing to the diffusion of dissolved oxygen via flooding water. However, the particularity of carbon and nitrogen transformation in this specific soil layer is not clear. A typical subtropical paddy soil was collected and incubated with 13 C-labelled rice straw for 100 days. The responses of exogenous fresh organic carbon( 13 C-rice straw) and original soil organic carbon mineralization to nitrogen fertilizer addition[(NH 4 ) 2 SO 4 ]in the micro-aerobic layer(0-1 cm) and anaerobic layer(1-5 cm) of paddy soil and their microbial processes were analyzed based on the analysis of 13 C incorporation into phospholipid fatty acid( 13 C-PLFAs). Nitrogen addition promoted the total CO 2 and 13 C-CO 2 emission from paddy soil by 11.4% and 12.3%, respectively. At the end of incubation, with the addition of nitrogen, the total soil organic carbon (SOC) and 13 C-recovery rate from rice straw in the anaerobic layer were 2.4% and 9.2% lower than those in the corresponding micro-aerobic layer, respectively. At the early stage(5 days), nitrogen addition increased the total microbial PLFAs in the anaerobic layer with a consistent response of bacterial and fungal PLFAs. However, there was no significant effect from nitrogen on microbial abundance in the micro-aerobic layer. Nitrogen addition had no significant impact on the abundance of total 13 C-PLFAs in the micro-aerobic and anaerobic layers, but the abundance of 13 C-PLFAs for bacteria and fungi in the micro-aerobic layer was decreased dramatically. At the late stage(100 days), the effect of nitrogen addition on microbial PLFAs was consistent with that at the early stage. The abundances of total, bacterial, and fungal 13 C-PLFAs were remarkably increased in the anaerobic layer. However, the abundance of 13 C-PLFAs in the micro-aerobic layer showed no significant response to nitrogen addition. During the incubation, the content of NH 4 + -N in the anaerobic soil layer was higher than that in the micro-aerobic soil layer. This indicates that nitrogen addition increased microbial activity in the anaerobic soil layer caused by the higher NH 4 + -N concentration, as majority of microorganisms preferred to use NH 4 + -N. Consequently, the microbial utilization and decomposition of organic carbon in the anaerobic soil layer were accelerated. By contrast, richer available N existed in the form of NO 3 - -N in the micro-aerobic soil layer owing to the ammoxidation process. Thus, the shortage of NO 3 - -N preference microorganisms in the paddy soil environment prohibited the microbial metabolism of organic carbon in the micro-aerobic layer. As a whole, nitrogen fertilization enhanced organic carbon loss via microbial mineralization in paddy soil with a weaker effect in the micro-aerobic layer than that in the anaerobic layer, indicating the limited microbial metabolic activity in the surface micro-aerobic layer could protect the organic carbon stabilization in paddy soil. This study emphasizes the heterogeneity of paddy soil and its significant particularity of carbon and nitrogen transformation in micro-aerobic layers. Consequently, this study has implications for optimizing the forms and method for the application of nitrogen fertilizer in paddy cropping systems.

Field investigation on the content of heavy metals in soils and three types of widely distributing compositae plants( Artemisia lavandulaefolia, Ageratum conyzoides L. , Crassocephalum crepidioides ) in lead-zinc tailings farmland of Yangshuo, Guangxi Zhuang Autonomous Region was carried out, and the differential centrifugation technique and sequential chemical extraction method were used to study the subcellular distribution and chemical forms of heavy metals in these plants. The results indicated that the soil in the tailings farmland was highly contaminated by Cd, Pb and Zn, and their concentrations were 37.7, 5.7 and 8.9 times higher than their respective values of national standard for soil environment quality(GradeⅡ). The contents of Cd, Pb and Zn in the analyzed plants exceeded the normal ranges. Ageratum conyzoides L. and Crassocephalum crepidioides showed strong capability in tolerance, accumulation and transport of Cd, and they could be used as pioneer plants for Cd-phytoremediation in study area and some related areas. In addition, Cd concentrations in the stem and leave of Crassocephalum crepidioides were 159.6 mg·kg -1 and 219.5 mg·kg -1 , respectively, which exceeded the threshold of Cd hyperaccumulator. Thus, Crassocephalum crepidioides can be regarded as a Cd-hyperaccumulator. Majority of total Cd, Pb and Zn were found in soluble fraction and cell wall in the plants, while only a small quantity were distributed in organelles. As for chemical forms distribution, Cd, Zn and Pb predominated NaCl-, HAC-, and HCl-extractable forms both in roots and shoots of the plants. Therefore, cell wall binding, vacuolar compartmentalization and distribution mainly in lower active chemical forms were supposed to be the main mechanisms for tolerance to heavy metals in the study plants. A difference of Cd subcellular distribution and its chemical forms in the three Compositae plants was observed. Compared with the shoot of Artemisia lavandulaefolia , more Cd was located in the vacuolus and cellular soluble parts in Ageratum conyzoides L. and Crassocephalum crepidioides . Moreover, the proportion of active chemical Cd in the shoots was lower than that in the roots. These results showed that the subcellular distribution and chemical forms of Cd were related to the plant species,and also indicated Ageratum conyzoides L. and Crassocephalum crepidioides were likely to possess a higher tolerance and accumulation of Cd.

Soil samples were collected from compound polluted tailings to analyze the contents of total heavy metals and their speciation in the soil. Laboratory batch tests were conducted to examine the effects of distilled water and different concentrations of oxalic acid, citric acid, acetic acid, HNO3 and EDTA on the removal of heavy metals from the polluted soils. The suitable eluent and its optimal conditions including liquid to soil ratio, reaction time and washing number were also optimized, and the total toxicity reduction index was proposed to evaluate the effect of the eluent on the remediation of polluted soil. The results showed that Cd and Pb were the most abundant heavy metals in the soil, reaching 52.2 mg x kg(-1) and 4836.5 m x kg(-1), respectively. There was significant difference in the removal efficiency for different heavy metals. Cr had a maximum removal efficiency of 2.7%, while the maximum Cd and Pb removal efficiency was both about 60%. Distilled water had little removal efficiency for heavy metals, with less than 0.1% removal rate; the heavy metal removal efficiency of oxalic acid and acetic acid was also quite low; EDTA in 0.1 mol x L(-1) was selected as the suitable eluent for the polluted soil. Evaluation of the total toxicity reduction index and the cost suggested that EDTA should be used with a liquid to soil ratio of 6:1, a reaction time of 3 h and 2 washings.

This study has taken advantage of the characteristics of concentration of soil organic matter (SOC) and delta13 C(SOC) values to provide proofs for environment quality assessment and to know more about polluted sources, sizes and processes in Beijing steel company area. delta13C values of SOC is good for tracing sources and documenting shifts in community composition and distribution. Two sections (Beijing steel company area and Yongledian, Tongzhou) which belong to two different soil types collected in Beijing, and organic carbon isotopic composition and total soil organic carbon were analyzed. These results shows that SOC of soil samples from Beijing steel company area are quite high, and even 9.7% at the surface sample, however SOC from unpolluted area (Yongledian area) is lower than those of industrial area. delta13 C(SOC) from soils of Beijing steel company area and Yongledian area respectively vary from -24.8 per thousand to -23.1 per thousand and -26.4 per thousand to -20.5 per thousand, the results are quite different. The results reflect that there are different organic carbon sources in different types' soil: Organic carbon from Beijing steel company area has been mainly affected by coal burning, soil organic carbon concentrations are quite high, and pollution can affect on soils 70 cm deep underground; and soils from Yongledian area, have been not polluted, and organic matter is from natural litter (C3 plants). Although there are different soil organic carbon concentrations and isotope compositions, two soil sections have similar variation trends. This study provides proofs for environment quality assessment and know more about polluted and natural sources, sizes in Beijing.