Interactive effects of hydrological conditions on soil respiration in China's Horqin sandy land: An example of dune-meadow cascade ecosystem.

Soil moisture (M) strongly influences dynamic changes in soil respiration (R) and is thus an important factor when predicting soil carbon emissions. However, the various sources of M (rainfall, groundwater, and condensation) exert complicated and uncertain effects on R. This study examined the growth seasonal variation (from April to October) of R and the diurnal variation in a cascade ecosystem consisting of sandy bare ground, a transitional artificial Populus forest, and a meadow Phragmites communis community in China's Horqin sandy land. Simultaneous measurements of the 0-10 cm depth soil temperature (T) and M, rainfall, the surface air relative humidity and the groundwater depth were collected. The results revealed that in sandy bare ground with M below field capacity, M had a greater impact on R than T, and rainfall could increase R The effect of condensation on R during periods of continuous drought could not be ignored. In the meadowlands with M above field capacity, the groundwater affected R indirectly by regulating M and the relationship with T, and rainfall had an adverse effect on R. The effects of rainfall, M and T on R were minimum as M approached the saturation water content. In the transitional forest, M and T were the main factors controlling R. The most favorable M for R was close to the field capacity. The results emphasize that field capacity and saturation water content are the demarcation points of a soil carbon emissions prediction model, and the effect of different hydrological conditions and T on R at each segment are reconsidered accordingly. Ultimately, the carbon emission patterns of the cascade ecosystems in arid and semi-arid areas are extremely complicated and have to be considered specially for estimating terrestrial carbon emissions.