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Plastic‐film mulch significantly improves maize (Zea mays L.) productivity in hydrothermally limited areas, but there is a lack of understanding of how mulch affects the partitioning of dry matter and nutrients to grain. This study was conducted to investigate the effects of plastic‐film mulch on the remobilization of biomass and nutrients accumulated in the presilking stage to grain during the grain‐filling period. A 2‐yr field experiment was conducted with five maize hybrids in a cold semiarid environment. The vegetative biomass in all cultivars at maturity was generally greater than that at silking regardless of whether the soil was mulched or not, indicating that there was no translocation of presilking assimilates to the grain. The increased grain yield in plastic‐film mulched plots compared with those that were nonmulched was ascribed to their enhanced partitioning of postsilking assimilates to grain. Conversely, increased accumulation of nitrogen and phosphorus in grain at maturity under mulch relative to no mulch mainly was due to increased remobilization from presilking vegetative organs. The benefit of plastic‐film mulch in increasing the partitioning of postsilking assimilates and the remobilization of nutrients from presilking vegetative organs was greater in a drier than in a wetter year. The results of this study strongly support the conclusion that in hydrothermally limited environments, maize generally does not remobilize presilking assimilates but does remobilize presilking nitrogen and phosphorus in vegetative organs to the grain during grain filling. Further, plastic‐film mulched, relative to nonmulched, maize increases the translocation to the grain of postsilking assimilates and presilking nitrogen and phosphorus accumulated in vegetative organs. Core Ideas: Mulch increased grain yield through enhanced partitioning of postsilking assimilates to grain.Mulch increased N & P remobilization from vegetative organs to maintain the grain concentration.Mulching efficiently alleviates effects of unfavorable climates on grain formation. [ABSTRACT FROM AUTHOR]

The effects of human activities on heavy metal pollution in soil have been less investigated on the Tibetan Plateau. The present study was designed to assess the effects of highway traffic on Cu, Zn, Pb, and Cd enrichments in the 0-60-cm soil profile in the eastern Tibetan Plateau. Soils were sampled at four transects (with an altitude range of 2643-2911 m) across the G212 highway and five transects (3163-3563 m) across the G213 highway. Background concentrations of Cu, Zn, Pb, and Cd to the 60-cm soil depth (measured at each transect 400 m away from highways) varied greatly among transects and between highways. However, this spatial variation in the heavy metal concentrations was not related to the altitude of the investigated areas. On each the left and right sides of G212 or G213, Cu, Zn, and Pb concentrations to the 60-cm depth, at 5, 10, 20, and 50 m away from the highway, were all generally greater than the respective metal background concentrations. Cd concentrations to the 20 cm on G212 or 60-cm soil depth on G213 increased prominently within a distance of 20 m away from the highways, compared to background values in different depths. From the curb to 400 m away from highways, concentrations of Cu, Zn, Pb, and Cd were generally higher in the upper than in the lower soil layers. This may suggest that other factors such as atmospheric deposition were also contributable to the accumulation of heavy metals in soil. The contamination factor (C f ) calculation showed that roadside soils to the 60-cm depth, within a distance of 50 m from the curbs of both G212 and G213, were moderately (1 ≤ C f  < 3) contaminated with Cu, Zn, and Pb. The contamination from Cd mainly occurred (1 ≤ C f  < 8) in the top 20 cm soil with a closer distance from the highways. Our results indicated that traffic effects in enriching heavy metals reached 60-cm depth in roadside soils on the eastern Tibetan Plateau. For assessment of heavy metal pollutions in soil in mountainous areas, it is necessary to in situ identify the background values.