On the Loess Plateau of China, a dry soil layer may form due to excess transpiration, leading to degradation of black locust(Robinia pseudoacacia) stands. In order to better manage projects involving black locust, this study was intended to investigate the response of black locust transpiration rate to soil water availability as affected by meteorological factors using two representative soils(loamy clay and sandy loam) on the Loess Plateau. Four soil water contents were maintained for black locust seedlings grown in pots initially outdoors and then in a climate-controlled chamber, by either drying or irrigating the pots. In both environments, daily transpiration rates were related by a power function to air temperature and by a logistic function to reference evapotranspiration(ET0). Transpiration rates were more susceptible to changes in the meteorological conditions in the sandy loam than in the loamy clay soil. The transpiration rate in the well-watered treatment was greater for black locust grown in the sandy loam than in the loamy clay soil. Normalized transpiration rates were unaffected by ET0 until a critical value of soil water content(θc) was attained; the θc value decreased significantly for the loamy clay soil but increased significantly for the sandy loam soil when ET0 increased. These suggested that the effect of the meteorological condition on the transpiration characteristics of black locust was dependent on soil texture. 相似文献
The extent and persistence of the effect of soil compaction in a system with annual ploughing were investigated in 21 long-term field experiments in Sweden with a total of 259 location-years. Crop yield, soil physical properties and plant establishment were determined. All experiments had two common treatments: control (no extra traffic) and compacted (350 Mg km ha−1 of experimental traffic in the autumn prior to ploughing), using a tractor and trailer with traditional wheel equipment and an axle load restricted to 4 Mg. During the rest of the year, both treatments were conventionally and equally tilled. The compaction was repeated each autumn for at least 7 years, and the yield was determined each year until 5 years after the termination of the compaction treatment.
Compaction decreased the porosity and the proportion of large pores and increased the tensile strength of dry aggregates. On clay and loam soils, it decreased the proportion of fine aggregates in the seedbed and the gravimetric soil water content in the seedbed.
The yield in the compacted treatment declined compared with the control during the first 4 years, after which it reached steady state. During this steady state, the compaction treatment caused a yield loss of 11.4%, averaged over 107 location-years. Within 4–5 years after the termination of the compaction treatment, the yield returned to the control level. The average yield loss at individual sites increased with increasing clay content.
Results from additional treatments indicated that yield loss was linearly correlated with the amount of traffic up to 300–400 Mg km ha−1. With greater ground contact pressure or a greater soil water content at time of traffic, there was a greater yield loss.
Soil compaction effects on yield were similar for all spring-sown crops, and the percentage yield loss seemed to be independent of the yield. In a few location-years with winter wheat there was on average no yield decrease.
There were 5.1% less plants in the compacted treatment than in the control. The yield decrease was significantly correlated with the number of plants.
Between years results were highly variable, and no consistent correlations between yield loss and soil water content at the time of traffic or the weather conditions during the growing period were found. Soil compaction affected yield during years with good as well as poor conditions for crop growth. 相似文献
Soils in urban areas are often artificially formed and preserved as cultural layers in which soil materials are mixed with artifacts. The vertical change of soil properties denotes the historical change of human activities, and therefore, urban soil can be considered as a record of history of urban development. We studied a deep profile of urban soil (600 cm) in Nanjing, China, by intensively sampling at every 5 cm. Soil samples were analyzed for lead content. Charcoal from several layers was found and was dated using 14C to recognize archaeological cultural layers that might be formed in different Chinese dynasties. Pb isotope ratios were determined by thermal ionization mass spectrometry (TIMS) technology. The study found that artificial deposition, i.e., the formation of cultural layer started about 1700 years ago and lasted until recently. All cultural layers above the original loess were heavily contaminated by Pb with total content varying from 100 mg kg− 1 to more than 2000 mg kg− 1. There were several historical periods in which Pb content peaked, indicating significant contamination by human activities, such as ancient ore smelting for various purposes and use of Pb-containing materials for handicraft manufacture. Various Pb isotope ratios of the cultural layers differed substantially from that of the original undisturbed loess in the deepest position. 206Pb/207Pb value decreased gradually from the bottom layer to top layer, although with occasional exceptions, indicating a long-lasting mixing of extraneous sources of Pb during the entire history since ca. 300 A.D. In comparison with the isotope ratios of lead ores of different sources and that of aerosols, it was illustrated that the source of Pb in cultural layers might come from lead ores of southern China in the earlier dynasties, while Pb from north China might have contributed to the Pb source in the more recent years, suggested by the lowering of 206Pb/207Pb ratio in the near-surface cultural layers. However, the impact of petrol burning on cultural layers was basically excluded, considering the relatively short history of petroleum use in this area. 相似文献