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.