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Little is known about the surface energy balance problem for a complex underlying surface. Taking data from the Loess Plateau Land-surface Processes Experiment (LOPEX) and investigating the characteristics of the surface energy balance over a complex underlying surface, this paper calculates the soil heat storage and vertical sensible heat advection, analyzes their contributions to the surface energy imbalance, and discusses the mechanism by which the vertical velocity and temperature gradient in the surface layer affect the vertical sensible heat advection transfer. We found that the vertical velocity in the surface layer provides the necessary dynamic power for vertical sensible heat advection, and a relatively strong temperature gradient is the energy source generating vertical sensible heat advection. Under an ascending condition, the effect of vertical sensible heat advection on the surface energy budget is more obvious. It is also found that when the soil heat storage term and the vertical sensible heat advection term are added to the energy balance equation, the imbalance significantly improves. The peak of average diurnal residuals decreases from 125.1 to 41.5 W m, the daily average absolute value of residuals falls from 59.0 to 26.4 W m, and the surface energy balance closure increases from 78.4% to 94.0%. [ABSTRACT FROM AUTHOR]

The nitrogen isotope of soil is of emerging significance as an indicator of climatic change and blogeochemical cycle of nitrogen in nature systems. In this paper, the nitrogen content and isotopic composition of modern ecosystems from arid and semiarid Loess Plateau in northwestern China, including plant roots and surface soil, were determined to investigate trends in δ15N variation of plant roots and soil along a precipitation and temperature gradient in northwestern China under the East Asian Monsoon climate condition. The δ15N values of surface soil from the study area vary from -1.2‰ to 5.8‰, but from -5.1‰ to 1.9‰ in the plant roots. Our results indicate that (1) although the isotopic compositions of both plant roots and surface soil change with a similar trend along the climate gradient, the apparent nitrogen difference between plant roots and soil existed, with Δδ15N values ranging from 0.3‰ to 7.2‰ with average of 4.1‰ and (2) mean annual precipitation (MAP) is the dominant factor for isotopic composition of plant-soil nitrogen in the Loess Plateau, and the δ15N values are less correlated with MAT; we suggest that nitrogen isotopic composition of soil is a potential tracer for environmental changes. [ABSTRACT FROM AUTHOR]