镉污染水平影响城郊菜地土壤氮循环和迁移的机制研究

There are two problems in the suburban vegetable soil,including nitrogen (N) enrichment and cadmium (Cd) contamination.It make a serious challenge for suburban agriculture development and urban ecological environment. As the Cd contamination increasing in soil, it is still not clear that whether the "high N holding"could be resulted from that soil N cycle chain was destroyed by high Cd contamination. In this project, large undisturbed soil core lysimeters, soil culture and root-box culture experments will be conducted to investigate the N transformation, transference such as nitrate leaching and nitrous oxide emission in acid yellow-brown vegetable soil, N absorption and utilization of vegetable cutivars with higher or lower nitrate accumulation affected by different soil Cd contamination levels. Soil microbial ecology and N metabolism of plant will also be studied to reveal the mechannism of affecting the nitrogen cycle and transference of suburban vegetable soil,the difference in utilization soil and fertilizer nitrogen between vegetable cultivars with higher and lower nitrate accumulation by soil Cd contamination.It will provide a theoretical basis and technical support for improving the N cycle in suburban vegetable soil with N enrichment and Cd contamination,safe using suburban polluted agricultural soils,guaranteeing the agricultural production safety and ecological environment security, and promoting the sustainable development of suburban agriculture.

城郊区菜地土壤普遍存在"氮富集、镉污染"问题，使城郊农业发展和城市生态环境面临严峻挑战。随着土壤镉污染加重，是否导致菜地氮循环"链断裂"和菜地"氮滞留"，尚不清楚。本项目拟以酸性黄棕壤和本课题组已经筛选获得的硝酸盐积累高和低的蔬菜品种为材料，采用原状土柱试验、盆栽试验和根箱试验，从镉污染水平对氮富集菜地土壤氮素转化、迁移（含硝酸盐淋失、氧化亚氮排放）、蔬菜氮吸收、转化的影响等方面揭示镉污染水平对菜地土壤氮循环和迁移的影响和高、低积累品种吸收、利用土壤和肥料氮的影响，并从土壤微生物生态、植物氮代谢的角度揭示其机制，为改善和加速城郊农业区"氮富集、镉污染"菜地氮循环，安全利用城郊农业区污染土壤，保障城郊区农产品生产安全和生态环境安全，推动城郊区农业可持续发展提供理论依据和技术支持。

城郊区菜地土壤普遍存在“氮富集、镉污染”问题，土壤镉污染加重是否会导致菜地氮循环“链断裂”和菜地“氮滞留”，尚不清楚。本项目采用原状土柱试验、盆栽试验和根箱试验，研究了镉污染水平影响城郊菜地土壤氮循环和迁移的机制，获得的主要结果如下：1）系统地揭示镉污染水平对城郊菜地土壤氮循环、迁移的影响及其机制。发现镉污染显著增加了菜地土壤硝态氮的淋失浓度和淋失量，而镉污染水平较低时（0.5-1.0 mg kg-1）土壤硝态氮的淋失浓度和淋失量更大；镉污染增加了蔬菜种植后土壤中硝态氮和铵态氮含量，加大了土壤中活性氮的积累风险；较低浓度镉（0.5、1.0 mg kg-1）污染增加土壤氧化亚氮排放量，而高浓度镉则显著抑制氧化亚氮日排放量。2）明确了阻碍土壤氮富集、菜地氮循环的镉污染阈值。15N同位素示踪表明，土壤镉污染增加了肥料在根际和非根际土壤中的氮素残留量，当根际与非根际土壤中镉浓度分别为7.22和6.16 mg kg-1时，土壤中肥料氮素残留量达最大。3）提出镉污染导致菜地土壤氮循环“链断裂”的生态风险。轻度镉污染增加了土壤微生物土壤氧化微生物AOA和AOB基因丰度，随着镉污染水平的提高，反硝化微生物nirS、nosZ基因丰度显著下降，是造成土壤氮循环“链断裂”的原因之一。4）明确了镉及其污染水平对高、低硝酸盐积累蔬菜品种吸收、转化、利用土壤和肥料氮能力的影响及其机制。15N同位素示踪结果表明，随着镉污染水平的提高，植株对土壤及肥料氮素的吸收减少，植株体内硝态氮含量下降、铵态氮和氨基酸态氮含量累积，硝酸还原酶活性下降，谷氨酰胺合酶活性先增后降，蛋白质含量下降，说明镉从吸收、同化等过程干扰了植株对土壤氮及肥料氮的吸收和转化。5）探明了蔬菜高、低硝酸盐积累品种推动菜地土壤氮循环、高效利用氮素的作用范围。低浓度镉促进小白菜生长，而高浓度镉抑制其生长，影响小白菜硝酸盐高、低积累品种氮素积累量的临界镉浓度分别为1.2 mg kg-1 和43.3 mg kg-1，当镉污染水平低于该临界值时，种植小白菜均能促进氮吸收，推动菜地氮循环，反之则不利于作物对氮的吸收利用。研究结果对于改善和加速城郊镉污染菜地土壤氮循环，保障农产品生产安全和生态环境安全，推动城郊区农业可持续发展具有重要科学意义和实践价值。