覆膜对土壤-莴苣体系氮素分布和植物吸收的影响

覆膜种植作为全球广泛采用的农作物栽培方式，能改变土壤的水热生态效应，并影响元素的赋存状态。本文采用野外栽培实验，研究了覆膜对土壤—莴苣体系中氮素分布和植物吸收的规律。相比于不覆膜方式，覆膜对土壤含水率、pH值及脲酶活性的影响不明显，但减少了6.0%的土壤有机质、10.4%的硝态氮和1.3%的全氮含量，增加了6.5%的土壤铵态氮。单因素方差分析表明，土壤有机质及全氮含量的组间差异达到了显著性水平。覆膜方式下，氮素生理群落微生物中反硝化细菌占生理群落总数的77.8%—96.2%，氨化细菌、亚硝化细菌及硝化细菌各占1.8%—16.5%、0.6%—5.1%和0.4%—2.8%。与不覆膜相比，覆膜使氨化细菌平均值减少了28.2%，亚硝化、硝化及反硝化细菌平均值分别增加了119.8%、26.7%和48.7%。莴苣植株中的全氮含量变化规律为叶片>茎部>根部。覆膜使根部全氮含量降低了2.8% ，茎部与叶部则分别增加了10.5%和6.8%。覆膜使莴苣根部全氮的富集系数平均值降低了1.5%，迁移系数TF1（莴苣茎部和根部全氮的比值）和TF2（莴苣叶部和根部全氮的比值）平均值分别提高了12.5%和9.5%，影响较小。相关性分析表明，有机质与土壤全氮呈显著正相关（P < 0.05），而脲酶、铵态氮及无机氮三者与亚硝化细菌均呈显著负相关（P < 0.05）。

Effect of plastic film mulching on the distribution and translocation of nitrogen in soil-lettuce system

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(P<0.05)with total nitrogen in soil, while nitrite bacteria had a significant negative correlation(P<0.05)with urease, NH⁺₄-N and inorganic nitrogen.