通过田间对比试验,采用逐级提取技术,研究覆膜栽培方式对莴苣茎和叶中Cu、Fe、Cd总量和形态的影响及其健康风险评价.相比于不覆膜方式,覆膜栽培方式促进了土壤有机质的矿化和莴苣的生长,改变了重金属Fe和Cd向莴苣迁移的行为.覆膜方式对莴苣茎和叶中Cu的总量影响很小,但显著增加了莴苣茎中的Cd含量,降低了叶片中Fe的含量,平均增值为0.2 mg/kg和-150 mg/kg.形态分析结果表明覆膜栽培使莴苣茎中的水提取态Fe向盐酸提取态Fe转化,降低了其在茎中的生物活性,由此导致叶片部位总Fe以及盐酸提取态Fe和残渣态Fe的减少.覆膜组茎中Cd的活性并没有改变,其总Cd的增加归功于活性最低的残渣态含量的增加;而叶片中显著增加的乙醇提取态Cd由于占总量的比例很少,因此覆膜组叶片中总Cd并没有显著增加.参照食品安全国家标准,2种栽培方式下的莴苣叶中Cd总量均超标,存在食品安全问题,但以每日最大允许摄入量作为参考,莴苣各部位中3种元素的人均日暴露量均低于WHO推荐的ADI值2.4、42、0.06 mg/d.覆膜栽培方式对人体莴苣叶摄入量贡献率的影响较大,使Fe的贡献率降低了13.1％,而Cd的贡献率增加了6.53％,即增加了摄入Cd的潜在风险.

Plastic film mulching(FM) is one of the commonly used agricultural practices worldwide in crop production.It significantly alters soil hydrothermal properties and processes such as conserving soil moisture,modifying soil chemical and physical environment,and affecting the forms of elements.Soil nitrogen plays an important role in providing necessary nutrients for crops and thus enhancing crop productivity.Its forms and transport in soil-crop systems are expected to be strongly influenced by FM.In this paper,field experiments were conducted during the winter of 2009—2010 to study the effects of FM on soil physicochemical and microbial properties,and subsequently on the distribution and translocation of nitrogen in a soil-lettuce system.The experimental plots were located in eastern China where it is typically rather moist in winter.Lettuce was grown in the plot soil and sampled following a 50-day growth.Compared to unmulching(UFM),FM had little influence on soil moisture,soil pH and urease.By comparison,FM decreased soil organic matter content by 6.0%,soil total nitrogen by 1.3% and soil NO-3-N by 10.4%,respectively,while increased soil NH+4-N by 6.5%.The one-way analysis of variation(ANOVA) showed that only soil organic matter and total nitrogen were significantly different between FM and UFM.Other soil properties were not statistically different between the two practices.In FM treatment,denitrifier accounted for 77.8—96.2% of the total nitrogen physiological microorganisms,and ammonifier,nitrite bacteria and nitrifier accounted for 1.8—16.5%,0.6—5.1% and 0.4—2.8%,respectively.As such,FM decreased ammonifier by 28.2% and increased nitrite bacteria,nitrifier and denitrifier by 119.8%,26.7% and 48.7%,respectively,as compared to UFM.In lettuce grown in the mulched soil,nitrogen was distributed the most in leaf while the least in root.While FM decreased total nitrogen in root by 2.8%,it increased total nitrogen in stem and leaf by 10.5% and 6.8%,respectively.This indicates that FM facilitated the translocation of nitrogen from root to above-ground parts.The calculated bioaccumulation factor(BCF) of total nitrogen in root,which is expressed as the ratio of nitrogen in lettuce root to that in soil,showed that FM decreased BCF by 1.5%,indicating that FM treatment decreased the total nitrogen in root.The average translocation factors TF1(TF of total nitrogen from root to stem)and TF2(TF of total nitrogen from root to leaf) were higher in FM treatment than in UFM treatment.Lower BCF may thus have resulted from higher translocation factors in FM.However,the influences of FM on the BCF of total nitrogen in root and the TFs of total nitrogen in root,stem and leaf of lettuce were statistically insignificant.This may be due to the abundance of moisture in the experimental location that FM and UFM could not be significantly distinguished in terms of BCF and TFs.The correlation analyses indicated that soil organic matter was positively significantly correlated(P0.05)with total nitrogen in soil,while nitrite bacteria had a significant negative correlation(P0.05)with urease,NH+4-N and inorganic nitrogen.

Outdoor pot or/and field experiments were conducted to investigate the influence of plastic film mulching on the soil temperature,soil pH,soil organic matter,soil microorganisms and soil activity on two types of vegetable.The pot experiments showed that the soil temperature was increased by 0.8 ℃ under plastic film mulching.And both pot and field experiments showed that under plastic film mulching,the soil moisture was increased by 6.18%~10.02%,while soil pH and soil organic matter were less affected.The amount of bacteria and fungi in mulching soil was increased by 29.0%~29.1% and 17.3%~51.9%,respectively.The behave of actinomycetes,however,was more complicated.It was decreased by 46.2% in field mulching soils,but less affected in the pot soils.The microbial community structure was less changed in pot soil,but the percentage of actinomycetes was decreased by 58.0% than the control group in field soil.Soil enzyme analysis showed that the urease was decreased obviously in the mulching soil,while the phosphatase and the catalase were less affected.